Comparing low light performance of different cameras

[Physics1]

You ignored my posts. Do you need help with the physics, Physics1? You should read Joseph James' and Richard Butler's articles. Then work out the exposures for yourself in the DPR tests, look at Bill Claff's input-referred noise data, and make the appropriate deductions. Start with Butler's articles.

http://www.josephjamesphotography.com/equivalence/

https://www.dpreview.com/articles/2666934640/what-is-equivalence-and-why-should-i-care

https://www.dpreview.com/learn/2799100497/equivalence-in-a-nutshell

Remember that the fraction of light collected from each subject point is proportional to the lens entrance pupil area and inversely proportional to the square of the distance to the subject. Remember also that the magnification is proportional to the focal length, and the entrance pupil diameter D = f/N, where N is the f-number. You can figure out how to normalize the DPR tests and figure out the appropriate exposures. You should find that the situation is what I already told you. Good luck. I'm traveling, and can't give any handholding.

Yes, I'm pretty sure that I could do it, though it does take me a bit of time and effort. Remember, I don't live and breathe this stuff like you and others here do and only started thinking about this stuff around a week ago

But ultimately you are likely to be in the situation where the DP Test Image taken under the conditions that you would like does not exist, and you need to choose the closest, which is a bit rough and approximate.

Also, how often, actually, are we in the situation where shot noise is not dominant? I don't usually care about DOF, but I take the point that if you do, then you may be forced to stop down the lens and compensate with high ISO. And if the ISO is high enough then you have very few measured photons, and the readout noise could be significant. In fact eventually it must become dominant, but before that happens the shot and readout noise could be so high relative to signal that the image is probably unusable anyway? Hey, I'm not a photographer or even a photographic enthusiast and have only my broad scientific background to guide me Am I understanding the broad idea here?

 

[Dem Bell]

In case anyone else is interested in buying a similar class of camera and cares about low light performance, here is a summary of 4 camera that I looked at. All focal lengths and f-numbers are quoted as 35 mm equivalent.

• My 2008 model Canon G10, used as a baseline
• 1/1.7"sensor
• Crop Factor = x4.6
• 28 - 140 mm (x5 zoom)
• f/12.9 - 20

• Canon G7X III
• 1" sensor
• Crop Factor = x2.7
• 24 - 100 mm (x4.2 Zoom)
• f/4.9 - 7.6

• Sony RX100 MK VII
• 1" sensor
• Crop factor = x2.7
• 24 - 200 mm (X8 zoom)
• f/7.6 - 12.2

• Sony A6500 / A6700 with Sony 27-202 mm lens
• APS-C sensor
• Crop factor = x1.5
• 27 - 202 mm (x7.5 zoom)
• f/5.25 - 8.4

To compare low-light performance the real aperture number, not the equivalent aperture number must used.

This depends on how one quantities "low-light performance". I would look at the total light as a proxy for image quality and this correlates with the equivalent f-number.

The equivalent aperture is used to compare the DOF.

DOF, background blur, diffraction blur and total light at a given shutter speed.

 

[Bill Ferris]

To compare low-light performance the real aperture number, not the equivalent aperture number must used. The equivalent aperture is used to compare the DOF.

We had better clear this up before proceeding. Yes, the equivalent aperture can be used to compare DOF. But it is also the correct choice for comparing low light performance.

Depth of field is related to the aperture diameter. Low light performance is also related to aperture diameter or, more precisely aperture area, which is what determines how much light the lens captures for a given FOV. So what is good for one is good for the other.

Surely you must have noticed that cameras with small sensors such as the Canon G15 with 1/1.7" sensor also have amazing low f-numbers, given that the lenses are physically very small. For example, the G15 sports a 5:1 zoom lens with f/1.8-2.8!! And it's small to boot. Wow. If that sounds too good to be true then it's because it is. As far as low light capability is concerned, you cannot validly compare f-numbers across different sensor sizes, but must instead convert to "35 mm equivalent" f-numbers by multiplying by the sensor crop factor. That is what I did, and it is the correct procedure for comparing low light performance.

One can alternatively calculate the lens aperture diameter (D=L/fnumber) and use that to compare low light performance. The two methods are mathematically equivalent. Sometimes one method is easier and more natural, sometimes the other.

I'm in a photo blind with another photographer. It's morning twilight and we're both focused on the same animal. We have the same make & model full-frame camera bodies but are using different lenses. I'm using an 800mm f/6.3 and my friend is using a 500mm f/4.

Which system captures more light from the animal?

 

[Horacecoker]

OK, so the question is, what methodology should be used for comparing the low light performance of different cameras. Clearly any methodology worth a pinch of salt must be able to determine that Camera B has the better low light performance.

In a previous posting I described how such testing would be done. Both cameras are set to the "best that they can do" to optimize their low light performance, being F/2 and F/4 in this case. They are then both pointed at the same dimly lit scene, at the same distance. Because they both have the same focal length of lens, they will both capture the same angular Field of View, and the scene image will be the same size on both sensors. ISO is set to a highish value to ensure that there is visible noise in the images, and both are set to the same shutter speed.

