At 03:14 PM 4/10/2002 -0400, you wrote:
>Thanks for the tip. The pipe is 3.5 inches long and just about 3/4-inch >deep. > >I'm not quite sure what you mean by "1/4th life sized". FWIW, I was >thinking of trying to fill the frame about 80% - 85% or so, in order to >leave some border around the pipe. I would like to fill an 11x14 sheet >with the image. 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 BUT - the DOF extends both in front and behind the focal plane. So if you focus on till the front edge of the pipe is in focus, about half of our DOF is wasted in the empty space between the pipe and lens. This is different from non-macro application of DOF, where DOF is weighted more towards the rear than the front. So when you take a landscape you have much less DOF in front of the focal plane compared to behind. In macros as the magnification goes up the DOF gets more and more evenly divided. At this magnification you are really looking at a close to 50/50 split in the DOF. You'll want to place the focal plane about 6.5 mm into the subject - so focus on the subject and then 6.5mm further in. The easiest was to do this is get something that is 6.5mm deep and focus on it, and then switch the pipe for it. I'd also stop down to at least f16 to be safe, and do some shots at f22 as well. For lighting - if you want a straight documentary shot get a decent sized cardboard box (a least the size of the ones that copier paper come in) and then paint it white or glue white paper inside. Put the pipe about half way in the box (which is on it's side) and set up two flashes at 45 degree angles, on pointing right at the pipe and one pointing up at the top of the box. When I do this I use a flash meter and just fiddle around with power levels till I get the combined output to correspond to my desired F Stop. Otherwise, I'm not sure how you would figure up the exposure with a set up like this. The direct flash will bring out the surface details, the one shooting up into the box will create a nice ambient light. I've never done this with a metallic object, so you may need diffusion on the direct flash to prevent overly bright highlights. Hope this helps - MCC - - - - - - - - - - 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 .
