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

  • From: Laurence Cuffe <cuffe@xxxxxxx>
  • To: "rollei_list@xxxxxxxxxxxxx" <rollei_list@xxxxxxxxxxxxx>
  • Date: Thu, 27 Sep 2012 22:48:05 +0100


Sent from an iPad, 

On 27 Sep 2012, at 18:24, Richard Knoppow <dickburk@xxxxxxxxxxxxx> wrote:

> 
> ----- 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 
> ---
Richard, I think the focus shift mentioned is that considered on page 7 of the 
following thesis
http://www.optimaxsi.com/PDFs/RulesOfThumbOptomechanics.pdf
its just easier to reference the formulae there than to try and convey them in 
an email.
All the best
Laurence Cuffe
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