Light readings with different focal lengths

[boarini2003]

I've never used a light meter, incident or reflected. I would like to try it, but I have a question: Let's say I take a reading out on the street. I get 1/250 and f16 for example. Is this reading applicable to different focal lengths? The wider the lens, the more light it gathers. There can be a great difference between a 50mm and a 24mm, for expample. So how does it work? Thanks...

[Sparrow]

The amount of light hitting the film is the same, so you set f16 regardless of length; the actual size of f16 is different at different focal lengths.

[jaapv]

The f-stop is a mathematical function for the focal length AND diameter of the aperture, so this factor has been taken into consideration. If you look through a telelens at say f8, the "hole"is much larger than if you look through a wideangle lens at the same f-stop. That is the effect of different focal lenghts (not counting the enlarging/diminishng effect of the lens elements of course, but you get the general idea)

[boarini2003]

Interesting. Didn't know that! Thanks.

[ampguy]

the answers above assume you are metering for a specific spot. If you are shooting landscapes and looking for an overall averaged exposure, you will get different readings in camera meter with averaging ttl metering at different focal lengths.

[PeterL]

Hi,

Actually, light can be different through different lenses. While it's true that f8 should give the same amount of light, that's only correct if the lens was perfectly transparent. I compared the exact same scene through a 50/f1.4 and a 90/f2.0. Instead of 1 stop exactly, the difference was 1 stop and 2/3 (if I remember correctly, it could have been more). The 90mm is a much more complex design and it seems that all the glass is absorbing some of the light. When one thinks of the "signature" of a lens, it's easy to understand some light must be absorbed.

There were two terms for the mathematical f-stop (lens diameter / focal length) and the f-stop with light absorption calculated in, but I forgot them. You can be certain that lenses show mathematical f-stop.


Peter.

[Gravatar]

"I compared the exact same scene through a 50/f1.4 and a 90/f2.0. Instead of 1 stop exactly, the difference was 1 stop and 2/3 (if I remember correctly, it could have been more). The 90mm is a much more complex design and it seems that all the glass is absorbing some of the light. When one thinks of the "signature" of a lens, it's easy to understand some light must be absorbed."

Were these lenses with there own leaf shutters or were the on a camera with a curtain shutter? Differences in shutters are GENERALLY far more likely to cause this degree of difference on the same given fstop and shutter speed. The second possibility is a difference in coating.


Grav

[Finder]

Originally Posted by PeterL

There were two terms for the mathematical f-stop (lens diameter / focal length) and the f-stop with light absorption calculated in, but I forgot them. You can be certain that lenses show mathematical f-stop.


Peter.

No, only one meaning for f-stop. The other is called a t-stop and takes lens transmittance into account. Movie cameras use t-stops. The photographic manufacturers did not want to use t-stops because their lenses would appear slower.