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02-20-2014, 09:20 AM   #16
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The example of the moon and a 1000/2.8 and a 300/2.8 is absolutely correct in that the 1000 will have more photons hitting the sensor. It is absolutely incorrect if you are thinking about how the moon itself is exposed. ISO 400 = 1/400 @ f/11 for a full moon. The moon is an object way out there, front lit by the sun. The stop difference from sunny 16 can be put down to the several miles of atmosphere obstructing the light path.

02-20-2014, 09:51 AM   #17
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QuoteOriginally posted by jon404 Quote
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?
the problem inherent to this question is that you asked several diff questions and are trying to associate them all as one.

light gathering refers to the accumulation of photons, just like a bigger bucket of water holds more water, a lens with a bigger element will gather more light. that's the answer to ONE of your questions. so a bigger lens will gather more photons, but what happens to those photons as they travel the length of that lens and land or miss the sensor at the end is another story.

this is the answer to your second question. f stop is in a poetic way, a description of what happens to those photons on their journey and how they are filtered, corralled, cajoled and otherwise manipulated by the lens elements and lens aperture and actual physical lens shape.

your exposure value is the final result of all the photon gathering and all the lens manipulation. f stop was created (solved) as a mathematical formula to assist in finding a uniform way of expressing exposure across a variety of circumstances (lens), because the behavior of photons is a known constant.

so although photon gathering is a component of exposure values, other factors come into play. therefore the end result is that f2.8 still IS f2.8 and f2.8 will REMAIN f2.8 across any lens on any sensor as long as the physical characteristics of photons remains the same. ie engineers did all the math for you based on the design of that particular lens, so you don't have to.

HOWEVER, if you were on the event horizon of a black hole, f2.8 wouldn't be f2.8 any more, but explaining that would REALLY confuse things.

Last edited by nomadkng; 02-20-2014 at 10:00 AM.
02-20-2014, 10:02 AM   #18
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QuoteOriginally posted by Adam Quote
Aperture/f-stop is proportional to the area of the front element ....
Just to be clear, it is the entrance pupil (an optical property), not the front element (a physical property). You can't just measure the front element with a ruler.

Someone might look at both the DA40 Ltd with the 40XS, and incorrectly assume the 40XS lens is slower. However, they are both f/2.8, and probably transmit about the same amount of light. So trust the markings on the lens.

Entrance pupil - Wikipedia, the free encyclopedia
02-20-2014, 10:13 AM   #19
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I was under the impression that F stop is just a formula of length/diameter, and whilst many people think it's light transmission, actually that depends on a number of other factors too. Ie number of elements, coating. Fact is every glass element only transmits a fraction the the light that comes through, so a 15 element lens must invariably transmit less than a 6 element lens. But how much of a difference that makes I don't know, if it's enough to even make a difference.

02-20-2014, 10:18 AM   #20
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QuoteOriginally posted by Nass Quote
I was under the impression that F stop is just a formula of length/diameter, and whilst many people think it's light transmission, actually that depends on a number of other factors too. Ie number of elements, coating. Fact is every glass element only transmits a fraction the the light that comes through, so a 15 element lens must invariably transmit less than a 6 element lens. But how much of a difference that makes I don't know, if it's enough to even make a difference.
That's where all the coatings of the various elements come in. The fraction of light that each element transmits is actually quite high, even with uncoated glass, somewhere North of 99%. So there is a loss, and that is where the t/stop comes in rather than the f/stop.

One must keep in mind the diameter talked about is the diameter of the aperture, not the diameter of any of the elements. It's the hole controlled by the aperture divided into the focal length of the lens that counts, with minor modifications due to lens sales "enhancements" (f/2.8 might actually be f/3.0 or 200 mm might actually be 183 mm).
02-20-2014, 10:20 AM   #21
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QuoteOriginally posted by Tanzer Quote
Just to be clear, it is the entrance pupil (an optical property), not the front element (a physical property). You can't just measure the front element with a ruler.

