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 .
