In article <4dut7s$l36@mars.hyperk.com>, glh@srv.net says...

>I recently did a rash of lens testing using an old Nikon FM
>body and an indoor (read: not very well lighted) testing
>setup. My eyesight ain't what it used to be, so I used the
>split image 'rangefinder' in the FM viewfinder for
>focusing. All the test shots appeared to be well focused
>SOMEWHERE, but with some lenses the focus appeared to be
>better at the corners of the image than in the center.
>
>Q: is this just a fluke, or does it mean the split image
>focusing aid somehow uses the edges of the lens rather than
>the center? Is the extent of this effect related to how
>much curvature of field the lens has? If so, is there
>anything to be done about it? (It's really tough for me to
>get a lens focused critically using a plain groundglass
>screen.)

I think the answers are: no, it is not a fluke; yes, the split
image uses the lens edge image to indicate focus; yes, the
extent of the effect is therefore related to the amount of field
curvature the lens has; no, there is nothing that can be done
about it (except resetting the screen position, which would make
that body useable with only one lens). I sympathize with your
predicament, since I find plain ground-glass focusing so much
easier and faster than using what I consider those dreadful
"focusing aids" that occupy prime territory in the middle of
the screen and get in the way of good viewing and focusing for me.
This may not be relevant to you, but have you tried a plain screen?
The center part of the screen is usually easier to use for GG
focusing, when it is available, especially in the FM type VF's
in which the GG image is quite soft near the edges and corners
with normal and shorter focal-length lenses. If you have not
tried focusing with the GG center of the 8008, N90, F3, and F4
bodies, give it a try (this is as good as GG focusing gets (in the
center) - VERY easy, for some of us). The F4 also allows diopter adjustment of the eyepiece lens - you can see if your sight is sufficiently correctable with the addition of an eyepiece diopter
lens on other bodies to make GG focusing easy and accurate. It may
be worth working with a good (and patient) eye doctor on this,to
find out if you can come up with a good prescription (for one eye,
at least) for glasses that would allow you to see the GG sharply.
Hope This Helps