Both cameras can't be set to the same ISO in that scenario as the f/2 camera image will appear two stops overexposed.

Yes, of course. I apologize for my careless wording. To quote from my original posting :-

Of course, all images need to be made equally bright by appropriate setting of ISO.

With that misunderstanding out of the way, do you agree with what I am saying?

You don't make it clear how the cameras are to be set to the appropriate ISOs. Is it by the user or the camera?

 

[Horacecoker]

To compare low-light performance the real aperture number, not the equivalent aperture number must used. The equivalent aperture is used to compare the DOF.

We had better clear this up before proceeding. Yes, the equivalent aperture can be used to compare DOF. But it is also the correct choice for comparing low light performance.

Depth of field is related to the aperture diameter. Low light performance is also related to aperture diameter or, more precisely aperture area, which is what determines how much light the lens captures for a given FOV. So what is good for one is good for the other.

Surely you must have noticed that cameras with small sensors such as the Canon G15 with 1/1.7" sensor also have amazing low f-numbers, given that the lenses are physically very small. For example, the G15 sports a 5:1 zoom lens with f/1.8-2.8!! And it's small to boot. Wow. If that sounds too good to be true then it's because it is. As far as low light capability is concerned, you cannot validly compare f-numbers across different sensor sizes, but must instead convert to "35 mm equivalent" f-numbers by multiplying by the sensor crop factor. That is what I did, and it is the correct procedure for comparing low light performance.

One can alternatively calculate the lens aperture diameter (D=L/fnumber) and use that to compare low light performance. The two methods are mathematically equivalent. Sometimes one method is easier and more natural, sometimes the other.

I'm in a photo blind with another photographer. It's morning twilight and we're both focused on the same animal. We have the same make & model full-frame camera bodies but are using different lenses. I'm using an 800mm f/6.3 and my friend is using a 500mm f/4.

Which system captures more light from the animal?

I'd say the the 800mm lens due to the animal being much bigger in the frame. 60% bigger in all linear directions.

 

[Icee258]

You are upgrading from a 16 year old camera with a very small sensor to a new(er) camera with an APS-C sensor. You plan a detailed experiment to tease out the best low-light performance in modern cameras.

But then you want a built-in flash, so you're going to sacrifice years of sensor and AF improvements to get an 8 year old camera? Seriously? Is that because you're overvaluing IBIS? Get any newer APS-C body and pair it with lenses with VR.

 

[ThrillaMozilla]

You ignored my posts. Do you need help with the physics, Physics1? You should read Joseph James' and Richard Butler's articles. Then work out the exposures for yourself in the DPR tests, look at Bill Claff's input-referred noise data, and make the appropriate deductions. Start with Butler's articles.

http://www.josephjamesphotography.com/equivalence/

https://www.dpreview.com/articles/2666934640/what-is-equivalence-and-why-should-i-care

https://www.dpreview.com/learn/2799100497/equivalence-in-a-nutshell

Remember that the fraction of light collected from each subject point is proportional to the lens entrance pupil area and inversely proportional to the square of the distance to the subject. Remember also that the magnification is proportional to the focal length, and the entrance pupil diameter D = f/N, where N is the f-number. You can figure out how to normalize the DPR tests and figure out the appropriate exposures. You should find that the situation is what I already told you. Good luck. I'm traveling, and can't give any handholding.

Yes, I'm pretty sure that I could do it, though it does take me a bit of time and effort. Remember, I don't live and breathe this stuff like you and others here do and only started thinking about this stuff around a week ago

You're doing pretty well so far.

But ultimately you are likely to be in the situation where the DP Test Image taken under the conditions that you would like does not exist, and you need to choose the closest, which is a bit rough and approximate.

With the DPR tests you can compare with the same exposure, or if you need to compare with the same total light, you change the ISO setting. Yes, that's an imperfect way to do it, but it can be quite accurate, depending on the read noise of the two cameras.

Also, how often, actually, are we in the situation where shot noise is not dominant? I don't usually care about DOF, but I take the point that if you do, then you may be forced to stop down the lens and compensate with high ISO. And if the ISO is high enough then you have very few measured photons, and the readout noise could be significant. In fact eventually it must become dominant, but before that happens the shot and readout noise could be so high relative to signal that the image is probably unusable anyway? Hey, I'm not a photographer or even a photographic enthusiast and have only my broad scientific background to guide me Am I understanding the broad idea here?

You can see the difference here. Those are all APS-C cameras, and the display is compensated for differences in pixel size (i.e., pixel pitch). Also, in practice you can sometimes see a big difference if you have a very large range of brightness in the scene, so you need to raise the shadows a bit to compress the range to your display device. The read noise is one of two factors that limits the dynamic range of brightnesses that the camera can record -- the other factor being how many photons per pixel can be recorded without clipping.

