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F/ number in relation to the sensor size

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lumigraphics said:
Equivalent photos do not have the same exposure? What? How are they equivalent?

If I need f2.8 on one camera to get the same DOF as f/5.6 on a different camera, my shutter speed or ISO sensitivity has to change. Don't you think that changing shutter speed makes a difference?
Click to expand...
But if you change shutter speed, that is no longer equivalent by definition. And ISO does not affect exposure.
 
lumigraphics said:
Great Bustard said:
lumigraphics said:
Equivalence is a false idea. If you factor in depth-of-field, then exposure changes and vice versa. You cannot and will not get the same results from different sensor or film sizes.
Click to expand...
There is nothing false about Equivalence. Equivalent photos do not have the same exposure, just as equivalent photos do not have the same focal length or same relative aperture. What equivalent photos have in common is the same perspective, the same [diagonal] angle of view, the same DOF, the same exposure time, and the same total amount of light projected on the sensor.
Click to expand...
Equivalent photos do not have the same exposure? What? How are they equivalent?

If I need f2.8 on one camera to get the same DOF as f/5.6 on a different camera, my shutter speed or ISO sensitivity has to change. Don't you think that changing shutter speed makes a difference?
Click to expand...
The photographic usage of the word Equivalence is specifically in regards regards to generating equivalent images that have virtually identical image attributes. To generate equivalent images you need different camera control setting. The important concept is to compare IMAGES and what camera settings will accomplish this.

One would never say that a FF Canon 1D4 camera is equivalent to an 4/3 Olympus OM-D EM1 for all of photographic shooting purposes, however is is possible for these cameras to create "equivalent images" that are virtually indistinguishable by suitable choice of lens, framing, point of focus, aperture, exposure and ISO. This is not magic.... it is simply the way that sensors, light, and camera setting work.

--
Charles Darwin: "ignorance more frequently begets confidence than does knowledge."
tony
http://www.tphoto.ca
 
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lumigraphics said:
Great Bustard said:
lumigraphics said:
Equivalence is a false idea. If you factor in depth-of-field, then exposure changes and vice versa. You cannot and will not get the same results from different sensor or film sizes.
Click to expand...
There is nothing false about Equivalence. Equivalent photos do not have the same exposure, just as equivalent photos do not have the same focal length or same relative aperture. What equivalent photos have in common is the same perspective, the same [diagonal] angle of view, the same DOF, the same exposure time, and the same total amount of light projected on the sensor.
Click to expand...
Equivalent photos do not have the same exposure? What? How are they equivalent?

If I need f2.8 on one camera to get the same DOF as f/5.6 on a different camera, my shutter speed or ISO sensitivity has to change. Don't you think that changing shutter speed makes a difference?
Click to expand...


87e09581297449df880ff422160e652b.jpg



--
gollywop
I am not a moderator or an official of dpr. My views do not represent, or necessarily reflect, those of dpr.
 
50mm f/1.4 lens is a 50mm f/1.4 lens, regardless of the sensor behind the lens). TRUE. Lots of newbies get confused by this.
Great Bustard said:
Derma pro said:
It is known that the aperture number have an unequal effect with different sensor sizes. For example, if the lens is f/2.8 on a APC-S sensor, it gives much more isolation of the subject than a f/2.8 lens on a 1 inch sensor. The depth of field will be much more for the 1 inch sensor.

I wonder if there is any equation to tell the equivalent f/stops for each sensor, say, similar to the crop factor for the focal lengths of lenses on different sensor sizes. For example, can we say that f/8 on FF sensor equals f/2.8 on 1 inch sensor? which f/number can give the same DOF for each sensor size measured with FF as a standard?
Click to expand...
The Equivalence Ratio (more commonly called the "crop factor") is the ratio of the sensor diagonals. For example, the Equivalence Ratio is 2.7 between FF and 1", 2 between FF and mFT, 1.5 between FF and DX, and 1.33 between DX and mFT, etc., etc., etc.

