At 18:46 5/31/03, Dan wrote:
> Zuiko 35-105/3.5~4.5: [snip]
> Only wish it
> were a stop faster as the viewfinder focusing aids black out in low
> light
I thought that the reason the focus aids go black were because you end up
looking at the insides of the lens, not because of external light
conditions -- am I wrong here?
-- dan
No . . . would help if I had a diagram. The easiest thing to describe is
the RF split spot . . . which goes "black" first, typically on one half,
sometimes on both in very low lighting. Unlike the matte on the focus
screen, the split spot is actually beveled on the bottom side of the
screen. The top half and bottom half are beveled in opposite
directions. The path of light for the top half is different from the path
of light for the bottom half allowing "triangulation" to make the two
halves line up. Since the light path is different for each half, it only
picks up a portion of the light passing through the lens compared to other
parts of the screen (the matte). For this reason, it's the first thing to
go "black." The next thing to go dark is the microprism ring . . . which
works somewhat in the same fashion with very small prisms. If you look at
the bottom side of a 1-13 or 2-13 screen carefully, you can see the prisms,
and the beveling for the RF spot.
BTW, the smoothness of the beveled surface is why the image always looks
"in focus" in the spot, even when the two halves are not lined up.
-- John
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