 

[Bill Ferris]

To compare low-light performance the real aperture number, not the equivalent aperture number must used. The equivalent aperture is used to compare the DOF.

We had better clear this up before proceeding. Yes, the equivalent aperture can be used to compare DOF. But it is also the correct choice for comparing low light performance.

Depth of field is related to the aperture diameter. Low light performance is also related to aperture diameter or, more precisely aperture area, which is what determines how much light the lens captures for a given FOV. So what is good for one is good for the other.

Surely you must have noticed that cameras with small sensors such as the Canon G15 with 1/1.7" sensor also have amazing low f-numbers, given that the lenses are physically very small. For example, the G15 sports a 5:1 zoom lens with f/1.8-2.8!! And it's small to boot. Wow. If that sounds too good to be true then it's because it is. As far as low light capability is concerned, you cannot validly compare f-numbers across different sensor sizes, but must instead convert to "35 mm equivalent" f-numbers by multiplying by the sensor crop factor. That is what I did, and it is the correct procedure for comparing low light performance.

One can alternatively calculate the lens aperture diameter (D=L/fnumber) and use that to compare low light performance. The two methods are mathematically equivalent. Sometimes one method is easier and more natural, sometimes the other.

I'm in a photo blind with another photographer. It's morning twilight and we're both focused on the same animal. We have the same make & model full-frame camera bodies but are using different lenses. I'm using an 800mm f/6.3 and my friend is using a 500mm f/4.

Which system captures more light from the animal?

I'd say the the 800mm lens due to the animal being much bigger in the frame. 60% bigger in all linear directions.

Well done. The increased magnification and slightly larger entrance pupil give the longer slower prime a subtle edge.

This is one scenario in which the full-frame equivalent f-numbers do not predict the outcome. Real world scenarios tend to be more complex than the narrowly-defined hypotheticals the OP prefers. Those have their place when discussing a single point but the real world is where the rubber meets the road.

 

[ThrillaMozilla]

To compare low-light performance the real aperture number, not the equivalent aperture number must used. The equivalent aperture is used to compare the DOF.

We had better clear this up before proceeding. Yes, the equivalent aperture can be used to compare DOF. But it is also the correct choice for comparing low light performance.

Depth of field is related to the aperture diameter. Low light performance is also related to aperture diameter or, more precisely aperture area, which is what determines how much light the lens captures for a given FOV. So what is good for one is good for the other.

Surely you must have noticed that cameras with small sensors such as the Canon G15 with 1/1.7" sensor also have amazing low f-numbers, given that the lenses are physically very small. For example, the G15 sports a 5:1 zoom lens with f/1.8-2.8!! And it's small to boot. Wow. If that sounds too good to be true then it's because it is. As far as low light capability is concerned, you cannot validly compare f-numbers across different sensor sizes, but must instead convert to "35 mm equivalent" f-numbers by multiplying by the sensor crop factor. That is what I did, and it is the correct procedure for comparing low light performance.

One can alternatively calculate the lens aperture diameter (D=L/fnumber) and use that to compare low light performance. The two methods are mathematically equivalent. Sometimes one method is easier and more natural, sometimes the other.

I'm in a photo blind with another photographer. It's morning twilight and we're both focused on the same animal. We have the same make & model full-frame camera bodies but are using different lenses. I'm using an 800mm f/6.3 and my friend is using a 500mm f/4.

Which system captures more light from the animal?

I'd say the the 800mm lens due to the animal being much bigger in the frame. 60% bigger in all linear directions.

Well done. The increased magnification and slightly larger entrance pupil give the longer slower prime a subtle edge.

This is one scenario in which the full-frame equivalent f-numbers do not predict the outcome. Real world scenarios tend to be more complex than the narrowly-defined hypotheticals the OP prefers. Those have their place when discussing a single point but the real world is where the rubber meets the road.

No, they are both virtually the same. Entrance pupil diameters: 800 mm /6.3 =126 mm. 500/4 = 125 mm. They are identical within the accuracy of the specifications.

At the same camera position, the fraction of light collected from the subject is proportional to the entrance pupil area.

The 800 mm lens, however, gives many more pixels on the subject because of the greater magnification. The image is larger but dimmer, however (= lower exposure).

 

[Bill Ferris]

To compare low-light performance the real aperture number, not the equivalent aperture number must used. The equivalent aperture is used to compare the DOF.

We had better clear this up before proceeding. Yes, the equivalent aperture can be used to compare DOF. But it is also the correct choice for comparing low light performance.

Depth of field is related to the aperture diameter. Low light performance is also related to aperture diameter or, more precisely aperture area, which is what determines how much light the lens captures for a given FOV. So what is good for one is good for the other.

