Re: DOF preview in OVFs of DSLRs is crippled

Paul Furman wrote:
David J Taylor wrote:
Ray Fischer wrote:
[]
Explain to us how the image seen through the viewfinder of a dSLR can
be different from the image recorded by the sensor given that the
image is produced by exactly the same lens under exactly the same
conditions.

Perhaps your source doesn't realize that there is a DOF button that
lets one stop down the lens. Perhaps he's incompetant. Perhaps it's
just a mistake.

But I don't blindly believe things that make no sense without even a
hint of an explanation. You shouldn't either.

The explanation was, that the sensor accepts a ray bundle with a larger angle than the viewfinder (if I understood the earlier commentary correctly). In other words, once the lens opening gets wider than f/2.8 (or whatever), the viewfinder will show no smaller depth-of-field. On that particular camera. If you changed the viewfinder things could be different.

That might be part of it, the part I understand is that the screen is only partially translucent so some of the image comes through. It's awkward to explain... if instead of a ground 'glass' focusing screen, if you had an opaque white projection surface and looked at that from the other side, that should look correct but you see that image combined with a clear view from your eye and your eye has a much smaller sensor than a DSLR (well the part used for viewing in this case anyways). Maybe I'm missing some terminology, the opening in your eye is much smaller than the 42mm opening in a DSLR so it's not capable of using the wider angle rays.

Entrance/exit pupil is the terminology I was searching for:
http://en.wikipedia.org/wiki/Exit_pupil
"To use an optical instrument, the entrance pupil of the viewer's eye must be aligned with and be of similar size to the instrument's exit pupil. This properly couples the optical system to the eye and avoids vignetting. (The entrance pupil of the eye is the image of the anatomical pupil as seen through the cornea.) The location of the exit pupil thus determines the eye relief of an eyepiece. Good eyepiece designs produce an exit pupil of diameter approximating the eye's apparent pupil diameter, and located about 20 mm away from the last surface of the eyepiece for the viewer's comfort. If the disc is much larger than the eye's pupil, much of the light will be lost instead of entering the eye; if smaller, the view will be vignetted."

It's like holding a video camera up to a DSLR lens, you won't get the shallow DOF so people invented a device that projects the lens image onto ground glass and the video camera focuses on the ground glass rather than focusing on infinity. Sorry, I know I haven't explained very clearly.

I can't comment if that's right in practice (as I no longer have such large aperture lenses).




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Paul Furman
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