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
Veteran
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
RFF Sponsoring Member.
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)
Interesting. Didn't know that! Thanks.
ampguy
Veteran
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.
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.
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"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.
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"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.
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Finder
Veteran
PeterL said:
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.