Surely you must have noticed that cameras with small sensors such as the Canon G15 with 1/1.7" sensor also have amazing low f-numbers, given that the lenses are physically very small. For example, the G15 sports a 5:1 zoom lens with f/1.8-2.8!! And it's small to boot. Wow. If that sounds too good to be true then it's because it is. As far as low light capability is concerned, you cannot validly compare f-numbers across different sensor sizes, but must instead convert to "35 mm equivalent" f-numbers by multiplying by the sensor crop factor. That is what I did, and it is the correct procedure for comparing low light performance.

One can alternatively calculate the lens aperture diameter (D=L/fnumber) and use that to compare low light performance. The two methods are mathematically equivalent. Sometimes one method is easier and more natural, sometimes the other.

I'm in a photo blind with another photographer. It's morning twilight and we're both focused on the same animal. We have the same make & model full-frame camera bodies but are using different lenses. I'm using an 800mm f/6.3 and my friend is using a 500mm f/4.

Which system captures more light from the animal?

I'd say the the 800mm lens due to the animal being much bigger in the frame. 60% bigger in all linear directions.

Well done. The increased magnification and slightly larger entrance pupil give the longer slower prime a subtle edge.

This is one scenario in which the full-frame equivalent f-numbers do not predict the outcome. Real world scenarios tend to be more complex than the narrowly-defined hypotheticals the OP prefers. Those have their place when discussing a single point but the real world is where the rubber meets the road.

No, they are both virtually the same. Entrance pupil diameters: 800 mm /6.3 =126 mm. 500/4 = 125 mm. They are identical within the accuracy of the specifications.

127mm to 125mm and, as I said, it's a subtle advantage.

At the same camera position, the fraction of light collected from the subject is proportional to the entrance pupil area.

Yes

The 800 mm lens, however, gives many more pixels on the subject because of the greater magnification. The image is larger but dimmer, however (= lower exposure).

And a bit more total light from the subject.

 

[Dem Bell]

To compare low-light performance the real aperture number, not the equivalent aperture number must used. The equivalent aperture is used to compare the DOF.

We had better clear this up before proceeding. Yes, the equivalent aperture can be used to compare DOF. But it is also the correct choice for comparing low light performance.

Depth of field is related to the aperture diameter. Low light performance is also related to aperture diameter or, more precisely aperture area, which is what determines how much light the lens captures for a given FOV. So what is good for one is good for the other.

Surely you must have noticed that cameras with small sensors such as the Canon G15 with 1/1.7" sensor also have amazing low f-numbers, given that the lenses are physically very small. For example, the G15 sports a 5:1 zoom lens with f/1.8-2.8!! And it's small to boot. Wow. If that sounds too good to be true then it's because it is. As far as low light capability is concerned, you cannot validly compare f-numbers across different sensor sizes, but must instead convert to "35 mm equivalent" f-numbers by multiplying by the sensor crop factor. That is what I did, and it is the correct procedure for comparing low light performance.

One can alternatively calculate the lens aperture diameter (D=L/fnumber) and use that to compare low light performance. The two methods are mathematically equivalent. Sometimes one method is easier and more natural, sometimes the other.

I'm in a photo blind with another photographer. It's morning twilight and we're both focused on the same animal. We have the same make & model full-frame camera bodies but are using different lenses. I'm using an 800mm f/6.3 and my friend is using a 500mm f/4.

Which system captures more light from the animal?

I'd say the the 800mm lens due to the animal being much bigger in the frame. 60% bigger in all linear directions.

But the exposure with the longer lens is lower by (6.3/4)^2=2.48. The answer is "about the same". The entrance pupil diameter is 125 mm for the 500 mm lens vs 127 mm for the 800 mm lens.

 

[Bill Ferris]

To compare low-light performance the real aperture number, not the equivalent aperture number must used. The equivalent aperture is used to compare the DOF.

We had better clear this up before proceeding. Yes, the equivalent aperture can be used to compare DOF. But it is also the correct choice for comparing low light performance.

Depth of field is related to the aperture diameter. Low light performance is also related to aperture diameter or, more precisely aperture area, which is what determines how much light the lens captures for a given FOV. So what is good for one is good for the other.

Surely you must have noticed that cameras with small sensors such as the Canon G15 with 1/1.7" sensor also have amazing low f-numbers, given that the lenses are physically very small. For example, the G15 sports a 5:1 zoom lens with f/1.8-2.8!! And it's small to boot. Wow. If that sounds too good to be true then it's because it is. As far as low light capability is concerned, you cannot validly compare f-numbers across different sensor sizes, but must instead convert to "35 mm equivalent" f-numbers by multiplying by the sensor crop factor. That is what I did, and it is the correct procedure for comparing low light performance.

One can alternatively calculate the lens aperture diameter (D=L/fnumber) and use that to compare low light performance. The two methods are mathematically equivalent. Sometimes one method is easier and more natural, sometimes the other.

I'm in a photo blind with another photographer. It's morning twilight and we're both focused on the same animal. We have the same make & model full-frame camera bodies but are using different lenses. I'm using an 800mm f/6.3 and my friend is using a 500mm f/4.

Which system captures more light from the animal?

