At 01:10 5/21/01, Donald Shedrick wrote (in part):
After thinking some more about this, I think the matte
screen is more sensitive to the different focus points
of different eyes. Have you noticed how someone else
using binoculars or a telescope will foucs them, and
when you look through them, they are slightly out of
focus for you? However, the split image screen is
using a technique of lining up of two images, which
may not be as different for different eyes. I guess
we all have to get diopter correction lenses in our
eyecups so we can obtain accurate focus, and not focus
the camera to compensate for our eye.
My conclusion is use a split image screen for most
accurate focus unless you have corrective lenses in
your eyecup. Thoughts?
I disagree. Unlike an SLR (and TLR) Microscopes, telescopes and binoculars
do not focus to a screen, but directly to the eye. The problem with
needing correction and not having it in the viewfinder is never being able
to see a razor sharp image in the microprism ring, and especially on the
surrounding matte. This leaves a very slight focusing range that all looks
the same.
Being a life-long eyeglass wearer, adding the dioptric correction in the
SLR's didn't change the critical focus point, it helped find it easier
(much easier). It allows seeing a singular point of focus with a razor
sharp image on the screen, and eliminates the ambiguous (small) focusing
range that looked the same no matter where you were in it. That's what my
earlier remarks referred to . . . being able to see that razor sharp image
when it snaps into view on the focus screen.
Before using dioptric correction, I focused using split image almost
exclusively. If it went dark due to a slower lens in lower light levels,
finding the correct focusing point using the microprism ring was difficult
and often a best guess. Using the matte was impossible. Afterward I could
use the microprism ring, and even the screen matte with some lenses, with
great accuracy.
Those who have astigmatism have the most trouble, in spite of dioptric
correction, because the vertical and horizontal axes in the eye have
different focal lengths. Even with dioptric correction, the eye will still
hunt for focusing on the screen. Adding prisms to correct for astigmatism
in the viewfinder isn't all that feasible either. The astigmatism
correction prisms must be oriented correctly to the eye. If you get it
right for holding the camera horizontally, it's suddenly 90 degrees off as
soon as you turn it vertical (making the problem twice as bad). I'm lucky
my astigmatism is so slight that it falls about halfway between none and
the first increment of correction eyeglass lens makers can use for it. I
usually tell the optician not to bother with it. Others with more severe
astigmatism are not so lucky and they have my sympathy.
-- John
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