F-stop

[Ivanaker]

I have a P&S Canon sx100is. its lenses are 6-60mm (i think that it is 30-300 on 35mm), and on the lens is said that the aperature is 2.8-4.3. I plan to by d3100 with a kit lens 18-55 (27-82.5 on 35mm) and the aperature on that lens is 3.5-5.6.

There is a few things i just cannon understand about aperature. I hardly belive that my sx100is can have larger aperature than 18-55 nikon. I understand that it is f/2.8, so what i want to know: on wide open canon f/2.8 if the focal lenght is 6mm does it mean that diameter of pupil will be 6/2.8 = 2.14mm, or do i need to calculate its 35mm focal lenght, 30/2.8= 10.7mm? But that means that on 300 it will be 300/4.3= 69mm, which is imposible. I really need some explanation on that.
Thanks.

 

[pixseal]

I have a P&S Canon sx100is. its lenses are 6-60mm (i think that it is 30-300 on 35mm), and on the lens is said that the aperature is 2.8-4.3. I plan to by d3100 with a kit lens 18-55 (27-82.5 on 35mm) and the aperature on that lens is 3.5-5.6.

There is a few things i just cannon understand about aperature. I hardly belive that my sx100is can have larger aperature than 18-55 nikon. I understand that it is f/2.8, so what i want to know: on wide open canon f/2.8 if the focal lenght is 6mm does it mean that diameter of pupil will be 6/2.8 = 2.14mm, or do i need to calculate its 35mm focal lenght, 30/2.8= 10.7mm? But that means that on 300 it will be 300/4.3= 69mm, which is imposible. I really need some explanation on that.

1. sx100is: 6mm/2.8 = 2.14mm and 60mm/4.3 = 13.95mm (no 35mm conversion necessary)
2. d3100: 18mm/3.5 = 5.14mm (no 35mm conversion necessary)

Thanks.

You're welcome.
--
-Dave
http://pixseal.com

 

[Draek]

First off, exposure is dependant on f-stop and not on the physical diameter of the aperture. Simply put, f/2.8 is f/2.8 is f/2.8, on a compact, APS-C dSLR or a medium format system, and if your light meter tells you the proper exposure is 1/400 at f/2.8 then that's what's gonna be regardless of which camera and lens you use.

And secondly, if you wish to know for curiosity's sake, the f-stop is the physical focal length (no 35mm equivalent anything) divided by the physical aperture, so the physical aperture is the physical focal length divided by the f-stop. But as I said, it has little relevance in the real world; it does give you a very rough approximation of the DOF you'll get (larger physical apertures usually mean less DOF), but the actual calculations are far more complex than that.
--
http://www.flickr.com/photos/Draek

 

[Graystar]

I have a P&S Canon sx100is. its lenses are 6-60mm (i think that it is 30-300 on 35mm), and on the lens is said that the aperature is 2.8-4.3. I plan to by d3100 with a kit lens 18-55 (27-82.5 on 35mm) and the aperature on that lens is 3.5-5.6.

Okay...got it.

There is a few things i just cannon understand about aperature. I hardly belive that my sx100is can have larger aperature than 18-55 nikon. I understand that it is f/2.8, so what i want to know: on wide open canon f/2.8 if the focal lenght is 6mm does it mean that diameter of pupil will be 6/2.8 = 2.14mm

Yes, that's what it means.

, or do i need to calculate its 35mm focal lenght, 30/2.8= 10.7mm?

No. The idea of 35mm equiv. is just for framing purposes, and truthfully, only has value if you are already familar with the framing of 35mm camera lenses. Otherwise it's as meaningless as saying "10 kilometer" to someone who doesn't know the metric system...there's no frame of reference.

But that means that on 300 it will be 300/4.3= 69mm, which is imposible. I really need some explanation on that.

Well you already have your answer, but I just want to point out that apertures can get quite large. Nikon's 200mm f/2 lens has a diameter of over 100mm to accomodate its 100mm aperture. Check out the dimensions...

http://www.nikonusa.com/Nikon-Products/Product/Camera-Lenses/2188/AF-S-NIKKOR-200mm-f%252F2G-ED-VR-II.html#tab-ProductDetail.ProductTabs.TechSpecs

One last note...when I say aperture I don't mean that thing in the lens that opens or closes...I'm referring to aperture in a theoretical sense. There's simply no advantage whatsoever to think about the actual going-ons inside the lens. Just compare F-numbers and don't worry about the actual physical sizes of things.