I'd say the the 800mm lens due to the animal being much bigger in the frame. 60% bigger in all linear directions.

But the exposure with the longer lens is lower by (6.3/4)^2=2.48. The answer is "about the same". The entrance pupil diameter is 125 mm for the 500 mm lens vs 127 mm for the 800 mm lens.

Yes, the exposure is 1 1/3 stop lower. Most folks would look at f/4 and f/6.3 and jump straight to the conclusion the 500mm lens is not just better but much better in low light. In scenarios where 500mm fills the frame with a usable composition, those folks would be right. However, there are scenarios - such as I've outlined - where the much slower lens with its larger entrance pupil has a slight advantage.

 

[ThrillaMozilla]

To compare low-light performance the real aperture number, not the equivalent aperture number must used. The equivalent aperture is used to compare the DOF.

We had better clear this up before proceeding. Yes, the equivalent aperture can be used to compare DOF. But it is also the correct choice for comparing low light performance.

Depth of field is related to the aperture diameter. Low light performance is also related to aperture diameter or, more precisely aperture area, which is what determines how much light the lens captures for a given FOV. So what is good for one is good for the other.

Surely you must have noticed that cameras with small sensors such as the Canon G15 with 1/1.7" sensor also have amazing low f-numbers, given that the lenses are physically very small. For example, the G15 sports a 5:1 zoom lens with f/1.8-2.8!! And it's small to boot. Wow. If that sounds too good to be true then it's because it is. As far as low light capability is concerned, you cannot validly compare f-numbers across different sensor sizes, but must instead convert to "35 mm equivalent" f-numbers by multiplying by the sensor crop factor. That is what I did, and it is the correct procedure for comparing low light performance.

One can alternatively calculate the lens aperture diameter (D=L/fnumber) and use that to compare low light performance. The two methods are mathematically equivalent. Sometimes one method is easier and more natural, sometimes the other.

I'm in a photo blind with another photographer. It's morning twilight and we're both focused on the same animal. We have the same make & model full-frame camera bodies but are using different lenses. I'm using an 800mm f/6.3 and my friend is using a 500mm f/4.

Which system captures more light from the animal?

I'd say the the 800mm lens due to the animal being much bigger in the frame. 60% bigger in all linear directions.

Well done. The increased magnification and slightly larger entrance pupil give the longer slower prime a subtle edge.

This is one scenario in which the full-frame equivalent f-numbers do not predict the outcome. Real world scenarios tend to be more complex than the narrowly-defined hypotheticals the OP prefers. Those have their place when discussing a single point but the real world is where the rubber meets the road.

No, they are both virtually the same. Entrance pupil diameters: 800 mm /6.3 =126 mm. 500/4 = 125 mm. They are identical within the accuracy of the specifications.

127mm to 125mm and, as I said, it's a subtle advantage.

You don't know that.

* the difference between 1.25 and 1.27 is photographically insignificant, and

* the numbers aren't right anyway. It's not f/6.3. It's f/6.3496.... rounded off to approximately f/6.3, and it's not 800 mm. It's approximately 800 mm. Manufacturers round off.

Besides, you forgot the difference in lens transmission, and the difference could easily be several percent.

So you don't know which lens transmits more light from subject to sensor, and the difference is probably photographically insignificant anyway. There's no sense in insisting on false precision and claiming one lens is slightly better than another, because (1) it makes no significant difference and (2) you don't know anyway.

Photographic lens specifications are approximate!

 

[ThrillaMozilla]

To compare low-light performance the real aperture number, not the equivalent aperture number must used. The equivalent aperture is used to compare the DOF.

We had better clear this up before proceeding. Yes, the equivalent aperture can be used to compare DOF. But it is also the correct choice for comparing low light performance.

Depth of field is related to the aperture diameter. Low light performance is also related to aperture diameter or, more precisely aperture area, which is what determines how much light the lens captures for a given FOV. So what is good for one is good for the other.

Surely you must have noticed that cameras with small sensors such as the Canon G15 with 1/1.7" sensor also have amazing low f-numbers, given that the lenses are physically very small. For example, the G15 sports a 5:1 zoom lens with f/1.8-2.8!! And it's small to boot. Wow. If that sounds too good to be true then it's because it is. As far as low light capability is concerned, you cannot validly compare f-numbers across different sensor sizes, but must instead convert to "35 mm equivalent" f-numbers by multiplying by the sensor crop factor. That is what I did, and it is the correct procedure for comparing low light performance.

One can alternatively calculate the lens aperture diameter (D=L/fnumber) and use that to compare low light performance. The two methods are mathematically equivalent. Sometimes one method is easier and more natural, sometimes the other.

I'm in a photo blind with another photographer. It's morning twilight and we're both focused on the same animal. We have the same make & model full-frame camera bodies but are using different lenses. I'm using an 800mm f/6.3 and my friend is using a 500mm f/4.

Which system captures more light from the animal?

I'd say the the 800mm lens due to the animal being much bigger in the frame. 60% bigger in all linear directions.

