No, Bill, DOF is computed from aperture diameter, and final image magnification. Not f-stop, and negative image size.
For a given f-stop a longer lens has a larger aperture and therefore less DOF. That means a 165 on a 6x7 would have about the same DOF as a 165 on 35mm if, and only if, the subject is the same size in the print (the subject would have to be farther away from the camera). However, if the shots were taken at the same distance the 35mm would have less DOF than the 6x7due to the higher image magnification. OTH, the 165 on a 6x7 would have less DOF than a 50 on 35mm which has about the same angle of view, and thus the same magnification, but a much smaller diameter aperture. Usually DOF is figured for an 8x10 print viewed at 10 inches, though some amateur only cameras had DOF markings for 5x7 prints (I do not think Pentax ever did that). It works out pretty well that way because larger prints are usually veiw from farther away. Of course here on the internet, everyone has their own opinion about DOF, but the above is what the mathematics breaks down to. Yes f-stop is often used, but only when divided into the focal length which of course factors into aperture diameter. Then focal length and subject distance and print size factor down into magnification. The only other number needed is the circle of confusion (COC), but that is taken as a fixed value normally. Ciao, Graywolf http://pages.prodigy.net/graywolfphoto ----- Original Message ----- From: "Bill Owens" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Thursday, July 24, 2003 8:20 AM Subject: Re: Shallow DOF with 6X7 lenses (was: 6x7 lenses - Brotherhood comments solicited) > Isn't DOF computed via f stop, regardless of format? I do know that a 165mm > lens projects an image the same size on the film regardless of format. In > other words, if a 165mm lens forms an image 1cm high on 35mm film, it will > also form an image 1cm high on 120 film, the difference being that the image > comprises a larger area on 35 than on 120. > > Bill > > ----- Original Message ----- > From: "Steve Larson" <[EMAIL PROTECTED]> > To: <[EMAIL PROTECTED]> > Sent: Thursday, July 24, 2003 7:28 AM > Subject: Shallow DOF with 6X7 lenses (was: 6x7 lenses - Brotherhood comments > solicited) > > > > Why do 6X7 lenses have a shallow depth of field even stopped down > > all the way? Chris mentioned it with the 165/2.8. Are they all like that? > > I could understand it if it was at close focus with tubes WO, but for a > > landscape shooting at infinity? Please enlighten me. > > Steve Larson > > Redondo Beach, California > > > > > > Chris Stoddart wrote: > > > Well I have the old Tak 105/2.4 and it's really sharp - not a complaint > > > against it. My 165/2.8 (newish model) is also sharp, but it has a > shallow > > > depth-of-field, even stopped right down which makes it a bit of a pain > > > for landscapes. I also have the final model 75/4.5, which is just > > > excellent apart from a dimmer viewfinder. One difference between the > three > > > is weight - as they get newer they seem to get lighter, so the 75, > depite > > > having the largest front element, is by far the lightest of my lenses. > > > The build quality doesn't seem to suffer though as the 75's focus is the > > > smoothest of any lens I've owned. Next purchase will be a late model 45 > or > > > 55 - by all accounts the newest 55 is among the sharpest lenses Pentax > > > have built. > > > > > > > Regards, Bob S. (considering joining the Brotherhood) > > > > > > Join, you won't regret it! > > > > > > > > >
