[rollei_list] Re: Hey, Rolleiwide users, is using the glass plate, or, not using the glass plate a big deal?

  • From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
  • To: <rollei_list@xxxxxxxxxxxxx>
  • Date: Thu, 27 Sep 2012 10:24:31 -0700
----- Original Message ----- From: "Peter Mattei" <petermattei@xxxxxxxxx> 
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Thursday, September 27, 2012 5:46 AM
Subject: [rollei_list] Re: Hey, Rolleiwide users, is using the glass plate, or, not using the glass plate a big deal?
Vick, There is no concern with "focus shift"" with the optical film plate. It fits recessed into the back of the light cavity. It guarantees 
that your film is at the perfect plane of focus.  Peter
I think it might help to clarify terms. Focus shift is an effect caused by residual spherical aberration in a lens. Spherical aberration occurs because the most common surfaces used in lenses are spherical and a purely spherical surfaced lens can not produce a sharp image. So, its corrected by using a combination of negative and positive surfaces to obtain the equivalent "correct" surface. Lenses can be made with non-spherical surfaces but they are much more difficult to generate, even with computer controlled grinding machines. They are sometimes used in very high quality lenses but the effect of an aspherical surface is only to simplify the lens for a given degree of correction. Focus shift occurs because the outer parts of the lens focus at a different point than the inner parts. The more the light is deviated or bent but the lens the more error it can have so the outer parts tend to produce most of the spherical. When you stop the lens down the image is formed by the light going through near the center of the lens so there is relatively little spherical. When the entire lens aperture is used and there is appreciable spherical the point of best focus will be in a different place (usually closer to the lens) than when the lens is stopped down. The effect in well-corrected lenses is small but can still be noticeable. A few lenses have enough residual spherical to require focusing when stopped down, the Goerz Dagor is notorious for this but all lenses of this general type share the problem. I think what is meant here by focus shift is deviation from focus caused by buckling of the film. The degree to which is this a problem is difficult to asses but I think it is often blamed for poor focus when other factors are really at fault. The use of a glass plate to insure film flatness is an old technique, often employed in aerial mapping cameras where film flatness is essential but, as has been pointed out here, it comes with a set if vices all its own. A flat glass plate _can_ generate aberrations. If the light striking the plate is not plane parallel (collimated) the plate can generate chromatic aberration and spherical aberration. Since the light striking the film is convergent a glass plate can cause some aberration although it may be very slight. The thinner the glass the better since such aberration is dependent on the light path through the glass. BTW, this is why gelatin or plastic filters are preferred to glass filters for use where preserving the optical quality of a lens is essential and why it is preferred that a filter be used on the subject side of the lens rather than in the optical path (in most cases). 


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk@xxxxxxxxxxxxx 
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