But the exposure with the longer lens is lower by (6.3/4)^2=2.48. The answer is "about the same". The entrance pupil diameter is 125 mm for the 500 mm lens vs 127 mm for the 800 mm lens.

Yes, the exposure is 1 1/3 stop lower. Most folks would look at f/4 and f/6.3 and jump straight to the conclusion the 500mm lens is not just better but much better in low light. In scenarios where 500mm fills the frame with a usable composition, those folks would be right. However, there are scenarios - such as I've outlined - where the much slower lens with its larger entrance pupil has a slight advantage.

No, no, no. You do not choose one lens over the other because of the supposedly larger entrance pupil, because you don't know, and you know nothing about transmission losses. That's false precision. See my previous post.

 

[Bill Ferris]

To compare low-light performance the real aperture number, not the equivalent aperture number must used. The equivalent aperture is used to compare the DOF.

We had better clear this up before proceeding. Yes, the equivalent aperture can be used to compare DOF. But it is also the correct choice for comparing low light performance.

Depth of field is related to the aperture diameter. Low light performance is also related to aperture diameter or, more precisely aperture area, which is what determines how much light the lens captures for a given FOV. So what is good for one is good for the other.

Surely you must have noticed that cameras with small sensors such as the Canon G15 with 1/1.7" sensor also have amazing low f-numbers, given that the lenses are physically very small. For example, the G15 sports a 5:1 zoom lens with f/1.8-2.8!! And it's small to boot. Wow. If that sounds too good to be true then it's because it is. As far as low light capability is concerned, you cannot validly compare f-numbers across different sensor sizes, but must instead convert to "35 mm equivalent" f-numbers by multiplying by the sensor crop factor. That is what I did, and it is the correct procedure for comparing low light performance.

One can alternatively calculate the lens aperture diameter (D=L/fnumber) and use that to compare low light performance. The two methods are mathematically equivalent. Sometimes one method is easier and more natural, sometimes the other.

I'm in a photo blind with another photographer. It's morning twilight and we're both focused on the same animal. We have the same make & model full-frame camera bodies but are using different lenses. I'm using an 800mm f/6.3 and my friend is using a 500mm f/4.

Which system captures more light from the animal?

I'd say the the 800mm lens due to the animal being much bigger in the frame. 60% bigger in all linear directions.

Well done. The increased magnification and slightly larger entrance pupil give the longer slower prime a subtle edge.

This is one scenario in which the full-frame equivalent f-numbers do not predict the outcome. Real world scenarios tend to be more complex than the narrowly-defined hypotheticals the OP prefers. Those have their place when discussing a single point but the real world is where the rubber meets the road.

No, they are both virtually the same. Entrance pupil diameters: 800 mm /6.3 =126 mm. 500/4 = 125 mm. They are identical within the accuracy of the specifications.

127mm to 125mm and, as I said, it's a subtle advantage.

You don't know that.

The math says 500mm/4=125mm and 800mm/6.3=126.984 or 127mm.

I'm applying the same equation to the published specs and sharing the answer. I've described the difference as subtle, which is accurate.

Besides, the main point of my post is that the f/4 lens is not better in low light than the f/6.3 lens. The f/6.3 lens is just as good and, based on the numbers, a skosh better.

* the difference between 1.25 and 1.27 is photographically insignificant, and

It's a calculable difference. All I've done is share what the math tells us.

* the numbers aren't right anyway. It's not f/6.3. It's f/6.3496.... rounded off to approximately f/6.3, and it's not 800 mm. It's approximately 800 mm. Manufacturers round off.

Where are you getting a precise f-stop of f/6.3496?

Besides, you forgot the difference in lens transmission, and the difference could easily be several percent.

Speculation.

So you don't know which lens transmits more light from subject to sensor, and the difference is probably photographically insignificant anyway. There's no sense in insisting on false precision and claiming one lens is slightly better than another, because (1) it makes no significant difference and (2) you don't know anyway.

All I've done is compare the lenses based on the official published specs. The math supports the statements I've made.

Photographic lens specifications are approximate!

None of what you've written changes the fact that the f/6.3 lens, based solely on the published specs, is at least as good in low light as the f/4 lens and, if anything, a skosh better.

 

[Bill Ferris]

To compare low-light performance the real aperture number, not the equivalent aperture number must used. The equivalent aperture is used to compare the DOF.

We had better clear this up before proceeding. Yes, the equivalent aperture can be used to compare DOF. But it is also the correct choice for comparing low light performance.

Depth of field is related to the aperture diameter. Low light performance is also related to aperture diameter or, more precisely aperture area, which is what determines how much light the lens captures for a given FOV. So what is good for one is good for the other.

