[rollei_list] Re: Veiling Flare


----- Original Message ----- From: <hoarcroft@xxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Saturday, June 25, 2005 9:39 AM
Subject: [rollei_list] Re: Veiling Flare


In <BEE2FE70.11FE0%egoldste@xxxxxxxxxxxxx>, on 06/25/05
at 12:28 PM, Eric Goldstein <egoldste@xxxxxxxxxxxxx> said:


The Xenotar and Planar are quite different in the specifics of their
designs, though of the same basic type and comparable in performance. The
many thousands who have shot these cameras over the years have not
reported a significant difference in flare between the two, so I think it
likely that there isn't much, but... I do think your basic premise is not
correct. Very small differences is design and execution can make
significant differences in all kinds of performance, including flare.

Oh, I'm well aware of the differences in design; I was referring to what
you quite rightly point out: they are comparable in performance. It is a
safe assumption -- I hope! -- that unit-to-unit differences in lenses in
Rolleiflexes are expected to be tightly controlled. That seems to be the
experience of most of those here, n'est ce pas? -
-----------------------------------------------------------
les clark / edgewater, nj / usa
-----------------------------------------------------------

Lens prescriptions can be analysed to give the performance of the design but that is not the same as the performance of actual lenses, it is only what they are capable of if made exactly according to the prescription. With this in mind I have analysis of several five element lenses included in the lens design survey program LensView. Included are the f/2.8 Xenotar, the f/2.8 Planar with the cemented element in front as used in Rolleiflex (there is at least one alternative design), and the Biometar of Zeiss Jena. The differences in the curves are slight. The Planar seems to have the least spherical aberration but only by a little. I suspect individual manufacturing differences are greater than the design differences.
The main consideration in overall flare is the number of glass-air surfaces, this is the same in all of these designs. Other flare, such as ghost images of bright objects in the image field and flare from extra-image light vary with the individual design. Some lenses simply do not have ghost images even when uncoated, others have serious ghost images. A ghost image is a spot of more or less focused light. Some lenses produce quite well focused ghost images of objects in the picture. The repeated images of the sun seen in some zoom lenses is an example of a very exagerated ghost image. Good coatings can reduce these but attention to the combination of surfaces in design remains important. The difference in overall flare among five element lenses of this type is probably not much but there may be differences in ghost images or images of extra image objects which is significant.
I also think that the condition of the lens surfaces and of the cement in the cemented surfaces can have an effect on flare. Lenses should be absolutely crystal clear. Most camera lenses pick up a certain amount of damage over time. The fine scratches sometimes called cleaning marks can cause flare because they act as a diffuser. Internal dirt and haze or haziness of the cement will cause a reduction in contrast, in the case of the cement it can be substantial. While everyone is familiar with the typical edge degradation of old fashioned Canada Balsam cement it should be noted that synthetic cement is also subeject to degradation, especially if there was a curing problem or if the surfaces were not prepared correctly. I've noted that some Kodak lenses develop a turbidity that looks a little like orange peel when examined with a magnifier. I just re-cemented a couple of old lenses (have to do one over again, ugh), they are amazingly clear. Anyway, I am rambling. I suggest checking out the interior of a camera for reflections. Just open up the lens and shine a flashlight through it from various angles and see how much light is reflected from the inside of the camera. Most of the time you will be surprized at how much there is.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 

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