Unit Focusing Tessar...

[jb7]

I've just separated a 13cm f/6.3 Zeiss Tessar from an old Kodak folder-
It's going to fill a niche in my 4x5 lens set...
Not a bad old lens, not perfect, but I'm looking forward to seeing what it does-
it's in quite an accurate sounding dial set shutter too-

This is a front element focusing lens, and I have a question...

If I focus the lens as a unit, what position should I set the front element to?
Infinity, closest focus, ⅓ in, ⅓ out?
Yes, I know, I should test- and I'll probably run a comparison at each end, and see if I can tell the difference, but perhaps someone here can point me in the right direction?

[GPS]

By moving the front element of this Tessar lens ahead you focus it at the closer range - with a small degradation of the image quality compared to the focusing of the lens as a unit for the same distance. So it seems to me that it would be advantageous for you to focus the lens at infinity before you do your unit focusing with it at a closer distance.

[jb7]

Thanks GPS- your hypothesis makes sense-
especially as the focal length of the lens should be calibrated for infinity, by definition...

Assuming optimum performance is delivered with the correct air space for the focal length, how might the quality of the image be altered by moving the front element for focus?
I presume that what is actually happening is that the focal length is being altered?

Should I be looking for less resolution overall, or at the edges, some field curvature, or something else?

I look forward to having a little test, though I'm quite certain that I'm not going to find any differences that couldn't be blamed on my usual errors...

[Dan Fromm]

I'd expect that the lens is best set for midway between infinity and the near focusing limit. This because that will give better performance than having it right at one end of the range and very wrong at the other.

FWIW, I have a 1912 vintage 130/6.3 CZJ Tessar extracted from a Folding Pocket Kodak. It is unit focusing, shoots very well on 2x3. I expect that yours will please you.

Which Kodak camera did yours come from? I ask because the f/6.3 Tessar, both Zeiss and B&L, was a premium lens in its day and was fitted to EKCo's top-of-the-line cameras. All of the cameras in catalogs at http://mgroleau.com/catalogues_kodak/ that had f/6.3 Tessars had unit focusing lenses. Also, if yours is echt Zeiss, not B&L, EKCo last used Zeiss lenses, at least in North America, in 1915. I've seen mid-20s Kodak UK folders with Zeiss lenses, all unit focusing.

1915 and earlier Tessars in EKCo (North America) cameras were all in Compounds. What shutter is yours in?

[EdWorkman]

In my experiences with kodak Monitors, the lenses are "adaequate" at f 11 and smaller holes- noticeably not very sharp at and near wide open, but way back then I had only VP. TX or HP5 should allow small holes with reasonable shutter speeds. The other question is what is the camera to subject distance and why not use that to set the lens- it does focus, to an extent, after all.

[jb7]

Thanks Dan, Ed-

Correcting the focal length for the middle of the range would seem to be a sensible approach too-

The lens came out of a 1A Junior Series 2- the camera is not in great condition,
and although I could cobble together a system for exposing 120 film in it (it takes A116 film)
I think I'd get better use out of it on 4x5. It's going to be much easier to focus using the camera than the lens...

I looked up the serial number on the lens- 1929. It's in a Compur Dial Set shutter-

I'm not necessarily looking for maximum resolution all the way to the edges,
though I seldom shoot at infinity anyway- but I'm looking forward to seeing how it looks-

Thanks for the responses-

[IanG]

I'd expect that the lens is best set for midway between infinity and the near focusing limit. This because that will give better performance than having it right at one end of the range and very wrong at the other.

FWIW, I have a 1912 vintage 130/6.3 CZJ Tessar extracted from a Folding Pocket Kodak. It is unit focusing, shoots very well on 2x3. I expect that yours will please you.

Which Kodak camera did yours come from? I ask because the f/6.3 Tessar, both Zeiss and B&L, was a premium lens in its day and was fitted to EKCo's top-of-the-line cameras. All of the cameras in catalogs at http://mgroleau.com/catalogues_kodak/ that had f/6.3 Tessars had unit focusing lenses. Also, if yours is echt Zeiss, not B&L, EKCo last used Zeiss lenses, at least in North America, in 1915. I've seen mid-20s Kodak UK folders with Zeiss lenses, all unit focusing.

1915 and earlier Tessars in EKCo (North America) cameras were all in Compounds. What shutter is yours in?

Are you sure EKCo last used Zeiss lenses in 1915 or do you mean Zeiss badged Zeiss lenses :D

I'm fairly sure Zeiss lenses with first their own name then later Kodak's were on EKCo's Recomars, OK the cameras were made in Germany at Kodak's Nagel factory but for the US market and finally assembled in the US (range finder & lens).

