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| p.1 #4 · p.1 #4 · 5D2 FW 2.0.4 - Iris jitter in M movie mode |
they mention changing iris in a macro when focusing.... isn't that mandatory once you focus closer than a certain distance
I think whoever made that statement misunderstood the situation with internal focus macro lenses, and the relationship between f-number (relative aperture) and effective aperture.
Recall that effective f-number is given by
N[eff] = N (1+M),
where N = f-number, and M = magnification ratio. So, at 1:1 and f-number set to f/2.8, the effective f-number is actually f/5.6, two stops less light. But the size of the entrance pupil is no different; the effective f-number describes the loss of light due to the fact that the subject in focus is not infinitely far away (and thus not projecting parallel light rays into the lens).
However, when you change the focus on an internal focus macro lens, you will notice that the entrance pupil diameter also appears to change. (To see this, mount the lens on the body, set the aperture to, say, f/8, look through the front of the lens, and while holding the DOF preview button, turn the focusing ring from infinity to MFD.) The reason for this change is simple--the true focal length of the lens does not remain constant in an internal focus design. In non-macro lenses, the focusing helical doesn't really move far enough to make this change in focal length apparent, so you won't really notice a significant change in focal length for an internal focus zoom like the 70-200/2.8L IS (though I can't say that about the Nikon 70-200/2.8 VR II, which has a serious problem @ 200mm @ MFD). But in an IF macro, at 1:1 your focal length really is more like 65mm.
Now, none of this is really a suitable explanation for why FW 2.0.4 exhibits this "iris jitter" problem. The reason why is simple:
1. I could not reproduce the issue with the 100/2.8L macro IS, which of course is an IF macro lens.
2. I was able to cause the issue on the 300/4L IS, which is not a macro lens.
3. The change in effective f-number only relates to the actual light-gathering ability, not the size of the entrance pupil.
4. The change in entrance pupil diameter as a function of focus distance for an IF lens is an intrinsic optical property of the lens' construction--you can see it even if the lens is not mounted to the body. It has no relationship to the rapid fluctuation of the iris caused by the video mode.