I can't get my head around how split screens work, so I'll gloss over
the details, but the gist of the situation is this:
When you're looking through the viewfinder, you're seeing the light from
the scene in front of you through the camera lens, the prism and your
eye's lens. When you're taking the shot, the light is going through the
camera lens only, and then striking the film.
For an object at a given distance to be in focus on the film, the lens
must be in one unique position (ignoring depth of field). But for that
object to be in focus on your retina, when you look through the
viewfinder, the situation is different - if your eye normally focusses
short of your retina (ie you're shortsighted), then what you see through
the viewfinder will focus perfectly on your retina if the camera lens
is focussing slightly longer than it should do, and vice versa. So the
image on your retina is in focus, but the image on film will not be.
I hope that's reasonably clear - it's surprising how clear the situation
is in my head and yet how difficult it is to make sense when I write it
down! Could do with drawing a diagram really.
Roger
I am probably over my head with this, but what you say would be true
if the image from camera lens actually came to focus on your retina
like binoculars or a microscope. However the focussing screen is
inserted into the optical path. The image from the lens comes to
focus on the focussing screen which is like a tiny rear projection
screen. You are not really looking through the lens with any SLR, you
are looking at the image projected by the lens onto the focussing
screen. The viewfinder is really more like a microscope focussing on
the screen instead of a specimen. The diopter correction comes in,
the way I understand it, to allow your eye to focus on the image on
the screen. Optically speaking that image appears to be only 2 or 3
feet away, so if you have close focus issues you need diopter
correction to see the screen clearly and to be able to use the
focussing aids on it. What happens is that the aids get more
difficult to use. The crank back and forth on the focus ring gets
larger and larger and it is difficult to guess where the middle of
the range of "sharp" focus is. That is why lots of people with
uncorrectable, impaired eyesight manage with some difficulty to still
take some good pictures(not counting AF). If it worked the way you
describe only someone with perfect eyes would be able to get an in
focus shot.
Winsor
--
Winsor Crosby
Long Beach, California
mailto:wincros@xxxxxxxxxxx
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