Unless you are using the F, F2, F3, F4, or F5
in the Nikon line (or some other old bodies, like
the Nikkormat...), the grid screen will be curved,
if you look at it carefully - and the outer lines
will not look parallel with the frame edges. If you
put on a lens with very low linear distortion
(Micro-Nikkors, most teles), you will see that
straight lines will agree with the grid, but not
the frame edges (they are "pre-distorted", to give
you a straight-looking rectangular edge to the image).
I credit both the desire for "bright" screens and
Olympus for this - the OM series was small, requiring
compromises in the VF optics (and other makers went
"small" and "bright", bringing the same problems).
If you look at the bottom of most prisms with the
screens removed, they are curved; the screen itself
often has the top surface as the real focus plane,
with the bottom surface also curved (or with a
similar-effect Fresnel lens); these introduce
both linear distortion and off-center focus errors
to these viewfinders, unfortunately...

On Thu, 05 Dec 2002 10:59:11 -0800, Gordon Moat wrote:
>
>I am curious about the viewfinder statement you made. With some removable
>screen Nikons, I notice that the viewing screen is curved. Is that the usual
>source of the distortion?
>
>One example that is a bit strange is the grid screen. There are lines etched
>into the screen. A casual look at this one shows it to have less curvature that
>the other screens I have, though they do look straight through the viewfinder.
>Thoughts?
>Ciao!
>Gordon Moat
>Alliance Graphique Studio
>

>Neuman - Ruether wrote:
>
>> Most zooms do have linear distortion, though many have
>> one point in the zoom range where distortion is minimal.
>> Most Nikkors have moderate distortion, and a few have
>> almost none (28-105mm f3.5-4.5, 100-300mm f5.6, and
>> 35-70mm f3.5, later version). Most WA and "normal"
>> non-zooms, and a very few teles, also have noticeable
>> linear distortion (exceptions: Nikkor 15mm f5.6, 21mm
>> f4, and late 50mm f1.8). BTW, most viewfinders have
>> linear distortion of the "pincushion" type (the others
>> are "barrel" and "wavy-line/moustache"), so it is
>> hard to use the camera's viewfinder for judging lens
>> distortion...
>> David Ruether