Archived posting to the Leica Users Group, 1999/12/01
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]1999-12-01-00:51:21 Eric Welch: > What we have been saying is correct, depth of field is independent > of focal length along [sic]. Magnification and aperture determine it. Magnification and aperture and optical formula. No, really. Take 35mm Summicron-Ms, both ASPH and pre-ASPH, and focus on something closeish at the same wide aperture. Look at nearer and farther things in the pictures you take. The ASPH lens dives out of focus at a more rapid rate than the pre-ASPH. And I'm talking about absolute sharpness at these places out of the plane of focus, not just differences made subjectively more dramatic by the greater snap of the ASPH when it *is* in focus. I asked Erwin about this some months ago -- theorizing that perhaps the ASPH had physically larger diaphragn openings for a given marked stop, and that the lenses were calibrated in T-stops rather than actual apertures -- but he told me that what I observed is actually because the rays within the ASPH design converge at a sharper angle. (I hope I'm paraphrasing right). So... depth of field being dependent only on magnification and aperture is probably just a good first approximation, or one valid across a family of optical designs similar in some as-yet-unspecified way, or... ?