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.

No problem. I realize that what I said is subject to more than one reasonable interpretation.

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.

I was only trying to offer an analogy (as outlined in the paragraph that begins with "What I am trying to say") that might be helpful to at least some readers. But as suggested by the paragraphs that follow, no simple analogy works in all respects. And in view of that, I am not sure how much it helps to bring focal length into the picture in the first place.

If we consider things from a practical photographic point of view, I'd say that there are two sets of parameters that are of primary interest in this context. The first is how high a reproduction ratio you can get and at what working distance (distance from the subject to the tip of the lens). The reproduction ratio is part of the lens specs and the working distance can be calculated from the specs (minium focus distance minus the flange distance and the length of the lens), although in some cases you may need to look at pictures of the lens in order to see how much its length increases when zooming in (for zoom lenses without internal zooming) and/or focusing close (for lenses without internal focus).

But although it is nice (at least I think so) to get as high a reproduction ratio along with as long a working distance as possible, there is also another couple of parameters to consider, namely the extent to which the angle of view and the location of the entrance pupil stay constant when you change the focus distance. For maximum ease of composition/framing and for the best results in focus stacking you wouldn't want either of them to change with the focus distance. But regrettably, it is not so easy to tell from the specs exactly what to expect from a lens with internal focus in this regard. Furthermore, it is quite conceivable that a lens that is preferable from the first point of view (reproduction ratio and working distance) may be less ideal from the second point of view (the extent to which AoV and the location of the entrance pupil changes with the focus distance) and vice versa.