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.
--- Donald Shedrick <shedridc@xxxxxxxxx> wrote:
> I recently bought the 35-70 f3.5-4.5 zoom. I have
> noticed that best infinity focus on distant objects
> with the split image on the 1-13 focusing screen is
> at slightly less than all the way to the infinity
>mark on the lens (about at the left loop on the
>infinity symbol). I am using the varimagni finder in
>the magnified position, and this is equally true for
>all focal lengths. However, when I use the fine
matte
> screen (1-8) it appears that the best focus is all
> the way to infinity, again using the varimagni
>finder. I have redone this several times making sure
>the focusing screens are well seated. Which is the
>more accurate indication of correct infinity focus?
>Thanks for the help.
>
>
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