[rollei_list] Re: Bright Screens
----- Original Message ----- From: "Kirk Thompson" <thompsonkirk@xxxxxxxxxxx> 
To: "Rollei List" <rollei_list@xxxxxxxxxxxxx>
Sent: Wednesday, January 04, 2012 7:19 AM
Subject: [rollei_list] Re: Bright Screens
Thank you Richard for this thorough explanation of pitfalls in the quest for sharpness.
Focus shift is even more a problem now that digital sensors (perfect 'film flatness') reveal small lens disparities. Leica recently re-designed the 35mm Summilux because it had earned a bad rap for focus shift. Sean Reid, on the best website for review of lenses for Leica, now tests all lenses for focus shift. Several of the Cosina Voigtlander lenses in Leica mount have done poorly in this respect.
One hears that Tessars can resolve as well as Planars if stopped down to f8. No doubt this is where DOF overcomes Tessar focus shift.
Fortunately for photographers, absolute sharpness isn't the only value in a lens. Other aspects of how a lens 'draws' or renders can be more important than how well it reveals pores & beard hairs. 

Kirk
Resolution has more to do with how well the several lens aberrations are corrected. Some aberrations are stop dependant and get better at smaller stops. Think of the angle of the light though the lens: the smaller the angle the light rays make with the surfaces of the lens elements the less they are deviated and the less effect whatever aberrations there are make on them. This is not true of all aberrations, for instance chromatic aberration is not improved by stopping down. The Tessar is capable of very good performance, for an f/3.5 Tessar both spherical aberration and its other aberrations are about gone at f/8. The ultimate limit of resolution is the diffraction limit which is dependant on the effective size of the lens: the larger the opening in the lens the higher its resolution is. So, as one stops a lens down there are two conflicting effects: the resolution due to diffraction becomes less; and the resolution due to reduction of aberrations becomes more. At some stop the resolution is maximum for that particular lens. For many practical photographic lenses of high quality this stop is around three stops down from maximum opening. However, it is also dependant on the coverage angle expected from the lens since many aberrations vary with this angle. As an example a Dagor, which is an inherently wide angle lens, is about optimum for "normal" coverage of about 60 degrees at about f/22 but, at its maximum angle, around 85 degrees, it must be stopped down to about f/45 for the margins to be sharp. Since some lenses have better correction of aberrations to begin with their "optimum" stop will be larger than another, poorer, lens of the same inherent speed. When lenses are stopped down beyond the "optimum" stop the resolution, in general, falls off so at small stops lenses of differing designs or even quality may measure the same since the resolution will be limited more by diffraction than by aberrations. 



--
Richard Knoppow
Los Angeles
WB6KBL
dickburk@xxxxxxxxxxxxx 
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