Light captured by a lens

[Rufus]

With all other things being equal, does a telephoto lens capture more or less light than a wide angle lens, especially at the same aperture of say f/8?

To answer my own question my starting point would be as follows (I am sure you others could put it in more precise scientific languaage, but I hope you get my drift.

1. Assume two prime lenses: a 35mm and 135mm
2. F/8 means divide the focal length by 8 which gives apperture diameters of 4.375mm and 16.875mm.
3. The overall angle of view for the lenses is in the region of 60 degrees and 15 degrees respectively.
4. The wider angle of view of the 35mm lens would capture 4 x more light, on the other hand the 135mm lens has 4 x the diameter so would capture more light.
5. Consequently both lenses capture the same amount of light as the 4 x the aperture diameter is ofset by the 4 x angle of view.

Is any of this correct?

[Manfred M]

Theoretical answer is that both lenses will let through exactly the same amount of light, at the same aperture setting.

The practical answer is a bit more nuanced. Lens materials and manufacturing processes come into play and any lens will transmit light at a lower level than that theoretical number. Cine lenses don't use f-stop markings, but rather t-stop ("t" stands for transmission).

If you go to the DxOMark website and look up a lens, this real number is shown for a wide open lens.

[xpatUSA]

With all other things being equal, does a telephoto lens capture more or less light than a wide angle lens, especially at the same aperture of say f/8?

To answer my own question my starting point would be as follows (I am sure you others could put it in more precise scientific languaage, but I hope you get my drift.

1. Assume two prime lenses: a 35mm and 135mm
2. F/8 means divide the focal length by 8 which gives apperture diameters of 4.375mm and 16.875mm.
3. The overall angle of view for the lenses is in the region of 60 degrees and 15 degrees respectively.
4. The wider angle of view of the 35mm lens would capture 4 x more light, on the other hand the 135mm lens has 4 x the diameter so would capture more light.
5. Consequently both lenses capture the same amount of light as the 4 x the aperture diameter is ofset by the 4 x angle of view.

Is any of this correct?

One way to think of the theoretical answer is to consider the simple formula for exposure value EV as found here. By setting each lens's f-number the same and keeping the shutter time the same, the theoretical exposures will be the same. If the exposures are the same, then the amount of light gathered must be the same.

[Rufus]

I had forgotten that many years ago I sometimes used a light meter to establish a shutter speed and aperture for a scene. The film's ASA had to be set on the light meter but there was never a requirement or facility for the focal length of the lens to be set on the meter. So intuitively, in the example I used, the exposure would have to be the same!

The information Ted supplied proves this - thank you Ted!

[rpcrowe]

There is perhaps another way to clear up this question. Shoot two images, using lenses of different focal lengths, at the same f/stop and shutter speed and see if the exposure is equal.

There is, of course, the question of transmission value but, I would expect that for the requirements of still photography, the exposures would be pretty near identical...

The "T" (Transmission) stops of cine lenses are designed t be absolutely the same so that when you cut a sequence shot with one lens next to one shot with another lens, the exposures would be identical and they would mesh seamlessly.

Sets of "T" stop cine lenses are exceptionally expensive.A set of six matched Cooke Cine Lenses can run over 42-thousand U.S. Dollars. Rokinon has some less expensive cine lenses but, even these are far-far more expensive than the standard Rokinon still photo lenses...

[xpatUSA]

There is perhaps another way to clear up this question. Shoot two images, using lenses of different focal lengths, at the same f/stop and shutter speed and see if the exposure is equal.

Perhaps, but only if the scene has exactly the same luminance within each angle of view ...

... or if the shots have exactly equal framing?