Hi Richard:
f8 - f11 for the Xenar sounds right. Thanks! -------------------------------------------------- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx> Sent: Wednesday, February 03, 2010 10:53 PM To: <rollei_list@xxxxxxxxxxxxx> Subject: [rollei_list] Re: xenar test roll is back
StephenFWIW, the main residual aberrations in a well-designed Tessar type lens are all reduced by stopping down. For many lenses of "normal" coverage the residual aberrations are essentially gone over the field when stopped down about two stops. For an f/3.5 lens this is around f/7 so stops of about f/8 or smaller should be quite good. The "optimum stop" may be somewhat smaller, probably about f/11. Beyond this the loss of resolution due to diffraction begins to become significant. This is the reason that lenses on small camras rarely stop down beyond f/22 while those on large cameras often go to f/32 or even f/45 to f/64. At f/64 a Rollei lens would be very blurred. Beside the improvement in sharpness there may also be some improvement in contrast as the optimum stop is approached. This is because spherical aberration and its relatives like oblique spherical and coma also tend to scatter some light and reduce contrast. For the most part well-designed Tessars do not have much residual spherical, mostly they have some oblique spherical. This is similar to coma and, like coma, increases with image angle and decreses as the lens is stopped down. Adding more elements is a way for the designer to control higher order aberrations. For fast lenses, or wide angle lenses additional elements over a Tessar are necessary if the lens is to have good performance and decent speed. Note that some quite simple lenses (wide angle Protar) will cover surprizingly wide angles (around 100 degrees) but onlyl at very small stops (c f/64).-- Richard Knoppow Los Angeles, CA, USA dickburk@xxxxxxxxxxxxx --- Rollei List - Post to rollei_list@xxxxxxxxxxxxx- Subscribe at rollei_list-request@xxxxxxxxxxxxx with 'subscribe' in the subject field OR by logging into www.freelists.org- Unsubscribe at rollei_list-request@xxxxxxxxxxxxx with 'unsubscribe' in the subject field OR by logging into www.freelists.org- Online, searchable archives are available at //www.freelists.org/archives/rollei_list
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