So, with that in mind (and apologies for not including 1" in the mix below):

Equivalence in 10 Bullets:

Neither the focal length nor the f-ratio of a lens change as a function of sensor (for example, a 50mm f/1.4 lens is a 50mm f/1.4 lens, regardless of the sensor behind the lens). However, the effect of both the focal length and the f-ratio on the visual properties of the photo very much depend on the sensor, and scale in direct proportion to the size of the sensor:

25mm f/1.4 on mFT (4/3) is equivalent to 31mm f/1.8 on 1.6x (Canon APS-C), 33mm f/1.9 on 1.5x (APS-C for everyone else), and 50mm f/2.8 on FF (FX), where "equivalent to" means:
  • The photos all have the same diagonal angle of view (25mm x 2 = 31mm x 1.6 = 33mm x 1.5 = 50mm) and aperture diameter (25mm / 1.4 = 31mm / 1.8 = 33mm / 1.9 = 50mm / 2.8 = 18mm).
  • The photos all have the same perspective when taken from the same position.
  • The photos all have the same DOF (as well as diffraction softening) when they are taken from the same position with the same focal point and have the same display size.
  • The photos all have the same motion blur for the same shutter speed (regardless of pixel count).
  • The same total amount of light falls on the sensor for the same DOF and shutter speed.
  • The same total light falling on the larger sensor will result in a lower exposure than the smaller sensor (the same total light over a larger area results in a lower density of light on the sensor).
  • The larger sensor system will use a concomitantly higher ISO setting for a given brightness of the LCD playback and/or OOC (out-of-the-camera) jpg due to the lower exposure.
  • The same total light will result in the same noise for equally efficient sensors (regardless of pixel count and regardless of the ISO setting).
  • If the 25mm lens at f/1.4 is 2x as sharp as the 50mm lens at f/2.8, 1.33x as sharp as the 33mm lens at f/1.8, 1.25x as sharp as the 31mm at f/1.8, the sensors have the same pixel count, and the AA filter introduces the same blur, then all systems will also resolve the same detail.
  • Other elements of IQ, such as bokeh, color, distortion, etc., as well as elements of operation, such as AF speed/accuracy, size, weight, etc., are not covered in this use of the term "equivalent".
Click to expand...
 
Easy to get sucked in Ellis. Makes you wonder. There are a few old pros on this forum - and a few very new and talented ones. Makes me wonder if some good folks ever read your qualifications! (or your articles)
Ellis Vener said:
Are you shooting video and filming the same take from different angles using cametas with different size sensors?
Click to expand...
 
lumigraphics said:
Great Bustard said:
lumigraphics said:
Equivalence is a false idea. If you factor in depth-of-field, then exposure changes and vice versa. You cannot and will not get the same results from different sensor or film sizes.
Click to expand...
There is nothing false about Equivalence. Equivalent photos do not have the same exposure, just as equivalent photos do not have the same focal length or same relative aperture. What equivalent photos have in common is the same perspective, the same [diagonal] angle of view, the same DOF, the same exposure time, and the same total amount of light projected on the sensor.
Click to expand...
Equivalent photos do not have the same exposure?
Click to expand...
Correct.
lumigraphics said:
What?
Click to expand...
The same total amount of light falling on different areas will *necessarily* result in different exposures, because exposure is *defined as* the total amount of light per area falling on the sensor.
lumigraphics said:
How are they equivalent?
Click to expand...
The same total amount of light falls on the sensor for Equivalent photos.
lumigraphics said:
If I need f2.8 on one camera to get the same DOF as f/5.6 on a different camera, my shutter speed or ISO sensitivity has to change. Don't you think that changing shutter speed makes a difference?
Click to expand...
Changing the exposure time does, indeed, make a difference. Equivalent photos have the same exposure time. Thus, you would [typically] change the ISO setting accordingly (or let the camera do it for you in Auto ISO) to achieve the desired brightness.
 
Last edited:
Astrotripper said:
lumigraphics said:
Equivalent photos do not have the same exposure? What? How are they equivalent?

If I need f2.8 on one camera to get the same DOF as f/5.6 on a different camera, my shutter speed or ISO sensitivity has to change. Don't you think that changing shutter speed makes a difference?
Click to expand...
But if you change shutter speed, that is no longer equivalent by definition. And ISO does not affect exposure.
Click to expand...
ISO does not affect exposure.

