>>Paul was puzzled (correct me if I'm wrong, Paul), by his observation that
>>everything in his viewfinder is in focus, even though the lens is wide open
>>(DOF button not depressed). Yet, if he were to take the photo at the
>>wide-open aperture, parts of the photo would be out of focus. If Paul's
>>observation is correct, something strange must be happening.
>
>Correct again. I have done some more focused observations based on the
>comments posted here and indeed I have discovered that the field is
>not in focus as much as I had thought it was (strange how my
>perception based on informal use was different than when I really
>focused (pardon the pun) on the issue). But I still maintain that the
>DOF you see in the viewfinder during normal focusing is greater than
>what you see in a final print if you shoot wide open. I'm puzzled as
>to why this is, but I appreciate everyone's comments. There is no
>reason this should be camera or lens specific is there?
As no one has replied to muy earlier message I will repeat it adding some
text:
There is something interesting about that:
I have a 50mm f/1.4 Pentax Super Takumar screw lens that I use in my 500n
by means of a M42 adapter. I think that DOF is greater on the focusing
screen that on film.
That coincides with the fact that open wide (at 1.4) the camera overexposes
1.5 stops.
That means that the light sensors over the focusing screen see a f/2.5 lens
instead of a f/1.4 lens.
In fact, if I stop from f/1.4 to f/2 I see no change on the viewfinder
brightness.
The brighntness difference, OTOH, is quite evident from f/2.8 to f/4.
So, from f/1.4 to f/2 the aperture as seen from the focusing screen does
not change.
Perhaps that's some kind of mirror vigneting, because stopping down at
f/2.8 or over the mettering is ok, but I think 1.5 stops is too much
vigneting.
So I think it's possible that DOF is greater on the focusing screen than on
film.
In fact, a lot of SLR's mirrors vignete at big apertures. So their efective
aperture on the focusing screen is smaller that on film.
You can test that with your camera: Defocus your lens and point to a
distant point source of light. You will see a circle on the viewfinder.
Move the circle arround the viewfinder and you will see that it get's cut
when you are near of one of the borders of the screen. That's mirror
vigneting.
I can see that on my EOS 500n with my 50mm f/1.8. So I'm pretty sure that
the mirror of the EOS 500n vignetes.
In order to test if there is mirror vigneting, or another light loss
associated to the mirror, you can also do that test:
Put a 50mm f/1.4 on the body:
Using DOF preview stop from f/1.4 to f/1.8. Is there is no change in the
viewfinder brightness, there is mirror vigneting at f/1.4. If the
brigntness change from f/1.4 to f/1.8, then there is no mirror vigneting.
As it depends on the body, the effect can be quite different from body to
body.
I will not be surprised to know that the vigneting effect I see on my EOS
500n is not present on a EOS 3.
As a conclusion I'm pretty sure that in my EOS 500n the efective aperture
as seen from the focusing screen is less that the one seen from the film.
But I'm not sure of the importance of it's effect on DOF. But In my camera,
using a EF 50mm f/1.8 I, the DOF on the focusing screen at f/1.8 must be
greater, perhaps slightly only, than on film.
Hope it helps.
Best regards
Vicente
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