On Mon, Jun 6, 2011 at 11:56 AM, Anthony Farr <farranth...@gmail.com> wrote: > On 6 June 2011 23:23, Matthew Hunt <m...@pobox.com> wrote: >> On Mon, Jun 6, 2011 at 9:08 AM, Anthony Farr <farranth...@gmail.com> wrote: >> >>> When you focus at any particular distance one third of the depth of >>> field (DOF) is between you and the focused distance, the other two >>> thirds is beyond the focused distance. Therefore when you focus at >>> infinity you squander two thirds of your DOF. >> >> That's an often-repeated statement, but it's not true. Or rather, it >> just happens to be true for some focus distances, but is not true in >> general. > > Actually, Matthew, you have it backwards. My statement is true in > general but just happens NOT to be true for some focus distances.
I am using "in general" in the mathematical, not colloquial, sense. Your statement is true for one focal distance. It is wrong, by some amount, for all other focal distances. > Lots of formulae get wobbly when they're pushed towards infinity, so I > won't apologise for this. When the numbers are single to three or > four figures the principle holds up. Did you happen to notice that we're talking about a lens set to infinity? It's certainly relevant to this discussion. > From moderately close-up (less than true macro) to near-infinity (but > measureable) distances the "one third in front, two thirds behind" > rule is as correct as you'll get. O RLY? At 10 feet, for the 135mm lens in question at f/11, the split is 0.48 : 0.52. At 20 feet, the split is 0.46 : 0.54. At 40 feet, the split is 0.43 : 0.57. Those are moderate distances; neither "true macro" nor infinity. Only at 88 feet is the split 0.33 : 0.67. > To make a long story short, my statement is true in general but just > happens NOT to be true for some focus distances. No, it's the opposite. It is true at a specific focus distance, namely 1/3 of the hyperfocal distance. It's wrong, to various degrees, at any other focus distance. The near:far ratio varies continuously from 0 to 1 as the focus distance goes from hyperfocal to 1:1 macro. There's no reason to pick out a 1/3 : 2/3 split as a "special" value to build a rule around; it's just one point in the middle. -- 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.
