Also to consider, at 1:1 your f8 50mm is really an f16 100mm and your f8 500mm is really an f16 1000mm. But with a close up lens it works the other way your lenses becomes something like an f8 25mm and an f8 250mm depending on the diopter. Another thing is using a COC for the negative size is confusing unless you are going to compare contact prints. Now if you at talking about a 50mm at 10ft and a 500mm at 100 feet, yes the DOF is going to be similar. Close up things become more problematic. And when you start comparing different formats it is still stranger, that is why I use an 8x10 enlargement as standard it helps reduce the confusion.
Ciao, Graywolf ---------------------------------------------------------------- ----- Original Message ----- From: Mark Cassino <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Tuesday, February 19, 2002 5:36 PM Subject: Re: DOF with close up lenses > At 12:45 PM 2/19/02 -0500, Graywolf wrote: > > >Wrong! > > LOL! Many a times, yes - but not today, not on this. :-) > > >You are confusing aperture (diameter of the opening) with f-stop (focal > >length divided by aperture). Also since DOF is normally figured for 8x10 > >prints viewed at 10 inches you can use 0.25mm (1/100 inch) as your COC for > >all calculations. > > I took the 0.033 figure for the circle of confusion from Kodak's workshop > book on close up photography. As the authors say - "it works for even the > most critical applications." I'd rather underestimate DOF than over estimate. > > >At f8 a 500mm lens has 10x as large an aperture as a 50mm so your DOF will > >be 1/10 that of the 50mm. > > This observation is correct only if your constant is distance, and as such > it is not inconsistent with what I said. > > Go here: > > http://www.shuttercity.com/DOF.cfm > > Plug in a 50mm lens, f8, 2 meters as the distance to the subject. DOF = > 0.77599 meters. > > Now plug in 500mm, f8, 20 meters as the distance. DOF = 0.7490 meters - > less than 3mm difference. > > My point was and remains: if the magnification of the subject is the same, > at any given f-stop, the DOF is the same. Focal length is irrelevant except > to the extent that longer focal lengths increase the magnification of a > subject at a given distance. > > John Shaw put it better than I can: > > "_If the image size and aperture remain the same, all focal length lenses > give the same depth of field._ Understanding this is very important for > closeup photography. If you photograph a subject at life-size with a 50mm > lens, and then move back and photograph it again at life-size with a 200mm > lens at the same f-stop, the depth of field will be the same. Photographs > taken with a wide angle lens and with a telephoto lens will have the same > depth of field if the image size and f-stop used are the same. The > backgrounds will look different due to angles of view and the perspective > will be different since the photos will be shot from different locations, > but the depth of field will be the same." > > Closeups in Nature, page 32. the first sentence is emphasized in the > original - probably the most important paragraph in the book. > > Cheers - > > 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 . - 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 .
