It's worth remembering that a great many folding rollfilm and not a few 35mm. cameras were supplied with "front element focusing" (in effect, variable focal length) lenses -- in the case of the (still expensive) Zeiss "Super Ikonta," an 80mm. f:2.8 "Tessar" had its front element coupled to the rangefinder.

Lenses of "Tessar" and "Cooke Anastigmat" design seem to have been particularly well suited to this sort of thing.

The purists, back then, worried that front element focusing lenses were not as sharp as their fixed-separation equivalents -- but it does suggest that the most prevalent second-generation anastigmats are relatively tolerant to spacing imprecision.

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On 2004-06-21 12:37, Dan Fromm wrote: Quote: On 2004-06-21 11:20, RichS wrote: The last note on this lens... And many other lenses... <large snip> After all this, the micro-exact lens spacing seems more an urban myth than science, at least with the old lenses... I'm no longer going to worry about it at all... And I just thought someone else out there may like this info to give them some peace of mind also... Depends on what you mean by old, and at least a little on the lens. My 38/4.5 Biogon was made in '69 so it seems relatively new, but first came to market in '52, so its approaching old. With this one, spacing is critical. My Ilex 65/8 SA clone, also known as the Super Fangulon, as in "sharper than a serpent's tooth," is very fussy about spacing. A silly millimeter off is enough to blunt it severely. Cheers, Dan

My "old" would be more the pre-70's computer lenses and just a guess.

But look at what you're comparing. One mm to a 38 is almost 100mm to my 300! I would have no doubt that the length of the lens matters. And whether or not it's a tele or wide-angle design. They did mention that the convertible designs I was talking about suffer much less from mis-spacings. And since non of us seem to really know. If it's a direct relationship, take the 3mm off on my 300 and apply that to the 38 and see what happens? That's .38mm spread in your spacing...

[quote]
On 2004-06-21 13:46, RichS wrote:

Quote:

On 2004-06-21 12:37, Dan Fromm wrote: Quote: On 2004-06-21 11:20, RichS wrote: The last note on this lens... And many other lenses... <large snip> <more snip> My "old" would be more the pre-70's computer lenses and just a guess. But look at what you're comparing. One mm to a 38 is almost 100mm to my 300! I would have no doubt that the length of the lens matters. And whether or not it's a tele or wide-angle design. They did mention that the convertible designs I was talking about suffer much less from mis-spacings. And since non of us seem to really know. If it's a direct relationship, take the 3mm off on my 300 and apply that to the 38 and see what happens? That's .38mm spread in your spacing...

Actually, what happened with my wide angles when the spacing was too, um, wide, was that the center of the image remained sharp and the outer parts got terrible. I'll ask some of my longer lenses what they do when I screw the cells partway out of the barrel or shutter. I expect they'll do much the same, and that on 2x3 I'll have trouble seeing the effects because they'll be off the gg.

Interesting, slightly relevant story. I've read that H. D. Lee didn't believe in ray tracing. He'd design a lens using algebra, then build a prototype and fiddle with spacing. And as we all know, lenses he designed do pretty well.

Cheers,

Dan

Maybe I'm tired, maybe not enough coffee, maybe another lost brain cell or two?...

TR: I don't remember knowing about front element focusing lenses. Very interesting!

Dan: I know I played with a few lenses. Trying them with single cells, unscrewing a cell to see what the extra spacing would do. As I remember, there weren't any suprises. Some lenses worked with a single cell, some didn't. Unscrewing the front cell produced not much that I remember...

In my weary mind, I could easily see a distortion produced by varying the cell spacing as rays meant for the central portion would now be hitting the outer edges (if increasing the spacing). I would guess this would be much worse as the lens itself got shorter in focal length. But I still think it's proportional to the focal length and that's why my 3mm has no apparent affect on the 300mm lens. I'll have to pull out a short lens and play again...

And here's another missing brain cell... Who's H.D. Lee???

I really should restrict my replies to times when I'm awake and alert

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On 2004-06-22 19:55, RichS wrote: Maybe I'm tired, maybe not enough coffee, maybe another lost brain cell or two?... TR: I don't remember knowing about front element focusing lenses. Very interesting! Dan: I know I played with a few lenses. Trying them with single cells, unscrewing a cell to see what the extra spacing would do. As I remember, there weren't any suprises. Some lenses worked with a single cell, some didn't. Unscrewing the front cell produced not much that I remember... In my weary mind, I could easily see a distortion produced by varying the cell spacing as rays meant for the central portion would now be hitting the outer edges (if increasing the spacing). I would guess this would be much worse as the lens itself got shorter in focal length. But I still think it's proportional to the focal length and that's why my 3mm has no apparent affect on the 300mm lens. I'll have to pull out a short lens and play again... And here's another missing brain cell... Who's H.D. Lee??? I really should restrict my replies to times when I'm awake and alert

My error, should have been H. D. Taylor. Designer at T. Cooke & Sons, where he invented the Cooke triplet, later at Taylor, Taylor, & Hobson, responsible for a number of more-or-less landmark designs. No a relative of the Taylors who founded TTH. I really should look things up before posting.

Cheers,

Dan