On Thu, May 16, 2002 at 11:51:26PM -0500, John A. Lind wrote:
> At 18:46 5/16/02, Moose wrote:
> >In addition to the f-stop, it hs to do with the angle of the wedges and
> >the angle of the light coming in from the lens. I have no idea what's
> >being done today, but back in the Nikon F & F2 days, Nikon made split
> >image focusing screens for tele (and WA?) lenses in addition to the
> >regular ones.
> >
> >Moose
>
> The "split image" rangefinder on the focusing screen works by slanting the
> surfaces of the two halves in opposite directions creating prisms out of
> them. Since the surface of them is smooth, the image in the split circle
> does not appear to be in or out of focus as it does on the matte
> area. IOW, the image in the circle is aerial, and is not formed on the
> screen.
This is what I think too.
> BTW, the "microprism" ring around the split image is formed of tiny
> versions of same type of prisms used in the split image. As the aperture
> is narrowed, the split image will go black first. As the aperture is
> narrowed further, the microprism ring will eventually "go black"
> also. Usually this isn't encountered except in very low light.
I don`t think that the amount of light has influence on darkening of
fosusing aids. When split image is black, then you see the
side of your lens mounting, or the apperture blades.
The angel of the prisms determine the angel between the two slanting rays.
When both rays pass through the appertue (smallest diameter)
then split image works = both parts are visible.
Stopping down (or a slower lens), the opening angel is getting less, and the
diverging
rays might not pass though the apperture anymore, and the microprism /split
image
is black (looking at the "wall"). This is why one part of the split image might
get
dark, when your eye is not centered.
This is my experience 2/13-screen:
F 1.4 -2.8 both split images + microprisms are always visible.
When your eye is too much off axis, first the viewfinder is unsharp,
before darkening occurs.
F 4-5.6 split-image/ microprisem might be dark,
depending on how centered viewfinder insight is
centered = both parts of split image visible (=works).
decentered = one is black.
F8 - ... at least one part will be black.
centered = both are black
decentered = one might be visible.
It is interesting to find out that even stopped down to F32 ist is possible to
decenter your eye
that way, that the slanted light of one part of the split image is parallel to
the optical axe, so
that it is not black. But with centered insight both parts are black!
Playing with the apperture preview button and varying the viewfinder insight,
is interesting.
Frieder Faig
Note 1: sometimes the effective apperture loss due to close focusing makes a
big difference
for the usability of the microprims and split image (e.g. 3.5/50 Macro, 4/80
macro, ....)
Note 2: O.k. I´m rather a technican, than an artist ..., but I like my F2-Zuiks
8-),
... still some focal lenghts missing 8-(.
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