> I've had some of those questions myself, see below for some info.
> Scott Gomez
>
> 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 :-(
>
I'll have a try (this isn't a real good write up, I know ...). If an image
is in focus then all light coming from a single point on the source of that
image must come together on one single point on the focusing screen even
when they travel different paths through the lens (f.i. one ray through the
left part of a lens and an other through the right part). A split image is a
small prism where the top part looks at those rays coming from the left and
the bottom part looks only at parts coming from the right (or the other way
round). If an image is off focus then these two images come together just
behind or in front of the focus screen. In that case the two half images
show a slightly different view and thus do not allign.
> > 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?)
>
They are just a lot of little split screens looking at all directions.
> > 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.
>
When the aperture closes those left and right views through the lens get
obstructed. A smaller angle of these prisms make it possible for them to
work with smaller working apertures.
Hope this helps,
Daan
Pictures of Insects at:
www.kalmeijer.net
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