That's got to be one of the best posts ever on here. Best as in most ridiculous.

Do I have to explain the exposure triangle?
 
Great Bustard said:
lumigraphics said:
Great Bustard said:
lumigraphics said:
Equivalence is a false idea. If you factor in depth-of-field, then exposure changes and vice versa. You cannot and will not get the same results from different sensor or film sizes.
Click to expand...
There is nothing false about Equivalence. Equivalent photos do not have the same exposure, just as equivalent photos do not have the same focal length or same relative aperture. What equivalent photos have in common is the same perspective, the same [diagonal] angle of view, the same DOF, the same exposure time, and the same total amount of light projected on the sensor.
Click to expand...
Equivalent photos do not have the same exposure?
Click to expand...
Correct.
Great Bustard said:
What?
Click to expand...
The same total amount of light falling on different areas will *necessarily* result in different exposures, because exposure is *defined as* the total amount of light per area falling on the sensor.
Great Bustard said:
How are they equivalent?
Click to expand...
The same total amount of light falls on the sensor for Equivalent photos.
Great Bustard said:
If I need f2.8 on one camera to get the same DOF as f/5.6 on a different camera, my shutter speed or ISO sensitivity has to change. Don't you think that changing shutter speed makes a difference?
Click to expand...
Changing the exposure time does, indeed, make a difference. Equivalent photos have the same exposure time. Thus, you would [typically] change the ISO setting accordingly (or let the camera do it for you in Auto ISO) to achieve the desired brightness.
Click to expand...
Funny how that's EXACTLY what I said. And I'm sure you are aware that changing ISO setting has its own side effects.
 
lumigraphics said:
Astrotripper said:
lumigraphics said:
Equivalent photos do not have the same exposure? What? How are they equivalent?

If I need f2.8 on one camera to get the same DOF as f/5.6 on a different camera, my shutter speed or ISO sensitivity has to change. Don't you think that changing shutter speed makes a difference?
Click to expand...
But if you change shutter speed, that is no longer equivalent by definition. And ISO does not affect exposure.
Click to expand...
ISO does not affect exposure.
Click to expand...
The ISO setting, by itself, does not affect exposure.
lumigraphics said:
That's got to be one of the best posts ever on here. Best as in most ridiculous.
Click to expand...
If a change in the ISO setting results in a change in the relative aperture, exposure time, and/or flash power, then it will result in a change in exposure. For example, photos of a given scene at f/2.8 1/100 ISO 400 and f/2.8 1/100 ISO 1600 will have the same exposure, but the ISO 1600 photo will be 4x brighter.
lumigraphics said:
Do I have to explain the exposure triangle?
Click to expand...
The "exposure triangle" is about brightness, not exposure. For example, a photo of a given scene at f/2.8 1/100 ISO 400 will have 4x the exposure as a photo at f/4 1/200 ISO 1600, but both photos will have the same brightness.
 
Last edited:
lumigraphics said:
Great Bustard said:
lumigraphics said:
Great Bustard said:
lumigraphics said:
Equivalence is a false idea. If you factor in depth-of-field, then exposure changes and vice versa. You cannot and will not get the same results from different sensor or film sizes.
Click to expand...
There is nothing false about Equivalence. Equivalent photos do not have the same exposure, just as equivalent photos do not have the same focal length or same relative aperture. What equivalent photos have in common is the same perspective, the same [diagonal] angle of view, the same DOF, the same exposure time, and the same total amount of light projected on the sensor.
Click to expand...
Equivalent photos do not have the same exposure?
Click to expand...
Correct.
lumigraphics said:
What?
Click to expand...
The same total amount of light falling on different areas will *necessarily* result in different exposures, because exposure is *defined as* the total amount of light per area falling on the sensor.
lumigraphics said:
How are they equivalent?
Click to expand...
The same total amount of light falls on the sensor for Equivalent photos.
lumigraphics said:
If I need f2.8 on one camera to get the same DOF as f/5.6 on a different camera, my shutter speed or ISO sensitivity has to change. Don't you think that changing shutter speed makes a difference?
Click to expand...
Changing the exposure time does, indeed, make a difference. Equivalent photos have the same exposure time. Thus, you would [typically] change the ISO setting accordingly (or let the camera do it for you in Auto ISO) to achieve the desired brightness.
Click to expand...
Funny how that's EXACTLY what I said.
Click to expand...
Well, no, it isn't. You said:

Equivalence is a false idea. If you factor in depth-of-field, then exposure changes and vice versa. You cannot and will not get the same results from different sensor or film sizes.

