I believe the reason that these lenses have movements
to focus "beyond" infinity is to allow for thermal expansion
of the optics and/or lens barrel, which will shift the focusing
point slightly as temperatures vary.
Then again, I may be wrong...
Mark Hammons
> After posting this question, and not getting a
> definitive answer, I continued to do some more
> investigations. I also noticed the focus difference
> in addition to my 35-70 f3.5-4.5, on my 75-150 zoom
> (not at 150, but at lesser focal lengths), and my 135
> f2.8 and 200 f4, with and without the 2x teleconverter
> - all having best focus with the split image at
> slightly less than infinity (amount of defocus from
> infinity varies with the lens, and with the focal
> length with the zooms) and at infinity with the fine
> matte screen.
> I decided to conduct some critical tests on film. I
> took shots of a distant group of buildings on which I
> could accurately focus with the varimagni finder with
> both the split image 1-13 screen and the fine matte
> 1-8 screen. Film was K64, and mirror lockup and
> selftimer were used on my OM1. Exposure times were
> 1/250 or 1/500 and at full open apeture to maximize
> the sensitivity to accurate focus.
> The results show in all cases the best focus was as
> determined by the split image screen at slightly less
> than infinity. The difference was clearly seen with a
> high quality 12X loupe on the slide.
> As a sidelight, I was amazed by the sharpness of all
> the lenses at best focus, especially the zooms. I was
> surprised the 75-150 zoom at f4 was the equal of the
> 135 f2.8 in sharpness except near the edges. It had
> slightly less contrast, however. Also, the sharpness
> of the 135 and 200 with the 2x teleconverter was
> excellent to the edges, with the 200 being only
> slightly better. I imagine the 135 stopped to f4
> (actually f8) might fully equal the 200 at f4, but I
> did not test that.
> All this is, I realize, subjective and your
> interpretation could be different.
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