.

 

[Ivanaker]

Thank you all.

Graystar wrote:

Just compare F-numbers and don't worry about the actual physical sizes of things.

.

Does it mean that with my sx100is which has f/2.8 on 6mm will let more light in then 18-55 f/3.5 on 18mm?

I'm actually an civil engineer, and i like to know technical stuff and mathematics on all things.

for example:
AF Zoom-NIKKOR
80-200mm f/2.8D ED

so on 80mm aperature will be 28.5mm and on 200mm will be 71mm. does it mean that it lets more light in on full zoom and i need faster shutter speed or lower iso?

 

[BIJ001]

Does it mean that with my sx100is which has f/2.8 on 6mm will let more light in then 18-55 f/3.5 on 18mm?

I do not think so. The slight difference in the f-number is more than offset by the size difference. The total amount of light let through will be more with the bigger lens.

--
Iván József Balázs
(Hungary)

 

[Graystar]

Graystar wrote:

Just compare F-numbers and don't worry about the actual physical sizes of things.

Does it mean that with my sx100is which has f/2.8 on 6mm will let more light in then 18-55 f/3.5 on 18mm?

I'm actually an civil engineer, and i like to know technical stuff and mathematics on all things.

for example:
AF Zoom-NIKKOR
80-200mm f/2.8D ED

so on 80mm aperature will be 28.5mm and on 200mm will be 71mm. does it mean that it lets more light in on full zoom and i need faster shutter speed or lower iso?

Do not think it terms of "amounts" of light. Exposure is not based on amounts...it's based on illuminance, which is a measure of intensity. Illuminance is dictated by the F-number, so two lenses of vastly different sizes will both have equal illuminance when set to the same aperture.

From the Wikipedia page on F-number...

"A 100 mm focal length lens with an aperture setting of f/4 will have a pupil diameter of 25 mm. A 200 mm focal length lens with a setting of f/4 will have a pupil diameter of 50 mm. The 200 mm lens's f/4 opening is larger than that of the 100 mm lens but both will produce the same illuminance in the focal plane when imaging an object of a given luminance."

http://en.wikipedia.org/wiki/F-number

Compact cameras, having smaller sensors, collect less total light for any given exposure level simply because the sensors have less area. It's like collecting rain...the rain is falling at the same rate ("intensity") everywhere, so the only way to collect more rain for a given time period is to collect it from a larger area. For a given shutter speed and aperture, larger sensors collect more light during exposure because of their greater area. But realize that for a given area, such as one square millimeter, both a tiny sensor and a large sensor collect the same amount of light for a given shutter and aperture.

.

 

[markkuk]

Does it mean that with my sx100is which has f/2.8 on 6mm will let more light in then 18-55 f/3.5 on 18mm?

Not if you measure the total light energy, but that's not relevant for photography. F/2.8 results in more light per area of sensor than f/3.5, and that's what determines the exposure.

so on 80mm aperature will be 28.5mm and on 200mm will be 71mm. does it mean that it lets more light in on full zoom and i need faster shutter speed or lower iso?

Assuming the subject is evenly lit then no change of exposure is needed, again because it's the flux per area on sensor that determines the exposure values, not "total light".

 

[mattr]

I'm actually an civil engineer, and i like to know technical stuff and mathematics on all things.

OK, here you go:

Google "joseph james equivalence"

Have fun reading!

 

[Graystar]

I'm actually an civil engineer, and i like to know technical stuff and mathematics on all things.

OK, here you go:

Good thing that URL is blocked, because it would not have helped. Illuminance isn't explained well, and the few lines where it is discussed are lost somewhere in the middle of that hundred-page-long web page.

Please stop giving that page as the answer to any question of exposure...because it's not.

.

 

[Barrie Davis]

Please stop giving that page as the answer to any question of exposure...because it's not.

Agree. Page not helpful.
--
Regards,
Baz

"Ahh... But the thing is, they were not just ORDINARY time travellers!"

 

[Ivanaker]

Your answers have all been very helpful and i thank you all alot.

So, bottom line, if i understood everything well is that with same FOV on both cameras, assuming a wide open lens and same ISO i can do faster shutter speed on d3100 over sx100 because it has larger sensor and captures more light.
18mm at f/3.5 beats 6mm at f/2.8.

 

[Graystar]

Your answers have all been very helpful and i thank you all alot.

