Strange phenomenon in A900 viewfinder

[01af]

Yesterday I noticed something strange---the apparent depth-of-field in the viewfinder is not even across the field. Instead it's smaller at the center and considerably wider near the edges. So when aiming at a subject detail with the lens somewhat defocused then in the viewfinder the detail will appear more blurred when it's at the viewfinder's center and almost sharp when it's near the edge. When aiming at a defocused flat subject filling the frame then it will appear more blurred at the center than at the edges, even though it's supposed to appear equally blurred across the whole field. It doesn't matter whether the lens is focused at a longer or a shorter distance, and it occurs with all focal lengths (albeit it's more obvious to the eye at longer focal lengths).

Do others see this, too---or is there something wrong with my camera? It took a moderate blow last week ... not hard enough to make me worry, until I noticed that phenomenon yesterday. Actual images are just fine, it's in the viewfinder only. I am using an L-type screen which however is just the same as the regular G-type screen, plus grid lines.

-- Olaf

[David Kilpatrick]

That's what a condensor lens viewfinder looks like - but it does depend on the lens. The more a lens vignettes naturally to the edges at full aperture, the more the apparent aperture may appear to be reduced and the narrower the bundle of rays available at the focal plane.

I imagine it will look worse with fast wide-angles, least noticeable with teles. You have to be very 'aware' to see it - or you must move your eye around.

David

[Greg Beetham]

I'm just wondering if field curvature (as well) has anything to do with it, there might be circumstances where the subject distance the viewing f-stop and the actual thickness of the focus screen itself plays a part, ie. it might be sharper on the front side of the screen, in places, than on the back....(field thickness), Olaf, what if you were able to have a play with the preview button at progressively deeper f-stops at the same focus position and watch what happens on the focus screen, just for interest sake. You would need a tripod and a very brightly lit subject tho...to see accurately what was on the screen at, say f8 and f11....
Greg

ps. Olaf don't mind me, I just enjoy a good mystery....

[01af]

That's what a condensor lens viewfinder looks like ...

I believe it has more to do with the focusing screen, not so much with the condensor. But that's just a speculation ...

I imagine it will look worse with fast wide-angles, least noticeable with teles.

While that seems logical indeed when thinking about it, it just isn't so. It's most obvious with slower telephoto lenses, less obvious with fast telephoto, least obvious (but still noticable if you look for it) with wide-angles.

I'm just wondering if field curvature (as well) has anything to do with it ...

No, it hasn't. That was my first thought as well, but then the focus being before or behind the subject would make a significant difference.

Of course, if the lens used had field curvature then it would add to the effect.

By the way, in the meantime I have also tried the old Dynax 7D (equipped with an ML-type screen), and there the same effect can be observed---however to a much lesser degree. Here it takes very close inspection to notice. So now I think my A900 is not defective but working as designed. I guess all SLR cameras with modern focusing screens are like this, more or less ... which is not a good thing. Oh well, technological progress ... I wish we still had simple ground-glass screens, at least as an option; they'd be darker than our high-tech spherical acute-matte screens but all those weird side-effects were missing. What is all the brightness good for when you can't properly use it?

-- Olaf

[Greg Beetham]

I'ts still interesting though, if I've got the concept right, the prism gathers the light from over a very large rather dullish area (too our eye in reality) and condenses it down too a bright-ish 6mm or so exit pupil (so the viewfinder can fit into our eye all in one go), and squirts it out of the VF as reproduction of what's on the focus screen....What about if the light coming from the peripheral area of the focus screen actually gain a little focal length and extra focus from having to travel a further distance...clutching at straws a little here...
Greg