moire in the viewfinder

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Ray - 25 Apr 2006 22:53 GMT

Yesterday I was looking at a bird's feathers through the viewfinder of
my EOS 350D, and noticed a moire pattern at certain focal lengths. I
tried looking at a computer monitor and got a stronger pattern. So
what's in the optical path to the viewfinder that could do this?

I wonder if the focusing screen isn't just ground glass, but has some
kind of pattern etched on it. For comparison, I looked at a monitor
through an old AE-1; there was a moire in the microprism circle, but
not in the rest of the viewfinder.

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Måns Rullgård - 25 Apr 2006 23:59 GMT

> Yesterday I was looking at a bird's feathers through the viewfinder of
> my EOS 350D, and noticed a moire pattern at certain focal lengths. I
[quoted text clipped - 5 lines]
> through an old AE-1; there was a moire in the microprism circle, but
> not in the rest of the viewfinder.

It is the focusing screen. Looking through the focusing screen from a
350D at an LCD monitor, I can clearly see moire patters.

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Ray - 26 Apr 2006 22:12 GMT

>> Yesterday I was looking at a bird's feathers through the
>> viewfinder of my EOS 350D, and noticed a moire pattern at certain
[quoted text clipped - 9 lines]
> It is the focusing screen. Looking through the focusing screen
> from a 350D at an LCD monitor, I can clearly see moire patters.

Presumably there's some advantage to a pattern versus random grinding.
Maybe better light transmission? But it was a bit disconcerting the
first time I saw it.

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Mike Coon - 26 Apr 2006 23:30 GMT

> Presumably there's some advantage to a pattern versus random grinding.
> Maybe better light transmission? But it was a bit disconcerting the
> first time I saw it.

It would make sense to use a Fresnel lens. You'll see a large one in any
OHP, to concentrate the light passing through the slide into the lens/mirror
unit.

Mike.

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Måns Rullgård - 26 Apr 2006 23:49 GMT

>>> Yesterday I was looking at a bird's feathers through the
>>> viewfinder of my EOS 350D, and noticed a moire pattern at certain
[quoted text clipped - 13 lines]
> Maybe better light transmission? But it was a bit disconcerting the
> first time I saw it.

It could also simply be cheaper to produce. Whatever the reason, I'm
not terribly bothered by it.

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