----- Original Message ----- From: "dave o'brien" <[EMAIL PROTECTED]>
(snip) > > DOF has to do with the focal length and the f-stop. Print > magnification has nothing whatsoever to do with it. > - But print magnification has EVERYTHING to do with it. Until now I have kept my silence WRT this subject but no more. Too frequently anecdote and intuition have been been offered as informed knowledge. The facts follow. The DOF concept is based on the circles of confusion (COC) of the out of focus part of a photograph being either smaller than the unaided eye can appreciate, in which case the subject matter will seem acceptably focussed, or large enough for the same unaided eye to see the unsharpness of it. Quite simply, when the enlargement factor of a print is increased then the COC of nearly focussed areas will become larger and will cross the threshold between small enough to pass as sharp and large enough to be deemed unsharp. Photographers who either criticise or praise different sized formats for their DOF characteristics are correct only as long as the developed film or a contact print (or 1:1 enlargement) is being viewed unmagnified. Otherwise, a 22mm standard lens on a 110 camera will produce the same DOF as will a 43mm lens on 35mm (the statistical standard lens), or a 75mm lens on 6x4.5cm, a 90mm lens on 6x7cm, or even a 320mm lens on 8x10in, as long as the shooting aperture is always the same and the prints being judged are always the same size, and regardless of differences in DOF of the unenlarged negatives. WARNING: ANECDOTE AND SUPPOSITION FOLLOWS. Digital cameras seem to bend theses rules somewhat because the imaging chips have optical characteristics of their own to confuse the equation. CCD pixels prefer to "look" straight ahead unlike film which easily accepts exposing light from oblique angles (the apparently fatal flaw of the full frame Philips chip proposed for the MZ-D). That narrow acceptance angle may lend CCD produced images more apparent DOF (my guess anyway) the same way as the CCD array in a scanner has great DOF even without a lens. That's my ~theory~ anyway for the long DOF that some list members have reported from their digicams. BTW the DOF scale on lenses for 35mm cameras are more suited to viewing as projections rather than as prints, and even when they're printed the overwhelming majority are 4x6in minilab prints. P&S 35mm and smaller or low end digicams most often have small aperture wide angle lenses so have greater inherent DOF to begin with. Medium and large format photographs almost always end up as prints for sale or for serious scrutiny by serious amateurs. It's no wonder that their DOF of is more critically judged. Regards, Anthony Farr - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
