Amount of light through lens divided by area coverage

I was pondering the question "if I am in low light and don't have a lens with sufficiently large aperture and the desired focal length, is it better to bring out the shorter lens with larger aperture or the longer lens with smaller aperture. How do they compare?"

So I divided the amount of light coming in (square of aperture) by the area covered (sine of lens angle) so that I have a method of ranking different lenses.

I do not claim to have a direct shooting use for this data but I though it's interesting nonetheless. Especially I was always wondering whether it's better to use the 135L or the 85L. According to this measure the 85L captures a lot more photons even if you divide by area covered.

200mm f/1.8 -> 1.77
 85mm f/1.2 -> 1.72
200mm f/2.0 -> 1.43
 85mm f/1.4 -> 1.27
 50mm f/1.2 -> 1.05
135mm f/2.0 -> 0.97
 50mm f/1.4 -> 0.77
 85mm f/1.8 -> 0.77
200mm f/2.8 -> 0.73

Wouldn't the T-stop give you a more accurate number? That's based on actual light loss through a lens, not a mathematical equation (like the f-stop, or your calculations), and I would think would give a better idea of how much light it passing. I dunno...

KirkS518 wrote in post #15884202
Wouldn't the T-stop give you a more accurate number? That's based on actual light loss through a lens, not a mathematical equation (like the f-stop, or your calculations), and I would think would give a better idea of how much light it passing. I dunno...

Good idea, actually. "Amount of light" is tricky. However at longer focal ranges (which this is about) it isn't too bad to use aperture-based values.

I think if you knew the t-stop for each lens, you could come up with an equation that presented the exact amount of light for each lens at different f-stops. You're better at math than I am.

I know, it's just that I didn't bother because I don't think that f-stop/t-stop is very different for those 50mm to 200mm L or almost-L lenses.

Didn't we just have a thread where somebody collected t-stop info?

You should be looking at a list of what lenses gather the most light for astrophotography. I believe the 200/1.8 is on top. Gene

For a uniformly illuminated scene, or for any specific subject, all lenses operating at the same f ratio (or using T stops if you prefer) will result is the same exposure, regardless of focal length. That's why we use f ratios.

I don't see what you are driving at here, nor do I follow your arithmetic. Are you using the square of the physical diaphragm, or the area of the front element as a measure of light gathering power? Why is the sine of the angle of view of a lens a measure of area? Isn't it proportional to the square of that?

xarqi wrote in post #15885675
For a uniformly illuminated scene, or for any specific subject, all lenses operating at the same f ratio (or using T stops if you prefer) will result is the same exposure, regardless of focal length. That's why we use f ratios.

I don't see what you are driving at here, nor do I follow your arithmetic. Are you using the square of the physical diaphragm, or the area of the front element as a measure of light gathering power? Why is the sine of the angle of view of a lens a measure of area? Isn't it proportional to the square of that?

Right, I should use the square.

The point really is "there's the beaver after sunset so small I have to crop a lot anyway, do I use the 135L or the 85mm f/1.4?".

So it is "light gathering power for that tiny to-be-cropped area".

There's a good explanation here showing the mathematics of f stops... http://www.uscoles.com​/fstop.htm

Since a long list of shutter speeds (Usually abbrev. without the 1/ part) and a long list of aperatures will have some numbers that look very much alike (15 16 for example,) Ansel Adams and his pals decided that we need an abbrev. for the apertures. They used F/. Does not stand for anything, just an indicator that the number following it refers to aperture, not shutter speeds. Gene

gasrocks wrote in post #15886153
Since a long list of shutter speeds (Usually abbrev. without the 1/ part) and a long list of aperatures will have some numbers that look very much alike (15 16 for example,) Ansel Adams and his pals decided that we need an abbrev. for the apertures. They used F/. Does not stand for anything, just an indicator that the number following it refers to aperture, not shutter speeds. Gene

Are you sure about this? It's my understanding that (as an example) f/8 meant that the aperture size was the focal length divided by 8. ie., on a 200mm lens at f/8, the aperture is 25mm. Now whether that is actual size or effective size (because of compound lenses) I'm not sure. So the "f" stands for Focal length. EDIT: It's relative size opening, not actual.

The part about Ansel Adams is wrong, as they term f/stop was used since the 1890's, whereas AA wasn't born until 1902.

Wiki is a great resource.

gasrocks wrote in post #15886153
Since a long list of shutter speeds (Usually abbrev. without the 1/ part) and a long list of aperatures will have some numbers that look very much alike (15 16 for example,) Ansel Adams and his pals decided that we need an abbrev. for the apertures. They used F/. Does not stand for anything, just an indicator that the number following it refers to aperture, not shutter speeds. Gene

I don't think so. The "f/" part means "Focal Length divided by" and the whole thing (such as f/8) is the focal length divided by the effective opening of the lens (I don't know the designated location for the opening but it has to do with the iris opening and possibly other optical elements).

uOpt wrote in post #15884723
I know, it's just that I didn't bother because I don't think that f-stop/t-stop is very different for those 50mm to 200mm L or almost-L lenses.

Didn't we just have a thread where somebody collected t-stop info?

It was my thread , I was asking for links to info on Canon lenses .
Kolour-Piker gave me a link to a DXO page :
http://www.dxomark.com …ngs/Optical-Metric-Scores

Not all the Canon line up was tested , but some are there and it does give you an idea .

I also found a page that "stated" that dslr cams will slightly adjust the ISO to cancel the effect of the difference between T and F stop .... I looked no further so I do not know of the facts in this instance .

If you square the dimension (actually making it an area) things change quite a bit:

200mm f/1.8 -> 10.16
200mm f/2.0 -> 8.23
 85mm f/1.2 -> 4.27
200mm f/2.8 -> 4.20
135mm f/2.0 -> 3.78
 85mm f/1.4 -> 3.14
 85mm f/1.8 -> 1.90
 50mm f/1.2 -> 1.59
 50mm f/1.4 -> 1.17

uOpt wrote in post #15886051
Right, I should use the square.

The point really is "there's the beaver after sunset so small I have to crop a lot anyway, do I use the 135L or the 85mm f/1.4?".

So it is "light gathering power for that tiny to-be-cropped area".

The 85/1.4. It is faster than the 135L, which is f/2. Focal length itself is irrelevant as far as exposure is concerned, when f ratios are used.