On 6 June 2011 23:23, Matthew Hunt <[email protected]> wrote: > On Mon, Jun 6, 2011 at 9:08 AM, Anthony Farr <[email protected]> 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. 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. > > For example: If you focus at the hyperfocal distance, the DOF in front > of the focused distance is finite, and the distance behind is > infinite. So 0% (finite/infinite) is in front of the focused distance. > While I concede the already mentioned problem of numbers getting rubbery near infinity, dont forget that in practical photographic terms infinity happens very early. Even with the longest lenses in common use we will in practice reach infinity in the four figure range at most, whether metres or feet. That pretty much explodes the concept of infinity as a bottomless pit from a photographic point-of-view. A lens doesn't have to focus all the way to infinity, either. It's enough for it to focus parallel rays from a point, e.g. a star, to a circle no larger than the circle of least confusion at the lens's largest aperture. For a number of reasons a lens doesn't need, could never achieve, and could never use an infinitely small circle of confusion anyway. But, and again in practical terms, by the time those "near infinity" distances are reached, DOF is generous enough that it doesn't matter. >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. > > At the other extreme, in macro photography, the DOF is split nearly > equally front/back. > Sometimes vagaries enter the calculations because we use "thick" lenses having two nodal points, sometimes outside the body of the lens itself, whereas simple optical principles assume a "thin" lens. All lenses behave as thin lenses at longer focused distances, the area of the original discussion. When we get into the realm of larger-than-lifesize macro, in other words exceeding 1:1 reproduction ratios, the rules get turned on their heads. The usual numerical relationships between object, lens and image get upended, the object and the image swap their places in the equations (but don't overthink it, they stay in their same relative positions). That's the reason we turn lenses backwards at larger-than-lifesize macro, to keep all the optical corrections on the correct sides of their equations. 1:1 is the tipping point where the relationship of everything is equal on either side of the lens. To make a long story short, my statement is true in general but just happens NOT to be true for some focus distances. regards, Anthony "Of what use is lens and light to those who lack in mind and sight" (Anon) -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
