canon 85mm 1.2 depth of focus

[siba]

For work I used to exclusively use my medium format contax + p45. Then a while ago for low light location jobs I took along my canon 5D and started shooting with the 85mm 1.2, and shot wide open. I was surprised how good this looked.

One thing I've noticed and don't understand, is that images have less depth of field than I see in the viewfinder. I'll reiterate that the other way round: when I look through the viewfinder, there is more depth of field than in the subsequently image which I see on the display and in the actual image in the computer.

How is this possible if I'm looking through the lens at it's most open?

Cheers for any thoughts

Stefan

[jjj]

A colleague of mine commented on this very issue only the other day.
It seems to be because of the design of modern focusing screens that with apertures wider than f2.8 the screen doesn't show much from difference from using f2.8. They are also a bit pants for manual focusing too. Non standard screens can be better.

[pw-pix]

The viewfinder lowers the detail level enough that the sharpest parts and the slightly less sharp parts look the same?
So when you see the images on the computer or printed and can see the sharpest parts clearly, the slightly less sharp parts are obviously different?

PS. In the subject line you used the incorrect term 'depth of focus', but in the body of the question you used the correct 'depth of field'.
Depth of focus refers to a characterictic similar to depth of field, but specifically where the image is formed on the sensor or film, behind the lens. (The tolerance of placement of the image plane in relation to the lens.)

[jonathanlung]

SLR viewfinders are optimized for to show brighter images when using zoom lenses. High(er)-end SLRs are usually optimized for faster zooms (~f/2.8 ) while low(er)-end SLRs for slower zooms (~f/4). A side-effect is that the DOF of the image you see in your viewfinder is limited to the f-stop for which the viewfinder is optimized.

Using your DOF preview button, you can stop down your lens' aperture until the viewfinder begins dimming; the last stop for which no change occurred is the f-stop your viewfinder is optimized for — probably f/2.8 on your 5D. Even though the image in the viewfinder has the DOF and brightness of a stopped-down lens when shooting wide open, your lens' aperture is actually wide open at this point, so if your lens experiences focus-shift, you're viewing the focus point at maximum aperture, not the reduced viewfinder aperture.

[Ken Bennett]

Replace your focusing screen with the "S" high-precision screen. Solves the problem completely. Costs about $40 in the USA. The only downside is that your screen will be darker with slower lenses. It'll be nice and bright with your 85.

[siba]

cheers guys.
You've confirmed what an acquaintance of mine has suggested as the reason.
But, you've been more specific, so I believe him now.
I'll look into that high precision screen, thanks.
Stefan