This is incorrect. For example, 25mm f/2.8 1/100 ISO 400 on mFT is equivalent to 50mm f/5.6 1/100 ISO 1600 on FF, despite the fact that the FF photo has 1/4 the exposure as the mFT photo. They will not be exactly the same, of course, but they will be fairly close, and the reason is because they will have the same perspective (if taken from the same position), the same [diagonal] angle of view, the same DOF, the same motion blur, the same brightness, and the same total amount of light projected on the sensor.
lumigraphics said:
And I'm sure you are aware that changing ISO setting has its own side effects.
Click to expand...
Sure. When a change in the ISO setting results in the camera changing the relative aperture, exposure time, and/or flash power, it changes the exposure. When a change in the ISO setting doesn't affect relative aperture, exposure time, and/or flash power (e.g. in manual mode with manual flash), it changes the brightness of the photo.
 
Also the exposure relative to f stop and shutter speed for a given situation are not affected.
 
"There's an App for that" Download DOFMaster. It gives accurate tables etc for every format within reason and lenses too. There is I believe also an Android App version....
 
Great Bustard said:
tbcass said:
Also the exposure relative to f stop and shutter speed for a given situation are not affected.
Click to expand...
Not sure what you mean.
Click to expand...
If the light requires, for example, iso100, f5.6, 1/200 shutter speed for a given exposure for FF it will be the same for any other size sensor. Some people don't understand that.
 
tbcass said:
Great Bustard said:
tbcass said:
Also the exposure relative to f stop and shutter speed for a given situation are not affected.
Click to expand...
Not sure what you mean.
Click to expand...
If the light requires, for example, iso100, f5.6, 1/200 shutter speed for a given exposure for FF it will be the same for any other size sensor. Some people don't understand that.
Click to expand...
Ah. Sure -- the same exposure settings result in the same exposure for all systems. However, the same exposure settings result in a different amount of light falling on the sensors of different formats, which is an important point that many do not understand.

Basically, the exposure is the density of light (the amount of light per area) falling on the sensor. Thus, a sensor with 4x the area receiving 1/4 the exposure will end up with the same amount of light falling on the sensor. This is why, for example, that f/5.6 1/100 ISO 1600 on FF results in essentially the same noise as f/2.8 1/100 ISO 400 on mFT despite the exposure being 1/4 as much.
 
lumigraphics said:
Astrotripper said:
lumigraphics said:
Equivalent photos do not have the same exposure? What? How are they equivalent?

If I need f2.8 on one camera to get the same DOF as f/5.6 on a different camera, my shutter speed or ISO sensitivity has to change. Don't you think that changing shutter speed makes a difference?
Click to expand...
But if you change shutter speed, that is no longer equivalent by definition. And ISO does not affect exposure.
Click to expand...
ISO does not affect exposure.

That's got to be one of the best posts ever on here. Best as in most ridiculous.
Click to expand...
In photography and sensitometry, "exposure" is defined as the amount of light falling on the sensitive medium per unit area. Lots of folks (mis-)use the word "exposure" to mean something else, which causes a lot of confusion, but there is only one technically correct definition of "exposure" in the field of photography. If you are going to talk about photography in a precise, meaningful way, you'd be well-advised to use this definition.