Someone might look at both the DA40 Ltd with the 40XS, and incorrectly assume the 40XS lens is slower. However, they are both f/2.8, and probably transmit about the same amount of light. So trust the markings on the lens.
f stop is measured based on the FOCAL LENGTH ratio to the entrance pupil size and really has nothing at all to do with the front element. the front element could be 1000mm away from the entrance pupil and the lens' effective focal length could still be 300mm, it all depends n the design of the lens. (look at a 500mm mirror lens and compare it to a 500mm "standard" lens for example) but that's a tangent from the original course of this thread.

this goes back to the OP's problem by mixing and matching different questions and expecting one answer. front element size DOES effect light gathering and light gathering requirements are effected by light transmission qualities and light transmission qualities do effect exposure values and exposure values are effected by f stop, phew!

HOWEVER, each lens design is unique, the 1 constant is the properties of photons. THEREFORE f stop was created as a way of summing up ALL the different things that happen to light on the way to the capturing medium (film/sensor et al). f stop is a HIGH LEVEL constant that is part of the overall exposure value equation (at the final destination) so that calculating the proper exposure for an image can be simplified and calculated based on three elements in a equation:

f stop (how many photons are available)
shutter speed (how long the media is exposed to the photons so that they can interact with it)
iso (how sensitive to the photons the media is)
02-20-2014, 10:54 AM   #22
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Without going through all the math to prove it. What you will find is that for a uniformly lit surface, and the angle of view of the lens, you ultimately have a constant factor fall out of the equation which is the diameter of the lens (or apature) divided by focal length. This is how F Stops got defined in the first place, the ratio was a mathematical one,

Don't get bent out of shape over sensor/film formats, they do not enter the equation. Consider only the light hitting the sensor. A bigger format has more light striking the sensor because the sensor area is bigger, but it also results in more light gathered by the lens because the field of view for the larger format is also bigger. As a result, the light per square area of sensor remains constant

Although T Stops or true light transmission has been used from time to time, they are not used a lot for normal photography because with the introduction of lens coatings, the transmission losses at each boundary inside the lens is now so greatly reduced it can be ignored. There are some cases where for very high element counts this is not the case, but when you consider TTL metering the camera manages these differences.

Last edited by Lowell Goudge; 02-20-2014 at 10:59 AM.
02-20-2014, 11:17 AM   #23
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To everybody -- thanks very much for the intelligent and helpful comments.

My takeaway for exposure -- f/2.8 is f/2.8 -- whatever the camera.

Again, thanks!

02-20-2014, 12:06 PM   #24
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QuoteOriginally posted by jon404 Quote
To everybody -- thanks very much for the intelligent and helpful comments.

My takeaway for exposure -- f/2.8 is f/2.8 -- whatever the camera.

Again, thanks!


Pretty much, in a very simple and short term.
02-20-2014, 04:10 PM   #25
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QuoteOriginally posted by Canada_Rockies Quote
One must keep in mind the diameter talked about is the diameter of the aperture, not the diameter of any of the elements. It's the hole controlled by the aperture divided into the focal length of the lens that counts ...
Actually, it is the effective hole diameter as seen through the front of the lens, i.e., the physical aperture, but subject to all of the optical magnifications/reductions caused by those elements. So, if you had direct access to the aperture blades and tried to physically measure them, that generally would not be the correct diameter to use.
Entrance pupil - Wikipedia, the free encyclopedia

QuoteOriginally posted by Canada_Rockies Quote
.... with minor modifications due to lens sales "enhancements" (f/2.8 might actually be f/3.0 or 200 mm might actually be 183 mm).
Ain't that the truth! There is similar confabulation that goes on with ISO settings. As an example, one manufacturer might try to make their ISO 16,000 look less noisy, by using a gain setting equivalent to ISO 12,000 instead. Reviewers won't always check the accuracy of the ISO values, they will just say that brand X has better high-ISO results than brand Y!

---------- Post added 02-20-14 at 06:32 PM ----------

QuoteOriginally posted by nomadkng Quote
... front element size DOES effect light gathering
This is true, if we assume the lens designers are not silly and wasteful.