So, bottom line, if i understood everything well is that with same FOV on both cameras, assuming a wide open lens and same ISO i can do faster shutter speed on d3100 over sx100 because it has larger sensor and captures more light.
18mm at f/3.5 beats 6mm at f/2.8.

No. That's wrong.

Exposure doesn't care about sensor size. If you point 10 different cameras of different lenses and sensor sizes (throw some film in there too) toward an evenly illuminated gray wall, and set them all to the same aperture and same ISO, then all their meters should indicate the same shutter speed. Also, if you throw a light meter in there and set it to the same ISO as the cameras, it will indicate the same aperture and shutter as the cameras. Exposure is based on luminance levels only.

So...if set to the same ISO, then your SX100 will have a faster shutter speed than the D3100 for no other reason than the fact that it's set at f/2.8 while your D3100 is set to f/3.5.

Now...having said that, the D3100 does have the advantage of collecting more light during an exposure. You can trade that light for speed by changing the ISO. This will introduce noise into the image. However, you should be able to set an ISO level that will give you a faster shutter and still have less noise than the SX100.

And of course...you can always buy a faster lens for your D3100.

.

 

[Ivanaker]

Now i wish i could mount my sx100 lens on that d3100. this is all much more complicated than i first thought it was. now i actually cant see the point of buying a dslr, and simply buy s30is which is 24-840mm f/2.7 - f/5.8 and a bit cheaper then 3100.

 

[Graystar]

Now i wish i could mount my sx100 lens on that d3100. this is all much more complicated than i first thought it was. now i actually cant see the point of buying a dslr, and simply buy s30is which is 24-840mm f/2.7 - f/5.8 and a bit cheaper then 3100.

If the technical side of photography were only about exposure settings then you might have a point. But there’s more to consider. For example, at f/2.8 your SX100 has the same Depth of Field as your D3100 at f/11. That’s very limiting when it comes to creative compositions. An inexpensive 35mm f/1.8 will let you take portrait shots with nicely blurred backgrounds (soon an inexpensive 50mm f/1.8 lens will be available for portraits.) Such shots are impossible with the SX100.

Also, image resolution is limited by diffraction effects. Your D3100’s larger sensor and lens will capture images with more detail and far better contrast than the SX100. Your D3100 has a Bulb mode for long exposures of night scenes. Your D3100 has more flexible control over exposure. These and other factors combine to make the D3100 a far more versatile photographic tool than the SX100.

I was shooting with a Canon A710 for a while. It has the same sensor size as your SX100. Now, the only time I use it is to take pictures of my Nikon D90.

.

 

[Barrie Davis]

Now i wish i could mount my sx100 lens on that d3100.

That wouldn't help. It is a small lens. It only puts more light per unit area on a small sensor. It doesn't have a circle of coverage big enough for the large sensor.

this is all much more complicated than i first thought it was. now i actually cant see the point of buying a dslr, and simply buy s30is which is 24-840mm f/2.7 - f/5.8 and a bit cheaper then 3100.

The point of the dSLR is that it has a larger sensor, and that means the pictures are of higher quality, and big prints easier to make. This is especially true if shooting in dim light, where even though the small cameras has a wider relative aperture and makes efficient use of the light available...

.... the large camera can use higher ISOs instead, and STILL get a better looking picture. This is because its high ISO noise is MUCH lower.

I hope that helps.
--
Regards,
Baz

"Ahh... But the thing is, they were not just ORDINARY time travellers!"

 

[Great Bustard]

I have a P&S Canon sx100is. its lenses are 6-60mm (i think that it is 30-300 on 35mm), and on the lens is said that the aperature is 2.8-4.3. I plan to by d3100 with a kit lens 18-55 (27-82.5 on 35mm) and the aperature on that lens is 3.5-5.6.

There is a few things i just cannon understand about aperature. I hardly belive that my sx100is can have larger aperature than 18-55 nikon. I understand that it is f/2.8, so what i want to know: on wide open canon f/2.8 if the focal lenght is 6mm does it mean that diameter of pupil will be 6/2.8 = 2.14mm, or do i need to calculate its 35mm focal lenght, 30/2.8= 10.7mm? But that means that on 300 it will be 300/4.3= 69mm, which is imposible. I really need some explanation on that.

