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
Alan wrote:
>
> I am still horribly confused by this issue (Gary's recent post has resulted
> in this) - if your eye site is bad won't the split screen still be limited to
> working when the subject is in focus? Even more so - no matter how poor your
> vision is won't the proper focus point result in the sharpest image ?
>
> There seem to be an implied comment that bad eye site causes people to focus
> at another (incorrect) point... Why is this ?
>
> Now I can understand (easily) that a misaligned mirror or focusing screen
> could
> cause this - but bad eyesite...
>
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