moire in the viewfinder

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.

--
Ray
(remove the Xs to reply)

Ray writes:

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.

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

--
Måns Rullgård

Måns Rullgård wrote:

Ray writes:

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.

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.

--
Ray
(remove the Xs to reply)

Ray wrote:
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.
--
If reply address = connectfee, add an r because it is free not fee.

Ray writes:

Måns Rullgård wrote:

Ray writes:

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.

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.

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

--
Måns Rullgård