My hyper-confused self and the f-stop

[BennoLF]

WARNING!!

In this thread I'm questioning things people have proven to be true not only by math and science but also by practice. It will not seem logical. If you want to close the tab and go about your day conniption-free, please do. I won't blame you.
If you're willing to continue on, thank you in advance for your patience. Also, buckle up

So I'm wondering about f/stops and brightness and different focal lengths and a whole load of other stuff.
NOTICE: All my assertions about brightness and stuff is based a metering system seeking to average the light and strive for an average of zone V. I haven't even thought about how it would change with, say, zone Vi'ing people's faces.


From what I understand about the f/stop, mostly from http://www.uscoles.com/fstop.htm , the f/number is a ratio between the diameter of the aperture and the focal length of the lens.
Longer lenses need bigger apertures diameters to pass the same amount of light, so we use f/stops to make everything simpler. f/8 on a 90mm is f/8 on a 300mm, etc.

Ok. Makes sense to me so far.
But. Two different lenses will give the same exposure for the same subject only if the camera is moved so the subject fills the frame
Right?

Let's say we have a wall 4x5 feet with an even luminous density of 30 cdl/sq. ft., suspended in an infinite blackness. Total output of the wall is (4x5)x30 = 600 foot-candles. Both a 300mm f/8 and a 90mm f/8 will give the same exposure: 1/30, asa64.
But they need to be at different distances so the wall occupies the entire frame.(we'll neglect light loss from distance)
Right? If the camera was fixed at a point where the wall filled the frame of the 300mm lens, the 90mm lens from the same distance would show tons of black around it, and would need a longer exposure to attain an average zone V, right? Because the bright wall would take up a smaller percentage of the frame? I'm already starting to confuse myself haha.

However, there are some subjects you can't really get closer or farther away from. Let's say the sky. A 300mm lens and a 90mm lens are picking up very different fields of view. So for a theoretically even luminous density sky, there is less light entering the front of a 300mm than a 90mm, by virtue of it taking light from a narrower field. So to give the same exposure for less total light, the 300mm needs to be more 'light-efficient', right? kinda like the slow, wide river or fast, narrow river distinction.
https://petapixel.com/2014/01/29/pic...y-photography/
^^ in the above link, about halfway down, they mention this thing called clear aperture, which is their word for aperture diameter.

"The 24mm f/2 lens collects light from a comparably wider field of view than the 100mm f/2. Since they’re both f/2, they both capture light at the same “speed”. So for equal shutter speeds, they should provide the same illuminance at the sensor." <-- this is because they're talking about astrophotography, so both lenses are viewing (essentially) a wall of even luminous density
here "speed" is intensity X area

"So in terms of exposure value, the 24mm lens will produce equivalent brightness images for any given ISO and shutter speed because it’s pulling light from more of the scene than the narrower 100mm lens, hence the identical f/number rating. The long lens collects more light at a time from a smaller area of the scene while the short lens collects less light at a time from a larger area of the scene."

So this would seem to say that a longer lens is brighter than a short lens for any given subject angle of view, even if they are the same brightness for their own lens angle of view.

Am I talking crazy? The more I think the more I'm confused. I realize that people have been using these principles for longer than forever, so that's what I'll use when I'm out in the field.
However, I'm curious and want to know more. If anyone has anything that would help sort me out it would be greatly appreciated.

Thanks!

Benno

[darr]

You are overthinking this. Measure of light calculations are based on the value of luminous on the subject measured. If you are measuring for a vase of flowers sitting inside a window frame in a large room, it does not matter if you use a wide, normal or long lens, the value of luminous falling on or falling off the vase of flowers is what is important. You may get a stop or two different metering results depending upon meters used, spot or incident, but the area of importance remains to be the vase of flowers. I am speaking in practical terms here. You might enjoy more scientific jargon, but I deal in practicalities.

[Luis-F-S]

Suggest you read Adams' The Negative and use a spot meter, not an in-camera meter, as your initial assumption that: "But. Two different lenses will give the same exposure for the same subject only if the camera is moved so the subject fills the frame Right?" is WRONG! Lenses don't give exposure, a meter does. Why I suggest using a spot meter!

[nicnilov]

I don't think you're confused. In fact, your logic seems pretty sound. Since f/stop is not an absolute measurement but a proportion, yes, long lens collects more light from the same subject angle than the wide one. This seemingly higher performance of the long lens is balanced by the diameter of the aperture opening. E.g. a 300mm lens with a 30mm opening has the f number of 10. A 50mm lens to get to the f number of 10 needs to have the aperture opening of 5mm. Thus, despite the ability of the wider lens to collect light from, well, wider angle, to get to the same f number it has to close down more, therefore restricting the total amount of light coming through.

One potential pitfall in the middle of your reasoning is that the optical performance of the lens has nothing to do with the subject size in the frame area. As soon as we agree to consider the average V zone exposure, it doesn't matter what combination of subject lightnesses in various parts of the frame creates it.

Focal lens and aperture opening are not the only factors governing the lens performance. One thing that may be relevant to this topic is a t/stop, which is a more meaningful way to compare performance of real lenses.

[Dan Fromm]

Benno, why don't you just get to the point and ask how best to meter?

