I've had some of those questions myself, see below for some info.
---
Scott Gomez
-----Original Message-----
From: Daniel J. Mitchell [mailto:DanielMitchell@xxxxxxxxxxxxx]
Subject: RE: [OM] Poll on Focusing Screens
A general question that's been bugging me -- how do focusing screens work?
> How does it 'know' to move the central split-image sideways if it's out of
>focus (and how does it know to move one side one way and the other the
>other?) What makes that bit go dark if the aperture's too small?
Haven't seen a good write up on this :-(
> How do microprisms work? (I know what they do, but how do they do it? Are
>they just like a whole lot of small split-image bits?)
Dunno. Shipman's book says it has to do with "microprism angles" but doesn't
elaborate.
> How is it that the
>1-1 screen goes dark at f5.6, but the 1-2 screen goes dark at f8, even
>though they're otherwise identical? (at least, that's what it looks like
>from the eSIF page on this). Does this imply they could make a screen that
>goes dark at f16?
Shipman: "Smaller microprism angles" in the 1-2 compared to the 1-1.
> The eSIF page also mentions that the completely clear screens don't
provide
>depth-of-field info -- again, what's going on there? How does a clear
screen
>work at all (and what is it about the texture on non-clear ones that _does_
>provide DOF?)
Shipman: "Some screens are transparent and do not intercept the image in the
plane of the screen. The image you sees is called an 'aerial' image because
it exists 'in the air.' It may be in focus at the screen, in front of it. or
behind it. We see the image in good focus because our eyes adapt..."
<snip>
I know, also that the difference between toe 1-4 and the 1-8 is how fine the
"ground" surface of the screen is. The 1-8 is finer.
thanks,
-- dan
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