Chip Louie wrote: > You know after reading this thread for a while, I'm wondering how Canon > picked .035mm as the standard of acceptable focus.
You're not the only one ... On page 193 of Lens Work II, Canon talk about 1/1000 to 1/1500 format diagonal, digress to viewing distance (a wide range) and enlargement, and finally arrive at 0.035 mm, as if they simply chose 1/1250 format diagonal as a compromise. It's not obvious why they did. Most sources cite values for an acceptable CoC of 0.2 mm ~ 0.25 mm in a final image viewed at 250 mm. If the final image is 8"x10", this translates to 1/1625 to 1/1300 of the diagonal. Because of the aspect ratio, it's difficult to enlarge a 35 mm negative to match the diagonal of 8x10; however, if it's enlarged to fill the 8" dimension (8.467x), the CoCs in the negative range from 0.024 mm to 0.030 mm. If you assume 5x7 rather than 8x10, by similar reasoning, the CoCs in the negative range from 0.038 mm to 0.047 mm. If you don't buy the original criteria for CoC in the final image, of course, none of the values just mentioned apply. There's a discussion on the EOS doc list about the appropriate value for CoC. Most sources acknowledge human visual acuity to be 1' of arc for reading high-contrast eye charts, yet cite values for CoC mentioned above (which translate to 2.75' to 3.44'), and offer explanations for the difference that range from none to somewhat less than rigorous ... it's possible that everyone simply copied what was printed somewhere else and that process perpetuated itself ... Jeff Conrad * **** ******* *********************************************************** * For list instructions, including unsubscribe, see: * http://www.a1.nl/phomepag/markerink/eos_list.htm ***********************************************************
