Katz Eye works well, its exactly what I expected. Happy camper.

According to Tom  <Tom@dontsendmespam.com>:

    [ ... ]

    I believe that the screen in the viewfinder simply lets more
light straight through, instead of interposing a ground glass screen or
a microprism screen.  As a result, your eye accommodates out-of-focus
objects by refocusing through the screen.  This can be reduced somewhat
by having the grid turned on, which gives your eye yet another thing to
anchor -- but if not careful, you will discover that the grid will go
out-of-focus, as your eye brings things which *should* be out of focus
into focus.

    A ground glass or a microprism will diffuse the light, so less
reaches your eye, thus producing a darker image.

    Part of the darkening can also be attributed to having to expand
the smaller sensor area to what looks like the same size image as
in the full-frame cameras.  Any magnification reduces light per unit
area.

    That may help -- but I suspect that you would do better with a
D70, or a D200 instead of the D50 under discussion.  I *know* that the
D200's viewfinder is much brighter than that on the D70, and the one
time I looked through a D50's viewfinder in the store, it was quite dark --
though that may be caused by there not being a battery in that camera at
that time -- I didn't think to check at that time.

    Enjoy,
        DoN.

The focusing screens are very clear, in order to be bright.  You look through
it, like a window, and focus with your eye.

With a coarser focusing screen, you're looking at the image projected onto
the screen, not the scene itself; your eye can only focus on that projected
image, and since at that point you're looking at the image being formed by
the lens, instead of the scene itself, you see a better indication of depth
of field.  However, this makes the viewfinder image darker.  A coarser
screen makes you see more of the projected image vs. the scene itself, but
as it gets coarser, it also gets darker.

Since the camera makers are pushing the ultra-slow zoom lenses these days
as an "all in one" solution, this is important.  You wouldn't want to look
through a D50 viewfinder and an 18-200 lens with a coarse screen designed
for manual focusing.

I replaced the screen in my D2x with a Katz Eye screen, without the optional
extra-brightness treatment (in order to get the best possible indication of
focus).  The D2x already has a much brighter viewfinder, and yet, I still
would not recommend using that screen if you regularly use lenses with max
apertures below f/4.  At f/5.6 it would be pretty unpleasant.  In a D50 it
would probably be unusable at f/5.6.  The extra-bright version would be a
requirement in that situation -- but it will still be darker than the
factory screen.