F4 vs F4

[fritz1]

Is there a difference bettween f4 on one lens vs another, in other words a 17-40 vs 24-105 due to longer f/l and more elements in say the 24-105 allows less light in, or is it the same effective measure of light entering at a given AV I hope this is making sense

[Wilt]

The aperture is always a ratio, so f/4 passes the same amount of light (approximately) regardless of focal length (this statement ignores the issue of relative light transmission of one lens design vs. another even with the same focal length, which is why T/stops were used on some lens quite a long time ago!).

[fritz1]

I kind of expected that but wasn't sure thanks for the clarification I suppose it was silly on my part after all f4 should be f4.

[Wilt]

Not so silly, which necessitated the T number for a while, until both lens design and lens manufacturing techniques eliminate the need! I have read that early zooms had light transmission that was 50% of fixed focal length lenses of the same FL, so an f/2 lens would also be labelled T/2.8.

[fritz1]

Now I feel even better not so silly, I remember something about T#s