Unit Focusing Tessar...

[GPS]

Or put it this way - the only distance the Tessar lenses used for unit focusing (our LF lenses) are "prefocused" with their front element is the - infinity focus. ;-)

[jb7]

"Or put it this way - the only distance the Tessar lenses used for unit focusing (our LF lenses) are "prefocused" with their front element is the - infinity focus. ;-)"


Thanks-

However, on this lens, the distances marked on the lens range from 6' to 100', and on to infinity.
The throw is minute, it seems a very elegant solution to the problem of focusing a lens,
and these cameras seem remarkably well designed overall.

It is probable that these images were meant to be contact printed, and that any reduction in image quality would be entirely unnoticeable- and acceptable.

I think Dan's hypothesis that optimum spacing was set toward the middle of the range seems more plausible;
image quality would be less degraded by focusing in or out by half the distance-
and if I were a lens designer, (which I'm not) then this might strike me as the best compromise-

Given that the main body of the lens remains fixed, I assume that moving the front element changes the focal length?
Focus might appear to be a by-product of this, and in that case, focal length must be variable-

The question is, at what point in the focusing range is infinity focus set for the correct air space, in order to achieve the best optical performance?

I think I'm going to have to test...

[jb7]

There's nothing like a good test in the morning…

First one is an easy one, focal length- based on making an image at 1:1.
I drew a 10cm line, and matched it to the grid on my ground glass.

Results are pretty conclusive, at one end of the focusing scale, object to image distance is 52cm- 52/4= F = 13cm.

At the other end, object to image distance is 55cm - 55/4= F = 13.75cm.
It's quite a short zoom...

So, I reckon that using it at the 13cm focal length would produce the best result.

This is an easy test, interpreting the results of images made at different focal lengths might be more difficult, and perhaps the effects will be quite subtle...

Thanks for the responses-


joseph

[BetterSense]

Also note that many of the lenses from those folders don't actually go to infinity. You may find that it is fudged to only focus to hyperfocal distance at some medium-small aperture (and using a generous CoC at that). I know that my Isolette folding cameras didn't focus to infinity, even when the front unit was set at the "infinity" mark; the infinity mark corresponded to something like the hyperfocal distance for f/16. I refocused them to infinity and made a custom scale, because I'm capable of making my own decisions about hyperfocal focusing.

[GPS]

There's nothing like a good test in the morning…

First one is an easy one, focal length- based on making an image at 1:1.
I drew a 10cm line, and matched it to the grid on my ground glass.

Results are pretty conclusive, at one end of the focusing scale, object to image distance is 52cm- 52/4= F = 13cm.

At the other end, object to image distance is 55cm - 55/4= F = 13.75cm.
It's quite a short zoom...

...
Thanks for the responses-


joseph

Sorry Joseph, but something is wrong. For cameras that use the front element focusing the flange focus distance is always the same (that is the point - this focusing doesn't need to change the image distance , i.e. the back of the lens to the image). The focusing changes only the distance between the front element and the back element of the lens by moving the front element only.
Now, if the elements distance is correctly put to have the whole lens focused at infinity (as it is with LF Tessars) you don't need to put them to a different distance when the lens is used for unit focusing. You only change the elements mutual distance for front element focusing. But then, as said, the back element doesn't move.

[rdenney]

My limited experience with 6x9 folders is that their lenses perform optimally at around 12-15 feet. I suspect that this is where the tessar design is most at home, and once focused there, might perform more consistently across the range if unit-focused after that.

This is somewhat in line with Dan's suggestion. But I think the motivation was based on the common use of these cameras for photographing people.

Cell focusing made rangefinders mechanically convenient for inexpensive folding cameras. I have a Russian folder with an Industar (tessar design) that works pretty well at 12 or 15 feet, but I've noticed the same thing in the results people get with Ikonta and other cell-focused folders.

It may be that the lens body is fix-focused at 15 feet, and then the front cell is adjusted either side of that to provide a focusing capability nearer and farther. If that is the case, then setting the cell focus position to 15 feet and then unit focusing after that seems like the best strategy.

My temptation would be to conduct some tests to determine where the cell focus setting is sharpest, and then lock it down there.

Rick "who found the lens not that satisfying when focused at infinity" Denney

[GPS]

Also note that many of the lenses from those folders don't actually go to infinity. You may find that it is fudged to only focus to hyperfocal distance at some medium-small aperture (and using a generous CoC at that). I know that my Isolette folding cameras didn't focus to infinity, even when the front unit was set at the "infinity" mark; the infinity mark corresponded to something like the hyperfocal distance for f/16. I refocused them to infinity and made a custom scale, because I'm capable of making my own decisions about hyperfocal focusing.

What you say makes sense. But in such a case I would say the focusing is probably just badly adjusted (especially when the infinity is marked on the lens focusing ring) as the Tessars in question don't use any enormous focusing distance for the front element to travel. And a difference in the travel between a hyperfocal distance and an infinity setting is so small that it would not make mechanically any difference to incorporate the true infinity setting to the front element focusing.
If it is the case with Joseph's lens too he'd better to adjust it for the true infinity exactly as you did.

[jb7]

Sorry Joseph, but something is wrong. For cameras that use the front element focusing the flange focus distance is always the same (that is the point - this focusing doesn't need to change the image distance , i.e. the back of the lens to the image).

I'm not sure if you've been reading my posts-

Just to re-iterate-
I've taken this lens off a folding camera, and have mounted it on a view camera.

Adjusting the airspace between the front elements of a Tessar alters the focal length,
that's how it focuses without moving the main body of the lens.

The lens is marked 13cm-
I've measured the conjugate distances for a same size image with the front element adjusted out, and in, and found, by measurement,
that the lens has a focal length of 13cm when the front element is screwed all the way in- marked 6' on the lens.
Screwing the front element out to a marked ∞ alters the focal length of the lens to 13.75cm, that's how it focuses without moving the main body of the lens.

Again, this lens is no longer on a folding camera, I can see what's happening on the glass of my LF camera, it's now an LF lens.
Infinity focus is achieved by adjusting the rear standard of the camera-
as is any other distance-

"Rick "who found the lens not that satisfying when focused at infinity" Denney"

According to what I found when I tested, the infinity mark on the front element of the lens produces the biggest deviation from the designed focal length, so perhaps your finding is not so surprising-

Thanks for the responses, they helped me to sort this thing out-

[Ernest Purdum]

Moving the front element of a Tessar would seem to lead to spherical aberration. I have long wondered (without getting up the energy to do anything about it) if running the front element all the way out would result in enough softness to be worthwhile.

[BetterSense]

What you say makes sense. But in such a case I would say the focusing is probably just badly adjusted (especially when the infinity is marked on the lens focusing ring)

No, because the threads on the front lens cell move the cell in and out a constant amount per turn, whereas the markings on the lens barrel are spaced unevenly, like all such numbers must be, with farther distances closer together than the closer distances. In this situation it's actually not possible to re-adjust the camera's focus. When I adjusted the front cell of the lens to true infinity (using ground glass in the back of my cell-focusing folding camera) and then re-zero'd the lens barrel infinity mark, none of the other, closer markings lined up, not even close. The only way to get the markings to all be correct throughout the scale was to set the infinity mark on the lens barrel to indicate infinity when the camera was actually focused at like 50 feet. There was no way around it but to make a new distance scale entirely.