What affects the amount of light falling on a unit area of a digital camera sensor is
  • The amount of light in the scene, (AKA "scene luminance")
  • how wide the lens aperture is (as denoted by f-number), and
  • how long you let the sensor be exposed to the light.
That's it. ISO does not affect exposure. What ISO affects is image brightness. Exposure also affects image brightness. Exposure also affects shot noise, and ISO can also affect camera-added noise. Lens aperture also affects Depth of Field and diffraction blur, and the exposure time affects motion blur (both camera and subject motion).
lumigraphics said:
Do I have to explain the exposure triangle?
Click to expand...
It might be amusing to see you try. Please, go ahead and explain it.

Meanwhile I'll give you a more accurate model.

There are indeed three parameters to exposure, as the use of "triangle" implies. Change any one of them and you have to make an offsetting change to one or both of the others to maintain the same exposure.

However, the erroneous "Exposure Triangle" one usually sees doesn't actually describe exposure, but image brightness. Therefore it is mislabelled. It has ISO take the place of scene luminance at one of the vertices of the triangle. Therefore, as a model, it is lacking, since it fails to show the effect of a change in lighting on image brightness.

The text that typically accompanies this bogus "exposure triangle" contains other common errors. The most obvious is the claim that increasing ISO increases noise. The only time an increase in ISO accompanies an increase in noise is when the camera is in P, A or S modes (or their equivalents), but the increase in noise in that case is actually caused by the decrease in exposure resulting from a change to shutter an/or aperture that the camera makes in that mode to counteract the effect of the ISO increase on targeted image brightness. In Manual mode, when you increase ISO there is no increase in noise. In fact, in most cameras, there is a small decrease in camera-added noise.

Another error commonly found in text accompanying the bogus "exposure triangle" is that an increase in ISO increases the sensitivity of the sensor. That's false. If you increase the ISO, the sensor will still count the same number of photons landing on it for a given exposure as it counted at the lower ISO. What an increase in ISO actually does is cause the camera to record a larger digital number in the image file for a given count of photons reported by the sensor.
 
lumigraphics said:
Astrotripper said:
lumigraphics said:
Equivalent photos do not have the same exposure? What? How are they equivalent?

If I need f2.8 on one camera to get the same DOF as f/5.6 on a different camera, my shutter speed or ISO sensitivity has to change. Don't you think that changing shutter speed makes a difference?
Click to expand...
But if you change shutter speed, that is no longer equivalent by definition. And ISO does not affect exposure.
Click to expand...
ISO does not affect exposure.

That's got to be one of the best posts ever on here. Best as in most ridiculous.

Do I have to explain the exposure triangle?
Click to expand...
I think you might want to look up the definition of exposure. There are only three variables controlling exposure: light level at the scene, shutter speed and aperture. ISO setting is just a way of making the image brighter or dimmer.
 
Now you're confusing things by using the term exposure when what you really mean is total light on the sensor. Exposure is determined by the light intensity per unit of area. That is constant regardless of sensor size for any given situation.
 
tbcass said:
Now you're confusing things by using the term exposure when what you really mean is total light on the sensor.
Click to expand...
No, he's effectively saying that the exposure multiplied by the sensor area determines the total amount of light. One has to multiply by the Quantum Efficiency to calculate the amount of light though, but QE is a constant for any given camera.
tbcass said:
Exposure is determined by the light intensity per unit of area. That is constant regardless of sensor size for any given situation.
Click to expand...
Read again. He hasn't said otherwise.
 
lumigraphics said:
Great Bustard said:
There is nothing false about Equivalence. Equivalent photos do not have the same exposure, just as equivalent photos do not have the same focal length or same relative aperture. What equivalent photos have in common is the same perspective, the same [diagonal] angle of view, the same DOF, the same exposure time, and the same total amount of light projected on the sensor.
Click to expand...
Equivalent photos do not have the same exposure? What? How are they equivalent?
Click to expand...
See above. It was pretty clearly stated.
lumigraphics said:
If I need f2.8 on one camera to get the same DOF as f/5.6 on a different camera, my shutter speed or ISO sensitivity has to change. Don't you think that changing shutter speed makes a difference?
Click to expand...
You just said "or ISO sensitivity" ... and "same exposure time" was clearly stated in GB's definition of equivalent photos. Sounds to me like you're being intentionally and unnecessarily argumentative.
 
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