Surely you must have noticed that cameras with small sensors such as the Canon G15 with 1/1.7" sensor also have amazing low f-numbers, given that the lenses are physically very small. For example, the G15 sports a 5:1 zoom lens with f/1.8-2.8!! And it's small to boot. Wow. If that sounds too good to be true then it's because it is. As far as low light capability is concerned, you cannot validly compare f-numbers across different sensor sizes, but must instead convert to "35 mm equivalent" f-numbers by multiplying by the sensor crop factor. That is what I did, and it is the correct procedure for comparing low light performance.

One can alternatively calculate the lens aperture diameter (D=L/fnumber) and use that to compare low light performance. The two methods are mathematically equivalent. Sometimes one method is easier and more natural, sometimes the other.

I'm in a photo blind with another photographer. It's morning twilight and we're both focused on the same animal. We have the same make & model full-frame camera bodies but are using different lenses. I'm using an 800mm f/6.3 and my friend is using a 500mm f/4.

Which system captures more light from the animal?

I'd say the the 800mm lens due to the animal being much bigger in the frame. 60% bigger in all linear directions.

But the exposure with the longer lens is lower by (6.3/4)^2=2.48. The answer is "about the same". The entrance pupil diameter is 125 mm for the 500 mm lens vs 127 mm for the 800 mm lens.

Yes, the exposure is 1 1/3 stop lower. Most folks would look at f/4 and f/6.3 and jump straight to the conclusion the 500mm lens is not just better but much better in low light. In scenarios where 500mm fills the frame with a usable composition, those folks would be right. However, there are scenarios - such as I've outlined - where the much slower lens with its larger entrance pupil has a slight advantage.

No, no, no. You do not choose one lens over the other because of the supposedly larger entrance pupil, because you don't know, and you know nothing about transmission losses. That's false precision. See my previous post.

Photographers make purchase decisions based on published specs all the time. In the world of professional lenses, there is no reason to think bench-tested transmission differences or differences between the measured entrance pupil diameters of specific copies will result in a noticeable departure in performance relative to that indicated by a simple mathematical evaluation based on the published specs for that class of optic.

By contrast, it is reasonable to expect that, in the scenario I've outlined, the shorter, faster optic will not have the significant light-gathering advantage a rudimentary comparison of f-stops might suggest. In fact, the longer, slower optic should perform just as well when filling the frame and, if anything, may have the edge by a skosh.

Since customers typically are not able to bench test specific copies of lenses - neither before nor after purchase - of what practical use are those factors in a purchase decision? Any assumptions one makes about transmission, for instance, would be pure speculation. All we have to go by are the published specs and whatever reviews one chooses to consider.

As far as reviews go, the folks at Photography Life often bench-test optics. They've bench-tested at least one copy of the 800mm PF and did not report any departure from the published focal length or the transmission one would expect of a professional lens.

The point you're missing, is that a superficial comparison of f-number would lead many to assume the 500mm f/4 would be significantly better in low light. In fact, if a photographer needs 800mm of reach for the photography they do, the 800mm f/6.3 PF will be just as good in low light and, perhaps, even a touch better.

 

[lumigraphics]

The only time I use flash is in studio with large studio lights. Otherwise, don't need it. This is experience of 40 years doing all sorts of photography. YMMV.

 

[Reilly Diefenbach]

:-

To get an overview of low-light performance use the test-scene compare in the DPreview tests.

Despite being a useful and wonderful resource in general, do we all agree that the images are not in general useful for ranking cameras in terms of low light performance?

We do not. It is as perfect a comparison of sensor low light performance at all ISOs as you're going to find. Between the Big Three, there is very little difference at high or low ISO for any of their APSC cameras. They are all good and will make a much better picture than your old camera. All three have similar zoom lenses available.

Low Light ISO 6400

 

[Physics1]

To compare low-light performance the real aperture number, not the equivalent aperture number must used. The equivalent aperture is used to compare the DOF.

We had better clear this up before proceeding. Yes, the equivalent aperture can be used to compare DOF. But it is also the correct choice for comparing low light performance.

Depth of field is related to the aperture diameter. Low light performance is also related to aperture diameter or, more precisely aperture area, which is what determines how much light the lens captures for a given FOV. So what is good for one is good for the other.

Surely you must have noticed that cameras with small sensors such as the Canon G15 with 1/1.7" sensor also have amazing low f-numbers, given that the lenses are physically very small. For example, the G15 sports a 5:1 zoom lens with f/1.8-2.8!! And it's small to boot. Wow. If that sounds too good to be true then it's because it is. As far as low light capability is concerned, you cannot validly compare f-numbers across different sensor sizes, but must instead convert to "35 mm equivalent" f-numbers by multiplying by the sensor crop factor. That is what I did, and it is the correct procedure for comparing low light performance.

One can alternatively calculate the lens aperture diameter (D=L/fnumber) and use that to compare low light performance. The two methods are mathematically equivalent. Sometimes one method is easier and more natural, sometimes the other.

I'm in a photo blind with another photographer. It's morning twilight and we're both focused on the same animal. We have the same make & model full-frame camera bodies but are using different lenses. I'm using an 800mm f/6.3 and my friend is using a 500mm f/4.

