Magnification is the key driver in DOF, and that does not change with format.
So if you take a 1/4th lifesized shot with a 6x6, you have the same DOF as if you take a 1/4th lifesized shot in 35mm. But of course, at that magnification your pipe fills the frame at 35mm but leaves a lot of empty space around the subject on the 6x6. So you fill the frame at 6x6 and in doing so you are shooting at more like 3/4th lifesized. The magnification goes up and the DOF decreases. Similarly, if you shoot a frame filling shot of the pipe on a small format digital camera with an 8.1 x 6.6mm sensor it's at about 1/10th lifesized. So the magnification is that much less and the DOF that much more, which is why small format digitals seem to have so much more DOF. So the math just works the same in any case - just compare the size of the image on the neg as a percentage of the size of the real object and you are set. - MCC PS: I think this is why we'll see larger sensors on digitals as digital technology matures. While getting great DOF is fine for some work (I love it for insects) it is a real detriment for other work where selective focusing is a key compositional element. So even if you can pack a zillion pixels in a tiny CCD, you can't get shallow DOF without the increase in magnification that only comes from recording a larger image. If Kodak invented a super sharp film that could be blown up far more than current film, I doubt that people would rush back to 110 cameras. 35mm, medium format, and large format offer sets of compromises built around the basic levels of magnification of the final image - compromises in DOF, focal length, and f stops - that make a lot of sense for different types of work. So ultimately I think we'll see digital sensors that are about the same size as film standards. Which is why a digital SLR's make a lot of sense. At 10:57 AM 5/4/2002 -0400, you wrote: >Hi Mark ... > >How does all this change when using a different format? If I were using >a 6x6 instead of a 35mm frame, would the results be any different? I >don't see a way of including the format size in these calculations. > >Mark Cassino wrote: > > > Hi Shel - > > > > I'm referring to the ratio of the pipe's physical size to the size of it's > > image on the film. > > > > So -- put it all in metric first: > > > > Your pipe is roughly 90mm long. You plan to use 80% of the 35mm frame, so > > the image will be 28mm long. The magnification ratio is 0.3 - so you are > > shooting at roughly one third lifesized (my 1/4th life sized was a > little off.) > > > > You want 1/2 inch DOF - so that is ~13mm of DOF. > > > > You now have all the info you need to solve the problem. The basic > formula is: > > > > DOF = ((2 * f_stop*circle_of_confusion_size) *(magnification + 1)) / > > magnification squared. > > > > I use 0.033 mm as the circle of confusion size. What this means is that > > I'll consider a point on the film that is 0.033 mm in diameter to be > > "sharp" - i.e. even though it is out of focus, it's so slightly out of > > focus it looks sharp. > > > > We know all the values except f stop, so: > > > > 13mm = ((2* f stop * 0.033) * (0.3 +1)) / 0.3 squared. > > > > You _could_ just solve that equation for the f-stop, but since the choices > > are limited I just plug in a guess and go from there. > > > > So - guessing f16 you have: > > > > ((2*16*0.033)*(0.3+1) / 0.09 = 15.25 mm DOF > > > > Guessing f 11: > > > > ((2*11*0.033)*(03+1) / 0.09 = 10.5 mm DOF > > > > At the half stop (f13) you get 12.4 > >-- >Shel Belinkoff >mailto:[EMAIL PROTECTED] >http://home.earthlink.net/~belinkoff/ >"When a man's best friend is his dog, >that dog has a problem." --Edward Abbey >- >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 . - - - - - - - - - - Mark Cassino Kalamazoo, MI [EMAIL PROTECTED] - - - - - - - - - - Photos: http://www.markcassino.com - - - - - - - - - - - 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 .