1. sx100is: 6mm/2.8 = 2.14mm and 60mm/4.3 = 13.95mm (no 35mm conversion necessary)
2. d3100: 18mm/3.5 = 5.14mm (no 35mm conversion necessary)

...you need some sort of explanation to accompany your arithmetic above. Your calculations give the aperture diameter, which, of course, is the relevant quantity for a given AOV (angle-of-view) in terms of both the DOF (depth-of-field) and the total light projected on the sensor (for a given shutter speed).

In other words, for the same perspective (subject-camera distance), AOV (angle-of-view), aperture diameter, and shutter speed, all systems will have the same DOF and project the same amount of light on the sensor (where, for the same total light projected on the sensor, the noise will be the same if the sensors are equally efficient).

Of course, not all systems will use the same ISO for the same apparent exposure, aperture diameter, and shutter speed. However, that's a non-issue for the most part. If that surprises you, you can read more about that here:

h ttp: www.josephjamesphotography.com/equivalence/index.htm#iso

The first paragraph of the above link is the meat of it:

A common myth is that higher ISOs cause more noise. In fact, what causes the increase in noise at higher ISOs is the lower exposure. That is, when we increase the ISO, the camera will either increase the shutter speed, close down the aperture, or both. The effect of this is that less light falls on the sensor, and it is the lesser amount of light falling on the sensor that increases the noise, not the higher ISO, per se.

However, it's not uncommon for the larger sensor system to use a slower shutter speed at base ISO than the smaller sensor system for a given AOV and aperture diameter, thus putting much more light on the sensor, resulting in less noise. The larger sensor systems can use the lower shutter speed (when motion blur is not a factor) because it can absorb more light before oversaturating.

Thanks.

You're welcome.

You're both welcome.

 

[pixseal]

There is a few things i just cannon understand about aperature. I hardly belive that my sx100is can have larger aperature than 18-55 nikon. I understand that it is f/2.8, so what i want to know: on wide open canon f/2.8 if the focal lenght is 6mm does it mean that diameter of pupil will be 6/2.8 = 2.14mm, or do i need to calculate its 35mm focal lenght, 30/2.8= 10.7mm? But that means that on 300 it will be 300/4.3= 69mm, which is imposible. I really need some explanation on that.

1. sx100is: 6mm/2.8 = 2.14mm and 60mm/4.3 = 13.95mm (no 35mm conversion necessary)
2. d3100: 18mm/3.5 = 5.14mm (no 35mm conversion necessary)

...you need some sort of explanation to accompany your arithmetic above. Your calculations give the aperture diameter, which, of course, is the relevant quantity for a given AOV (angle-of-view) in terms of both the DOF (depth-of-field) and the total light projected on the sensor (for a given shutter speed).

Well, I was answering his question about aperture size...

Thanks.

You're welcome.

You're both welcome.

You're welcome too!
--
-Dave
http://pixseal.com

 

[Graystar]

...you need some sort of explanation to accompany your arithmetic above. Your calculations give the aperture diameter, which, of course, is the relevant quantity for a given AOV (angle-of-view) in terms of both the DOF (depth-of-field) and the total light projected on the sensor (for a given shutter speed).

The OP asked only about exposure. An explanation is needed, but on optical geometry. The things you mentioned are completely irrelevant to exposure.

Your fanatically unceasing attempts to interject your essay into discussions where it has no application has gotten your link blocked. I guess now you’re trying to get yourself banned...again.

.

 

[Great Bustard]

...you need some sort of explanation to accompany your arithmetic above. Your calculations give the aperture diameter, which, of course, is the relevant quantity for a given AOV (angle-of-view) in terms of both the DOF (depth-of-field) and the total light projected on the sensor (for a given shutter speed).

The OP asked only about exposure. An explanation is needed, but on optical geometry. The things you mentioned are completely irrelevant to exposure.

Hmm, here's the OP (bold emphasis mine):

http://forums.dpreview.com/forums/read.asp?forum=1018&message=38250332

There is a few things i just cannon understand about aperature. I hardly belive that my sx100is can have larger aperature than 18-55 nikon. I understand that it is f/2.8, so what i want to know: on wide open canon f/2.8 if the focal lenght is 6mm does it mean that diameter of pupil will be 6/2.8 = 2.14mm, or do i need to calculate its 35mm focal lenght, 30/2.8= 10.7mm? But that means that on 300 it will be 300/4.3= 69mm, which is imposible. I really need some explanation on that.

Your fanatically unceasing attempts to interject...

...to disregard what was asked, and lash out against anyone explaining what is going on and what relevance it has, is a bit tiring. But, well, it's what you do.

Ta.