Which system captures more light from the animal?

I'd say the the 800mm lens due to the animal being much bigger in the frame. 60% bigger in all linear directions.

Well done. The increased magnification and slightly larger entrance pupil give the longer slower prime a subtle edge.

This is one scenario in which the full-frame equivalent f-numbers do not predict the outcome.

I would word things differently. The 800 mm lens has a slightly larger aperture diameter (127 mm vs 125 mm) so it will collect more light for a given field of view (FOV). Period. If both cameras to which these lenses are fitted have the same sensor size, then to get the same FOV in the image on the sensor, you will need to crop the image for the 500 mm lens.

Contrary to what you claim above, of course you will draw the same conclusion using equivalent f-numbers, though doing it that way is more messy and less intuitive for this example. Recall that to get equivalent f-number, the usual way is to take the real f-number and multiply by the crop factor. In this case, the 500 mm lens image is cropped to get same FOV. If we define the crop factor of the 800 mm lens image as 1.0, then the crop factor for the 500 mm lens image is 800/500 = x1.6

F_equiv for 800 mm lens = 6.3 x 1.0 = 6.3

F_equiv for 500 mm lens = 4.0 x 1.6 = 6.4

Well what a surprise, we get exactly the same result, that the 500 mm f/4 lens does just slightly better with an equivalent f-number of 6.4 versus 6.3

Of course, if you do the algebra then the two methods are identical, and therefore always give identical results.

And I think that we would all agree that while calculating this result (by either method) is important for the purpose of showing how the calculation is done, the difference for this particular example is negligible and of no consequence in the real world.

 

[Bill Ferris]

To compare low-light performance the real aperture number, not the equivalent aperture number must used. The equivalent aperture is used to compare the DOF.

We had better clear this up before proceeding. Yes, the equivalent aperture can be used to compare DOF. But it is also the correct choice for comparing low light performance.

Depth of field is related to the aperture diameter. Low light performance is also related to aperture diameter or, more precisely aperture area, which is what determines how much light the lens captures for a given FOV. So what is good for one is good for the other.

Surely you must have noticed that cameras with small sensors such as the Canon G15 with 1/1.7" sensor also have amazing low f-numbers, given that the lenses are physically very small. For example, the G15 sports a 5:1 zoom lens with f/1.8-2.8!! And it's small to boot. Wow. If that sounds too good to be true then it's because it is. As far as low light capability is concerned, you cannot validly compare f-numbers across different sensor sizes, but must instead convert to "35 mm equivalent" f-numbers by multiplying by the sensor crop factor. That is what I did, and it is the correct procedure for comparing low light performance.

One can alternatively calculate the lens aperture diameter (D=L/fnumber) and use that to compare low light performance. The two methods are mathematically equivalent. Sometimes one method is easier and more natural, sometimes the other.

I'm in a photo blind with another photographer. It's morning twilight and we're both focused on the same animal. We have the same make & model full-frame camera bodies but are using different lenses. I'm using an 800mm f/6.3 and my friend is using a 500mm f/4.

Which system captures more light from the animal?

I'd say the the 800mm lens due to the animal being much bigger in the frame. 60% bigger in all linear directions.

Well done. The increased magnification and slightly larger entrance pupil give the longer slower prime a subtle edge.

This is one scenario in which the full-frame equivalent f-numbers do not predict the outcome.

I would word things differently. The 800 mm lens has a slightly larger aperture diameter (127 mm vs 125 mm) so it will collect more light for a given field of view (FOV). Period. If both cameras to which these lenses are fitted have the same sensor size, then to get the same FOV in the image on the sensor, you will need to crop the image for the 500 mm lens.

Contrary to what you claim above, of course you will draw the same conclusion using equivalent f-numbers, though doing it that way is more messy and less intuitive for this example. Recall that to get equivalent f-number, the usual way is to take the real f-number and multiply by the crop factor. In this case, the 500 mm lens image is cropped to get same FOV. If we define the crop factor of the 800 mm lens image as 1.0, then the crop factor for the 500 mm lens image is 800/500 = x1.6

F_equiv for 800 mm lens = 6.3 x 1.0 = 6.3

F_equiv for 500 mm lens = 4.0 x 1.6 = 6.4

Well what a surprise, we get exactly the same result, that the 500 mm f/4 lens does just slightly better with an equivalent f-number of 6.4 versus 6.3

Of course, if you do the algebra then the two methods are identical, and therefore always give identical results.

And I think that we would all agree that while calculating this result (by either method) is important for the purpose of showing how the calculation is done, the difference for this particular example is negligible and of no consequence in the real world.

That's your catch phrase. You write it and folks come along to disagree with what follows ]

As I've mentioned in other posts, the notion that an f/6.3 lens can equal or potentially better one of the exotic primes - the venerable 500mm f/4 - runs contrary to what most of us are taught.

On paper, the difference is minor. In reality, that concept challenges the status quo.