[cowanw]

Your fifth paragraph is where things fall down, because your assumption of a averaging and variable metering system which cannot be a correct value for consistancy in exposure in both your examples of the wall and the wall in a dark space

[BennoLF]

Suggest you read Adams' The Negative and use a spot meter, not an in-camera meter, as your initial assumption that: "But. Two different lenses will give the same exposure for the same subject only if the camera is moved so the subject fills the frame Right?" is WRONG! Lenses don't give exposure, a meter does. Why I suggest using a spot meter!

Oh sorry, I didn't phrase that very clearly. What I meant by lenses giving an exposure was a lens passing enough light to properly expose a film to a given density.

Benno, why don't you just get to the point and ask how best to meter?

This is all just idle curiosity, partly piqued by my noticing that longer lenses 'look' brighter through the GG than wider lenses at the same aperture.
I at least 'think' I know how to meter ahah. Maybe after all this I'll reconsider

Your fifth paragraph is where things fall down, because your assumption of a averaging and variable metering system which cannot be a correct value for consistancy in exposure in both your examples of the wall and the wall in a dark space

I don't quite understand you. Do you mind rephrasing? Sorry :>

[Dan Fromm]

This is all just idle curiosity, partly piqued by my noticing that longer lenses 'look' brighter through the GG than wider lenses at the same aperture.
I at least 'think' I know how to meter ahah. Maybe after all this I'll reconsider

If that's what you wanted to know, you should have asked the question directly instead of raving at us. Please try to be more direct.

To answer your question, long lenses look brighter off-axis on the GG because they're narrow angle lenses. The rays they project on the GG are nearly perpendicular to it. They go through back straight through it to the viewer's eyes. W/A lenses' off-axis rays are more oblique to the GG, don't go straight back to the viewer's eyes. In addition, w/a lenses have worse cos^4 falloff off-axis that narrow angle lenses. Its called optical vignetting.

[C. D. Keth]

From what I understand about the f/stop, mostly from http://www.uscoles.com/fstop.htm , the f/number is a ratio between the diameter of the aperture and the focal length of the lens.
Longer lenses need bigger apertures diameters to pass the same amount of light, so we use f/stops to make everything simpler. f/8 on a 90mm is f/8 on a 300mm, etc.

First assertion is almost right but not quite. It's not the diameter of the aperture but the diameter of the "entrance pupil." That is the circle of light that, entering the front element of the lens, is completely allowed to pass through the iris. It seems pedantic but some lenses have very strong magnification or reduction in the front group that makes the iris look much larger or smaller than it is in reality. One would measure this by placing the rear of the lens facing a window or other broad, bright light source and measuring the circle of light that projects from the front of the lens.

Two different lenses will give the same exposure for the same subject only if the camera is moved so the subject fills the frame
Right?

No. Exposure and field of view are different. We are not dealing with automatic or through the lens metering. I can expose a photograph precisely the same from the same vantage point whether I do it on a 15mm lens or a 500mm lens. The perception of brightness may be different, however. A black dog in a snowy field will look different if he fills 80% of the frame than when he fills 2% of the frame, even when exposed the same.

"So in terms of exposure value, the 24mm lens will produce equivalent brightness images for any given ISO and shutter speed because it’s pulling light from more of the scene than the narrower 100mm lens, hence the identical f/number rating. The long lens collects more light at a time from a smaller area of the scene while the short lens collects less light at a time from a larger area of the scene."

So this would seem to say that a longer lens is brighter than a short lens for any given subject angle of view, even if they are the same brightness for their own lens angle of view.

Lets continue to imagine this clear sky scenario with a 25mm lens and a 50mm lens. The field of view of the 25mm lens will be twice the angular field of view both horizontally and vertically.

Yes, the 25mm lens does gather light from a larger area of sky, 4x larger by area in fact. That does mean that it gathers 4x more light as a gross amount of photons. However, it then passes that light through an aperture that is 1/4 the area of the longer lens' aperture.

Lets do the math with theoretical f/2 figures, to make relatively small numbers:

area of the entrance pupil, a circle, is found with the equation area=pi*(r^2) where r is the radius of the circle

pi*(12.5^2)=491 square mm area for the 25mm lens

pi*(25^2)=1963 square mm area for the 50mm lens

Let’s check the relationship between our f2 entrance pupils. Remember that the areas of sky photographed were different by a factor of 4x, the 25mm lens “saw” 4x more area of sky.

491*4=1963

The 25mm lens that saw 4x more sky let only 1/4 of the gross light through its aperture than did the longer lens.

Net result? The same amount of light gets through each lens and is recorded onto the film. Being a completely featureless, clear, blue sky the images look exactly the same.

This is essentially a simple proof of why the f-stop system exists: it is self scaling to any lens. f/8 on a 150 year old lens for whole plate will pass the same intensity of light as a brand new, state of the art cinema zoom lens set at f8.

[cowanw]

I don't quite understand you. Do you mind rephrasing? Sorry :

If the wall is a constant and the lens f stop is a constant, then the variable of what your light meter says is different in your two scenarios. You can't change one thing arbitrarily using your own rules of how to measure light intensity and and conclude the other variables change too. Assuming that The median reading of a averaging reflective light meter is always the correct exposure setting is your error
Alternate scenario; if you used an incident light meter the reading would be the same in both of your scenarios.