Hi, well, now you've put me on the spot! These numbers are useful for predicting how big you can go before you start to lose apparent sharpness, but they depend on what you as the photographer are trying to achieve. That's why they can't be treated as rules. In the end you decide how big you want your prints to be and that's that. Whether they are sharp or not is for the viewers (who include you) to decide for themselves. You probably want the critical parts to look sharp from about 15cm away because people peer closely at exhibition prints, instead of being good and looking from the regulation distance. People also typically want to look at quite large prints, but don't adjust their viewing distance, so you have to consider these human factors, rather than just relying on the numbers - you want it to look sharp from about 15cm away, but you want it blown up to about 60x40cm (24x16"). You can't necessarily have both because if you shoot 35mm your CoC's too small. 0.009mm. Sorry.
I suppose in theory you could measure some important points and scale them up to see if they would look sharp in a bigger print. More practically you could look at the depth-of-field in an existing proof print, say 15x10cm (6x4") and work out approximately how big you can reasonably go before it starts to look unsharp, or keep zooming in with the enlarger until the critical parts start to look unsharp. There's a nice simple explanation and diagram of CoC here: http://www.tpub.com/content/photography/14209/css/14209_37.htm. You need to grasp this to understand depth-of-field, which depends on it. In your example below, you will have reduced depth-of-field in the prints compared to smaller versions, or the same size seen from a different viewing distance. This may or may not be important - only you can decide. At some magnification and distance nothing will look sharp; at others, everything will. 0.036mm is not a number you have to try and conform to, it's just a pragmatically useful reference point used as a constant in optical calcualtions. You probably could conform to it by shooting for a specific size of print, being very careful about your depth-of-field calculation, and controlling the viewing distance, but really this is only likely to be practical (if at all) if you're shooting for billboards or something. -- Cheers, Bob Sunday, December 28, 2003, 2:04:33 PM, you wrote: > Thanks Bob. That more or less forced me to relook through old bookmarks > which at the time, I looked at, cringed, then consciously decided to revisit > sometime in the future :-) I can't say I more than vaguely grasp the concept > of CoC. So say I've made an A1 sized print (594x820?) from 35mm- viewing > from 1.5m to 2m would (I think) look like: > c = (36 * 1500) / (1000 * 594) = 0.091mm. > c = (36 * 2000) / (1000 * 594) = 0.121mm > Now for the more ignorance-revealing bit: How then would I make use of those > figures? Does it mean I need to readjust one of the variables to conform to > 0.036mm? Like for 4x5 it'd be closer to 0.1 wouldn't it? > Rgds, > Ryan
