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02-19-2014, 10:54 PM   #1
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F/2.8 ain't what it used to be

F/2.8 sure ain't what it used to be. Thinking today that a f/2.8 telephoto lens on a K-30, with its large glass, must be capturing a LOT more light than the very small lens in my GR, when also set at f/2.8. Same sensor size on both cameras. Which makes me wonder, if I put an even bigger-glass f/2.8 645 lens (if such a thing even exists) on the K-30 -- would that one collect even more light?

So -- if I'm understanding this correctly, f/2.8 isn't the same across different lenses, right? It's relative to each lens separately... and I guess to the size of the sensor the light falls on also?

Or -- is the f/2.8 exposure the same across all lenses and cameras?

02-19-2014, 11:04 PM - 1 Like   #2
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It isn't exactly the same between lenses, but it is close enough. True measure of light transmission is measured in t-stops instead of f-stops, no one really publishes this except for cine lenses. The f-stop is nothing more than the ratio between the focal length and the diameter of the aperture opening, this doesn't always mean the physical diameter though. If you are shooting with a 50mm and a 28mm lens at f2.8 then you should be able to use the same shutter speed and ISO and the exposures will be very similar, not exact, but within the ballpark.

The 645 lens will let in more light, but it doesn't mean anything on a smaller frame because that light doesn't hit the sensor. The image it projects is larger, that is all.
02-19-2014, 11:05 PM   #3
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QuoteOriginally posted by jon404 Quote
So -- if I'm understanding this correctly, f/2.8 isn't the same across different lenses, right? It's relative to each lens separately... and I guess to the size of the sensor the light falls on also?
Aperture/f-stop is proportional to the area of the front element and the focal length. Long lenses like a 200mm need a much bigger piece of glass to gather the same amount of light as wide lenses like the one on your GR.

QuoteOriginally posted by jon404 Quote
Or -- is the f/2.8 exposure the same across all lenses and cameras?
By the logic above, this is correct as long as we ignore sample variation/rounding.

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02-19-2014, 11:15 PM   #4
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You still get more light through a 200/2.8 than a 18/2.8 due to the 'clear aperture' on the 200 being muuuch bigger. Useful for astrophotography.

02-19-2014, 11:34 PM   #5
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So -- help me out with this -- if "You still get more light through a 200/2.8 than a 18/2.8 due to the 'clear aperture' on the 200 being muuuch bigger..." then -- on the same scene at the same ISO -- wouldn't you be able to use a faster shutter speed at f/2.8 with the telephoto?

What am I missing here?
02-19-2014, 11:40 PM   #6
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The telephoto has (takes) a much smaller angle and thus needs a larger opening to get the same (image) exposure.
02-19-2014, 11:43 PM   #7
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It's not so straight forward. Here is a good explanation The f/ratio myth
02-20-2014, 12:15 AM   #8
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An f/2.8 medium format lens lets in more light than an f/2.8 FF lens, and in turn an f/2.8 APS-C lens. However, that extra light doesn't fall on the sensor, as the sensor's too small.

Assuming all have the same T stop, they will all let in the same amount of light per cm squared, and will therefore give the same exposure at the same aperture setting.

02-20-2014, 02:11 AM   #9
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The idea of larger format lenses capturing more light (as they have longer focal lengths in order to achieve the same FOV and therefore larger clear apertures), but that most of that light would never reach the sensor/film as the projected image is larger, is the idea that all those metabones speed boosters and so on work on.

All they do is focus the larger image (with all its extra photony goodness) down to a smaller area, thereby giving you 'extra' light.
02-20-2014, 03:01 AM - 3 Likes   #10
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QuoteOriginally posted by jon404 Quote
So -- help me out with this -- if "You still get more light through a 200/2.8 than a 18/2.8 due to the 'clear aperture' on the 200 being muuuch bigger..." then -- on the same scene at the same ISO -- wouldn't you be able to use a faster shutter speed at f/2.8 with the telephoto?

What am I missing here?
QuoteOriginally posted by Woolcott Quote
The idea of larger format lenses capturing more light (as they have longer focal lengths in order to achieve the same FOV and therefore larger clear apertures), but that most of that light would never reach the sensor/film as the projected image is larger, is the idea that all those metabones speed boosters and so on work on.

All they do is focus the larger image (with all its extra photony goodness) down to a smaller area, thereby giving you 'extra' light.
I don't know if it helps, but here is a very specific set of examples:

A. Suppose I want to shoot the Full Moon with a 100mm f/2.8 lens and a 300mm f/2.8 lens
My 100mm lens will have an open aperture (entrance pupil) of almost exactly 10 square centimeters (1001.78 square milimeters to be exact) and my 300mm lens will have an open aperture area of 90 square centimeters. Thus for each unit of time, my 300mm lens will capture 9 times more photons from the Moon that will my 100mm lens.

Now, the Moon is an extended object with an angular diameter of 0.5 degrees as seen from Earth. Thus the image formed by my 100mm lens on my sensor will be a circle with an area of 0.6 square milimeters (0.598 to be exact) while the image formed by 300mm lens will have an area of 5.4 square milimeters - that is a 9 times larger area of the image on my sensor.

Therefore, the 9 times more photons that my 300mm lens captures relative to my 100mm lens will also be spread over a 9 times larger area, and thus, exposure times will have to be the same with both lenses.

