[pure-silver] Re: unsharp lens

  • From: Aaron Reece <oboeaaron@xxxxxxx>
  • To: pure-silver@xxxxxxxxxxxxx
  • Date: Mon, 7 May 2007 22:55:25 -0400


On May 7, 2007, at 9:38 PM, Shannon Stoney wrote:

It also says that maximum sharpness for this lens is at f/11-22. I usually stop down quite a bit more than that. Could that be the problem?


Yes. Diffraction is a limitation that is quite real. Some informal tests run several years ago by a member of the Rollei list demonstrated that the resolution of the Tessar, Xenar, Planar, and Xenotar lenses in those cameras took a rapid nose-dive, in some cases falling from a peak at f/11 to half that resolution at f/22. That's right, only two stops difference but the resolution was reduced by half. My own even more informal experiments with a view camera confirmed the tendency. Remember that view camera lenses are reputed to be best corrected around f/22 on average, unlike smaller camera lenses.

At apertures this small (beyond f/22) your shutter speeds must also be quite slow, which could further compound the problem, as several people have pointed out. Even a small 4x5 monorail camera requires a very sturdy (read: heavy) tripod.

If you don't currently make use of camera movements with your 4x5, buy a book on the subject and read up. The Camera by Ansel Adams is good, although there are clearer treatments of the subject available. Using movements to alter the plane of sharp focus allows you to use larger apertures and shorter exposures. Search the net for "Scheimpflug effect" or "Scheimpflug principle" to get you started.

Best of luck,
Aaron Reece
Oswego, NY, USA
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