Basic light measurement question

MTL · September 22, 2004

I'm not used to measuring light with a incident/reflective meter that's not TTL like in most manual 35mm still cameras. My question is when I get a reading on the meter, and I ajust my exposure accordingly, does the focal length of my Angenieux lens (for eclair acl) have an affect on how much light is reaching the film? If so, what's the genral rule for compensating.

Dominic Case · September 23, 2004

does the focal length of my Angenieux lens (for eclair acl) have an affect on how much light is reaching the film?

No!

There is a relationship between the focal length of the lens and the diameter of the lens and the amount of light reaching the film. But it is all dealt with in the single value of the f/stop (or more strictly the t/stop).

In short, you will get the same effective exposure with any lens of any focal length if it is set to the same t/stop.

Phil Rhodes · September 23, 2004

Hi,

Probably worth mentioning that there are zoom lenses where you lose stop towards the long end. Video zooms routinely do this but I have seen it on film ones too.

An exceptional case, though.

Phil

Laurent Andrieux · September 23, 2004

it's called ramping. The F stop number is defined as N=f/D where f is the focal length and D the optical diameter of the lens. There usually is no problem because when f increases, D does as well. There is a problem when the real front lens diameter is less than this D value (that depends on the focal length...)

This definition doesn't take count of the transmitance of the lens. So the manufacturers engrave a print for T stops, that give you the same amount of light as if it was perfect, with the stop you set.

Look, there shoudn't be anyproblem, since you hace T stops prints on your lens. This the case of may Angénieux zooms, like 12/120 that one being certainly the one you have... It would great us to know, we could help much better. But, mind that these T stops were engraved when the lens was brand new. Sometimes, you have a red mark close to the stops mark, that shows you the compensation you hacve to make at the longest focal length.

The best would be you to make keylight tests at different focal lengths, with yourlight meter and find out : 1) what sensitivity to compute in your light meter that gives you the best result for one consider film stock 2) if you have somehow the need to compensate at some focal lengths. I would be worth doing this since you own the material.