40-150 PRO focus breathing?

whumber wrote:

Anders W wrote:

I think the problem is basically that we are talking past each other rather than substantively disagreeing. Let me try to clarify what I am and am not trying to say.

You're correct, I was misunderstanding what you were trying to say.

What I am trying to say is that the 40-150/2.8 set to 150 mm and shot at its miniumum focus distance has a reproduction ratio, and thus a field of view (expressed in linear rather than angular terms, e.g. in millimeter à la Christof/CrisPhoto), that is identical to that of a thin lens with a focal length of 100 mm shot at the same focus distance. It also has a reproduction ratio/linear FoV at those settings that is approximately (but not exactly) the same as that of an old-fashioned, reasonably symmetric 100 mm lens that focuses by moving the entire array of lens elements in and out, without any use of floating elements.

What I am not trying to say is that either a thin lens with an FL of 100 mm or the 40-150/2.8 set to 150 mm has an effective focal length of 100 mm with both shot at a focus distance of 0.7 meters. If we define the effective focal length of lens X shot at focus distance Y as the focal length of a lens that gives the same angle of view as X shot at Y when shot at infinity, then the effective focal length of the thin 100 mm lens at 0.7 meters is F(1 + R) = 100(1 + 0.21) = 121 mm. For a thick and potentially asymmetric lens like the 40-150/2.8 set to 150 mm and 0.7 meters, the effective focal length instead becomes F(1 + R/P) where P is the pupil magnification (the ratio of the diameter of the exit pupil to the entrance pupil). But since I don't know what F and P are in this case, I can't calculate the result.

Ok, I'm in complete agreement with everything here. I think the only thing we (maybe?) disagree on is that the relationship initially posted is not very useful at close focus distances, at least not in a very general sense.

I am well aware that it is based on a thin lens model but that doesn't make it useless for the purpose at hand. Moreover, the formula is useful precisely at close focus distances.

If it was just a matter of it being off a bit, then that would be one thing but you can easily end up in situations where it's not even giving you the right trend depending on the lens design. Again though, I think maybe I'm misunderstanding your intention here.

have to jump in a say, a rare thing for my posts, glad you guys ended up agreeing.

Also, the Canon 70-200mk2 is a rare zoom lens to have so little focus breathing at 200mm and yet be IF and have the same size and weight as its competition.