On Thu, Nov 13, 2008 at 10:09 PM, JC OConnell <[EMAIL PROTECTED]> wrote:
> dof HAS ZILCH to do with format, its all about > magnification which is object size vs image size. > object size vs image size ignores format. > That said, if you are using the same lens > focal length, and same distance of the object, > and same f-stop, the DOF will be identical > in the image regardless if its APS or 4x5 format. > the format only changes the angle of view, not > the DOF. DOF is not a fundamental image property. It's based on a judgment of what constitutes acceptable sharpness in the final product (print, projected slide, image on screen, etc.). From that assumption, you can get the acceptable CoC on the film or sensor, which will vary by image format. There is a good treatment in "Applied Photographic Optics" starting on Page 216. You can get a suitable free preview from Google Books: http://books.google.com/books?id=cuzYl4hx-B8C&printsec=frontcover&dq=applied+photographic+optics#PPA216,M1 The author discusses the importance of enlargement factor and assumed viewing distance. 8x10 contact prints are contrasted with 8x enlargements from 35mm. Prints from 35mm are contrasted with projected 35mm. Finally, a table is provided showing allowable CoC for various formats, under certain standard assumptions. Here is another such table, from the author of DOF-calculating software: http://www.dofmaster.com/digital_coc.html and a Circle-of-Confusion calculator for various formats: http://www.dofmaster.com/digital_coc.html#coccalculator Here is another discussion of the topic: http://www.wrotniak.net/photo/tech/dof.html#DIGITAL "What's so special about digital cameras? Actually, not much, except that most have sensors smaller (or much smaller) than the 35-mm film frame. Therefore the CoC size used to define "acceptably sharp" also has to be proportionally smaller." Here is a reprint of a Kodak pamphlet, which also contains a table of different CoCs for different formats: http://people.rit.edu/andpph/text-kodak-aa-3.html Here is another tutorial: http://photo.net/learn/optics/dofdigital/ One of Bob Atkins' (the author's) comments may be particularly helpful to you: "The circle of confusion value is NOT an inherent property of the image recored on film. Neither is DoF. As I carefully explained above DoF depends on the circle of confusion value, which in turn depends on how much you enlarge the image." -- PDML Pentax-Discuss Mail List PDML@pdml.net http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