If you add a step-up ring with huge filter to the front of a lens, it won't change the f/stop rating of the lens, of course. Similarly, it is possible to design a giant oversized front element that looks big and impressive, but does not actually send more light through the physical aperture of the lens down to the sensor. This would be a poor design, because it adds more glass for nothing, and makes the lens more expensive to manufacture, and heavier. Plus the lens would be more prone to flare, with light bouncing all around internally.

I'm not trying to assert that lens manufacturers are deliberately vain or wasteful. However, just looking at the size of a front element alone really doesn't tell you the light gathering capabilities of the lens, as evidenced by this little guy:
https://www.pentaxforums.com/lensreviews/data/68/large/smc_Pentax-DA_40_2_8_XS.jpg
https://www.pentaxforums.com/lensreviews/data/68/large/smc_Pentax-DA_40_2_8_XS_aux.jpg

Last edited by Tanzer; 02-20-2014 at 10:02 PM.
02-21-2014, 05:15 AM   #26
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One thing i note here is the debate of the front element diameter and aperture

For tele lenses and many simple primes this is true. For zooms, not always
02-21-2014, 08:41 AM   #27
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One of the largest variations in objective lens sizes is the Pentax 50mm F1.4 FA with 49mm filter size and the Sigma 50mm F1.4 with 77mm filter size.
02-21-2014, 09:42 AM   #28
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QuoteOriginally posted by mrNewt Quote


Pretty much, in a very simple and short term.
Think how complicated it would be if every camera/lens used different values for the same exposure. It would be pixel-peeper's nirvana...
02-21-2014, 10:33 AM   #29
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@jogiba -- that is a very interesting picture! Unless I'm wrong, all four lenses are designed to cover a FF sensor. And, as I've learned from this thread, they all produce the same exposure at, say. f/2.8.

So why is the Sigma lens glass so large? As another post noted, lens designers certainly aren't stupid. Off the top, I'd think that the larger glass must cost more to produce that the smaller glass on the other lenses. Is that true? If so -- what optical advantages do they get, if any? Or, is it just a marketing thing aimed at folks like me -- "bigger lens, must give you better pictures?"
02-21-2014, 11:57 AM   #30
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QuoteOriginally posted by jon404 Quote
So why is the Sigma lens glass so large? As another post noted, lens designers certainly aren't stupid. Off the top, I'd think that the larger glass must cost more to produce that the smaller glass on the other lenses. Is that true? If so -- what optical advantages do they get, if any? Or, is it just a marketing thing aimed at folks like me -- "bigger lens, must give you better pictures?"
Maybe you can find diagrams for those lenses. They have a different number of elements and groups. With more elements you can get more corrections, but you lose some T. These days, lenscrafting is a very complex thing, using a variety of materials to remove things like ghosting, coma, CA, flare resistance etc. Those 50mm lenses will give you (for all purposes) identical brightness at the same aperture, but they will render quite differently. This means the colours, tones, contrasts, sharpness, bokeh would be different.
This also depends on the priorities of the brand. Pentax makes compact lenses with classic quality. Size is a priority. Sigma on the other hand, is trying to sell its lenses by making them as best as they can be on the given budget - even if huge. The Zeiss Otus is an even larger lens, with even more correction (and higher cost). Is its IQ better than the smaller lenses? Yes. Would viewers immediately notice it? Probably not. Most brands either try to strike a balance between important factors (IQ, size, cost, AF). In the hands of a skilled photographer with the proper process, photos taken with FA 50mm f1.4 and Sigma 50mm f1.4 would be almost indistinguishable to the untrained eye. Photographers might know what to look for to find the differences. Differences might show up in extreme conditions, though, where one lens might exhibit flare and the other wouldn't, due to different coatings or fewer lens elements. Or one might allow a closer minimum focus distance than the other. Or one has faster or silent AF. Or one shows tones that are more pleasing for portraits than the other. Or one might have smooth background blur, the other might have jagged or swirly blur. One might have softer corners. One might vignette more. One might have CA. Its a whole science
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