B. Suppose I want to shoot some stars with a 100mm f/2.8 lens and a 300mm f/2.8 lens
This is to complicate matters a bit, but also to give the full story:

Stars are so distant that we don't see them as extended objects - only as point-like sources of light of varying brightness. Our camera lenses see them the same way and they do NOT form images of stars as extended objects the same way as with the Moon. Rather images are small diffuse dots and faint rings (: diffraction circles) which size only depends upon the f-number, irrespective of the other lens characteristics.

Therefore, in THIS case my 300mm lens will again capture 9 times more photons from any given star than will my 100mm lens. But the photons from both lenses will form images (diffraction circles) of identical sizes. If I can just and barely form an image of a faint star in one minute with my 300mm lens, I shall need 9 minutes to capture the same star with my 100mm lens. But mind you: This is for point-like sources of light only!
02-20-2014, 05:05 AM   #11
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So, simply put, aperture is what it used to be and f2.8 is f2.8. There might be sample variation or variation between lens types, but it is basically insignificant.
f2.8 is a fraction, a ratio, so it is equivalent across everything, in terms of exposure value. The confusion comes when people start doing equivalence based on DoF or based on "net amount of light" (of the whole projected circle instead of the intensity of brightness on the sensor)

100mm f4.0 made for medium format, 100mm f4.0 made for film, and 100mm f4.0 made for crop sensor will give you the same brightness on the actual sensor, so you can use the same shutter and ISO to get the same exposure at the same aperture. The one for crop sensor captures "less light" since it projects an image circle for the size of the crop sensor, while the 100mm f4.0 for MF makes a bigger image circle, one that fits Medium format.

The ideas behind "aperture" make sense and are consistent.
02-20-2014, 05:51 AM - 1 Like   #12
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Not enough answering the OP's question in this thread.

Bottom line:

F-number is a ratio that would theoretically produce uniform light intensity per unit area for any focal length, assuming uniform incoming light. T-stop is the practical equivalent value after taking into account light losses through the lens.

If you have a wall of completely uniform color and lighting that is big enough to completely fill the frame, a picture taken at the same f/t-stop, ISO and shutter speed will have exactly the same exposure (look exactly as bright) on any lens/camera combination regardless of focal length or sensor size (ignoring individual lens/camera characteristics such as vignetting, manufacturers lying about ISO, etc.).

If you want to deal with the concept of how absolute aperture size affects imaging, that's a whole different area which gets into things like why larger sensors based on the same technology have lower noise (total light gathered), why larger absolute aperture telescopes can resolve smaller and fainter objects, regardless of f-number (total light gathered and diffraction), etc.

Last edited by Cannikin; 02-20-2014 at 06:08 AM.
02-20-2014, 06:57 AM   #13
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f2.8 is pretty much f2.8, but the size of the lens will dependent on whether the lens is a prime or a zoom (zoom will be bigger), the focal length (longer will require a larger lens), the size of the image circle, and the registration distance of the mount. In addition, highly corrected lenses will tend to be bigger -- modern way is to have smaller lenses with less correction and then just deal with issues with in camera corrections.
02-20-2014, 07:11 AM   #14
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QuoteOriginally posted by Cannikin Quote
If you have a wall of completely uniform color and lighting that is big enough to completely fill the frame, a picture taken at the same f/t-stop, ISO and shutter speed will have exactly the same exposure (look exactly as bright) on any lens/camera combination regardless of focal length or sensor size (ignoring individual lens/camera characteristics such as vignetting, manufacturers lying about ISO, etc.).
Exactly, Aperture is a mathematical formula that gives a rough estimate of a starting point for determining exposure. If it was precise, you wouldn't need an EV button. This article is getting so lost in it's own thought train, it's conclusions become un-intelligle and absolutely false. I'll blame it on the current disposition to measure everything, noise, DoF etc. in terms of f-stops. This is just some poor guy wallowing in his own, self induced confusion. Clean up your thinking,

QuoteOriginally posted by Adam Quote
Aperture/f-stop is proportional to the area of the front element and the focal length. Long lenses like a 200mm need a much bigger piece of glass to gather the same amount of light as wide lenses like the one on your GR.
Dwell on that for a second. Every attempt to deviate from that definition, and assign magical properties to an ƒ-stop to define whatever attribute of the day you want to discuss, will only contribute to confusion like this man's. If you listen to those who should know better ascribing all kinds of other factors to f-stops, you're entering an un-resolvable world of confusion. It's not a measure of light transmission, it's not measure of DoF, it's not a measure of noise, it's a mathematical expression of focal length to lens opening and that is exactly what it always was, anyone who has tried to add to that definition in the end, only causes confusion , as does every oversimplification, and I must say, many of the most knowledgeable posters technically, are the most guilty of this.

You only believe f-2.8 isn't what it used to be, if you bought into the BS.
02-20-2014, 07:32 AM   #15
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Oh, and f-stops are also useful because you can quickly adjust the exposure if you know the numbers. With digital cameras and continuous auto metering, this is no longer something photographers learn, but in the past we would know the full stop of shutter speeds and aperture, so that one could easily just dial the aperture in one direction and shutter in the other - keeping the same exposure, but changing the photo (DoF, movement blur). With half stops and third stops things get a little more confusing, but aperture numbers are standard for a reason. They allow quick reference, adjustment, and even imagining the exposure/brightness of the final photo.
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