Where is the 'grain' coming from?

[S Taylor]

Hey folks,

I've been wondering about something I'm seeing when I look through my 20D viewfinder when using L series lenses. There appears to be a blackish grain present, especially noticeable when looking at light backgrounds, such as a sky.

First noticed it with my 17-40 but never saw the grain present in the images, so I wasn't too concerned. It was present on my 70-200 f/2.8 as well, so I figured it was either something on my viewfinder glass, mirror or a characteristic of the lens. I recently just purchased a 400mm f/5.6 L, and again, looking at a sky I see the blackish grain.

One interesting note - when I put my EF 1.8 50mm on, I don't see the grain. ???

Anyone know what I am seeing when I see this grain? I'm VERY glad its not present in my images, but my curiosity has gotten the best of me so I came here.

Any help is greatly appreciated.
Thanks
William

[internationalmanofmystery]

Perhaps the viewfinder has a ground glass surface?

[Tom W]

I believe that it's the grain of the focus screen's matte finish. If so, it will be more visible on smaller-aperture lenses like f/4 or f/5.6 than on faster lenses like f/2 or f/1.4.

[S Taylor]

ah, ok. Thanks for the explanation.