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John_4185
26-Sep-2005, 07:56
Under a different thread, joerg krusche said, in part, [...] the only thing you have to do is to get two rings to adjust for spacing, which is not critical since [it is] a symmetrical lens design. Optical performance should satisfy all needs.

Is it true, then, that Biogons (8 and 10 element later designs) can well tolerate a variance from the factory cell-spacing at the shutter? Or are they not symmetrical enough?

Dan Fromm
26-Sep-2005, 08:55
John, I have a cross-section of a 38/4.5 Biogon posted over my desk. That lens is far, far from symmetrical. Two big elements on one side, one on the other. And the two cells' inner cemented groups aren't that much alike.

I'm not sure Joerg's statement about symmetry eliminating the need for accurate spacing is correct. I don't think that my four element double Gauss type wide angles agree with him and I'm pretty sure that my dialyte type lenses disagree strongly.

Cheers,

Dan

Ole Tjugen
26-Sep-2005, 13:18
Accurate spacing is still crucial even with symmetrical designs. Especially with wide-angle lenses!

John_4185
26-Sep-2005, 13:23
Dan: I have two diagrams of later Biogons: 8 and 10 element lenses. :( I agree they are not symmetrical, but people, including Zeiss, seem to be rather casual with the term 'symmetrical', sometimes qualifying it as 'almost' so that I have to test the waters from time to time for a reality check.

Dan Fromm
26-Sep-2005, 15:41
John, I agree completely with you that many people and firms describe lenses as "symmetrical" that are anything but. In this case, I don't think the word is applied correctly. And I don't think that repetition -- you're innocent, please don't be offended -- makes it right.

Cheers,

John_4185
26-Sep-2005, 18:11
dan: And I don't think that repetition [...] makes it right.

symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical .... sayeth Zeiss

Okay, lemmie look at the lens now. Yer right, Dan. It's still not symmetrical! 'least not yet.

symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical symmetrical ... sayeth others

Nope. Still the same.

Google gives 309 hits on Biogon & symmetrical, so it's GOTTA be true!

I'll go offline now and work on it some more. symmetrical symmetrical ...

Struan Gray
27-Sep-2005, 00:52
The computars are optimised for different repro ratios by changing the spacing by a smidgin, so a small error obviously has measureable consequences, even for symmetrical plasmats. On the other hand, changing the spacing won't make a symmetrical lens unsymmetrical, so at 1:1 you'll still get automatic cancellation of coma, lateral colour and (I think) other aberrations. Shifting the iris may break symmetry, but my fiddling with oddball lenses indicates that small shifts have negligible effect.

My stock of old wive's tales says that Biogons and other wide angles are very sensitive to spacing changes. Easy enough to test on the ground glass - just unscrew one element a tad and see what happens.

Joerg Krusche
27-Sep-2005, 03:53
Hello,

my statement that spacing is not that critical with symmetrical lenses applies to symmetrical repro lenses only, e.g. dialyte types etc. The possible amount for varying air space between front and rear cells depends on the focal length, i.e. for focal lengths of 300mm and ... more.

To optimize infinity performance off the image center within a symmetrical design that is optimized for 1:1, for example at 300 mm focal length the increase in air space may be in the range of less than one millimeter and increase to several mm's as you go to 600 mm focal length. This is info from Carl Zeiss optical designers, the ones that made the Apo Germinars etc. Applying such adjustment to lenses with max. opening of e.g. f=19,5 does not make sense. So far the expert from Carl Zeiss Jena.

From above one may conclude that with short focal lengths and with non-symmetrical designs spacing of front and rear cells is very critical to extremely critical, down to hundredths of a millimeter i.e. in the multi-micron range. And the Biogons are very short focal length wide angles .. and they are not absolutely symmetrical, a look at their lens cut will confirm this.

Best regards

Joerg

John_4185
27-Sep-2005, 07:10
First, thanks to Jorge for clarification of his original statement. I regret that people had to elaborate upon the obvious for my silly question. But this is not the end of silliness...

BUT NOW FOR SOMETHING DIFFERENT - I did it! I have a couple identical Biogons and just because it is already cold and dark and I was bored, and possibly because the Bob Dylan TV movie was playing in the background making me crazy, I reassembled each into two lenses: one with two front clusters and one with two back clusters. (Yes, I labeled them so they can be put back with their original mates). They are petty goofly looking lenses. Then I went to bed late and had bad dreams.

If I get off the day-job while there is still light, I'll bring home a digital snapper and post pictures and work on a mount to see whazup when one makes a 'symmetrical' lens of late Biogons. I suspect the outcome will be a mess only Sally Mann could appreciate.

The alchemist,