Kodak's other pre-WWII European plants came under Kodak Limited (UK) a separate company. Kodak did shift their headquarters for a short time to the UK after taking over Wratten & Wainright, who's research department under Mees provided the backbone for both EKCo (Rochester) and Kodak Ltd (harrow) research.

I must mount my pre-WWI (1913) 165mm f6.3 Tessar on a lens board again, it's the sharpest Tessar I've used on a 5"x4" camera. Just got a load of cheap LF film from someone going fully digital so will do the Tessar/Xenar/G Claron/Sironar tsets next week :D

Ian

[Dan Fromm]

Ian, EKCo was and is a multinational firm. The North American branches (US, Canada) last used Zeiss-made lenses in 1915, at least on Vest Pocket and Folding Pocket Kodaks. After 1915 B&L-made "Zeiss" lenses were not engraved "Zeiss." B&L continued making Tessars and Protars until some time after WW-II.

After WW-I Kodak North America used B&L and then their own lenses. If you wade through the catalogs on Mario Groleau's site -- tedious -- you'll see that during the '20s EKCo phased out B&L and other makers' lenses, phased in their own.

As I reported, Kodak UK used lenses by Zeiss (and other makers, which I didn't mention) after WW-I. Kodak France used French-made lenses, presumably for tariff reasons.

I have several f/6.3 CZJ Tessars, also some B&Ls. The B&Ls aren't all the same and not all of them all shoot really well. Some of this may be due to the cells being (my opinion) in the wrong barrels but my two 85/6.3s, both extracted from Premo #12s, really are different and one is much better than the other.

[GPS]

Thanks GPS- your hypothesis makes sense-
especially as the focal length of the lens should be calibrated for infinity, by definition...

Assuming optimum performance is delivered with the correct air space for the focal length, how might the quality of the image be altered by moving the front element for focus?
I presume that what is actually happening is that the focal length is being altered?

Should I be looking for less resolution overall, or at the edges, some field curvature, or something else?

I look forward to having a little test, though I'm quite certain that I'm not going to find any differences that couldn't be blamed on my usual errors...

Lenses, Tessar being no exception, are made to be unit focused. Only some of them, Tessar being among them, can be focused by moving the front lens element. But in such a case, their optical performance is slightly impaired, if still to an acceptable degree. On a higher end of these small cameras, Tessar lenses were unit focused!
There is no doubt that using the front element focusing the lens has less good optical performance than that of unit focusing. It was a poor man's focusing, to exaggerate a little bit, accepted in some cameras for its mechanical advantage.
I cannot tell you exactly what optical corrections are hit most when you use the front element focusing but the general knowledge speaks about the overall optical performance.
Optically, you have nothing to loose, only to gain, when you focus your lens as a unit - lenses like it more that way...:)

[GPS]

Focusing a Tessar as a unit to infinity doesn't make it worse for focusing - as a unit - at a closer range. It is simply the most correct way of focusing. Tessars are not "half way prefocused" with their front element before they are put to the photographer's hands to be unit focused, that would be simply a silly idea.

[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.

[GPS]

Joseph, sure I read your posts. It's just that we were speaking apples and oranges. You said that "object to image distance" was changing.
I'm used to know that with the front cell focusing the image conjugate is not changed, what changes is the subject conjugate.
And yes, the front cell focusing focuses by changing a little bit the focus length of the lens, making it a short zoom.
Anyway, if it clears things for you, it's all right...

[jb7]

Thanks- yes, I think those tests helped clear up my original question-

Ernest, that may be the case, I'll have a look and see if I can find any evidence...

Bettersense, I've calibrated a helical to a camera before, and I think I'd be more concerned about accurate focus in the near scale, rather than infinity-
50 feet can be quite close to infinity anyway, depending on your focal length and maximum aperture-

[GPS]

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.

Yes, the front cell focusing affects the SA. Normally it is considered that the front element focusing can be acceptable up to 1m of close distance focusing. It seems to me, that what is worthwhile for you is exactly on the opposite side of the scale, so perhaps you could have that 1m as a starting point...:)

[Bernard Kaye]

All of us (including British armed forces in WWII) who used Super Ikontas and Ikontas and their front element focusing cousins must have been very forgiving and Zeiss wasted engineering effort in designing Zeiss wedge incorporating coupled range finders for these front focusing Tessars. Again, much depends on having front standards parallel to film plane: my experience in camera retailing and "dealing with" pros and advanced amateurs was to seldom find parallel to each other front standards and film planes. Correcting that in large and medium format produces improved images. Bernie