Focus of a Franka Solida with a Trinar lens

[SalveSlog]

The Trinar is f:2.9 with front cell focusing. The camera seems pretty original and has 1 thick cardboard ring at the back of the front standard and 1 thick plus 2 thin ones in front of it. If I adjust the focusing ring for correct at infinity, at 1.5 meter true distance, the ring reports the distance is circa 1.7. The other way around: when setting ring correctly at 1.5 meter, infinity is not acceptable.

Can the cardboard rings be moved to make focus correct at both long and short distances?

 

[AgX]

I do not undertand these shims at all. For gaining enough extension a thicker piece would make more sense.
Fine adjustment can be made by adjusting the scale and arrest at the helicoid.

What then remains as explanation is adjusting the lens in barrel seperately coarsely and then doing the fine adjustments by shims instead after mounting the assembly to the body.
But what benefit does this have?

 

[BradS]

A spacer is expected between the shutter and the front strut (aka front standard). This is supposed to set the proper lens-to-film distance when the lens is focused on an object at infinity.

So set the lens at infinity and add or subtract spacers as needed to get the image at the film plane in focus.

 

[SalveSlog]

Would this lens' infinity be when the front cell is turned all the way to the bottom? (Without the scale ring.) This was not the case with this camera when I got it, and I think that would throw off infinity completely, even if placing all those shims in front of the front standard.

This lens in its current position on the front standard requires less turning of the front cell than the focusing ring ask for, to go from infinity to 1.5m in focus. So my question is: Do I have to move the lens closer to, or farther from the film plane, to increase the required turning of the front cell?

 

[Ian Grant]

Yes Infinity is when the cells screwed right in (to the bottom), It's best to use some ground glass to check focus.

Ian

 

[AgX]

I still do not get the idea behind the thin shims, in case there is a adjustable scale with arrest.

 

[bernard_L]

1. Without optical knowledge. These cardboard spacers were clearly not intended by the designer of the lens/camera.
   
2. With elementary optical knowledge. The spacers as you describe them are altering the spacing between the front and rear groups of the triplet. (please confirm that here is indeed glass, and not just shutter/diaphragm, on either side of the standard and added cardboard spacers). As a consequence, the focal length of the assembly has changed, and the engraved focus scale cannot be correct over its range.

My guess is that these spacers result from an ill-founded attempt at DIY focus adjustment. Accordingly, my advice is to remove these spacers, and to perform the focus adjustment the usual and proper way: separate the focus ring (with the scale) from the front cell (probably three grub screws). Adjust infinity focus either with a ground glass and an object at >=100m, or using the Rick Oleson method http://rick_oleson.tripod.com/index-123.html (recommended). Then gently slip the focus ring back in place and secure the grub screws. Careful with the grub screws: unscrew them only as far as needed to get the focus ring free; if they fall off you may lose them and it may be tricky to re-insert them; do not apply excessive force when screwing back, as the slotted end is fragile. While you are servicing the camera, check that the front standard is parallel to the film gate; possibly the cardboard thing was an inappropriate response after the camera was dropped?

When you cross-check the near focus, bear in mind that with vintage folders, the distance scale is often from the lens and not from the film plane; see: https://www.photrio.com/forum/threads/folder-distance-scale-origin-point.160778/

And... please report back on the final outcome.

 

[AgX]

With elementary optical knowledge. The spacers as you describe them are altering the spacing between the front and rear groups of the triplet. (please confirm that here is indeed glass, and not just shutter/diaphragm, on either side of the standard and added cardboard spacers). As a consequence, the focal length of the assembly has changed, and the engraved focus scale cannot be correct over its range.

Interesting point. BUT all three spacers just change the distance of the complete lens/shutter assembly to the front standard.

To me here a thick spacer might make sense, to keep the full throw of the helicoid. But not the thin ones as fine infinity adjustment should be possible at the helicoid.

 

[bernard_L]

BUT all three spacers just change the distance of the complete lens/shutter assembly to the front standard.

Correct. I should have looked more carefully;

 

[bernard_L]

Inner negative element has been, during a previous "repair", re-inserted wrong way around ???

 

[Ian Grant]

I still do not get the idea behind the thin shims, in case there is a adjustable scale with arrest.

I have a Trinar with front cell focussing, there's no adjusting the Focus stop, taht has to be done with spacers.

Ian

 

[BradS]

It may help to realize that almost every type of camera other than a folder, the focal length of the optic is fixed and focus is achieved by changing the image distance. A folder however, achieves focus by changing the focal length of the optic. The image distance is fixed!

In a folder, the image distance is fixed. It cannot change!

For this reason a spacer (of appropriate thickness) is very definitely intended and necessary due to the facts that the distance between film plane and lens is fixed and the infinity stop of the lens is also fixed.

So we must insert a shim between the lens and front strut if we are to be able to achieve infinity focus when the lens is set to its longest focal length - when the front element is screwed all the way in.


EDIT: Note that many large format field cameras achieve focus by moving the front standard - simultaneously changing both the image distance and the object distance. As can be imagined this is a design compromise that is not without problems. It makes it difficult and sometimes nearly impossible to achieve focus on very close objects. When doing macro work with one of these cameras, it is often much easier to achieve fine focus by moving the whole camera (changing only the object distance) instead of using the on camera focus control that moves the front standard.

 

[SalveSlog]

Brads, thanks for making me aware of the functioning of front cell focusing! Might be the key to solve this problem.

 

[BradS]

Brads, thanks for making me aware of the functioning of front cell focusing! Might be the key to solve this problem.

Thanks. I hope it helps. I typed up several paragraphs of mostly theoretical background but abandoned it in favor of a simple summary statement.
Please be assured that you need to use some thickness of shim between the shutter and the front strut to set the infinity focus image distance.

Don't loose your mind over the loss of accuracy at the close focus end of the scale. If it is off by a little, it is likely due to the fact that it is really difficult to focus on something at infinity (!). you have to use a small piece of ground glass in the position of the film and a powerful loupe and a dark cloth and then, its still really difficult to "nail" it. You may also not be able to achieve the exact offset necessary with the shims you have.

You have to have a relaxed attitude about old folders...after all it is just a simple camera, made for the masses, with a simple lens that uses "guess" focus. Folders are convenient and fun. have fun and don't worry about the small stuff.

 

[AgX]

Brad, whether one moves just the front cell or the whole lens assembly with its shutter, makes no difference concerning shims.
Furthermore countless 35mm models with front cell focusing were mode. Without sucg shims.

Ian however brought up a valid explanation (post #11).

 

[BradS]

Brad, whether one moves just the front cell or the whole lens assembly with its shutter, makes no difference concerning shims.
Furthermore countless 35mm models with front cell focusing were mode. Without sucg shims.

Ian however brought up a valid explanation (post #11).

I am trying to HELP the OP. I am not speculating about what is possible...

I don't think you understand the situation with this specific camera. Please re-read what I have written here. . I've explained the issue as best I can. I've worked on these cameras -both the Franka Rolfix and the Franka Solida with the Trinar, Radionar and Frankar lenses. They all have a fiber shim (at least the ones I've seen do). I assure you that a shim is necessary on this camera....without it, you cannot hope to focus at infinity. I am not hypothesizing about some general case. I am speaking of the specific camera and lens that is the subject of this thread....

You may be assuming something about the camera or the lens that is simply not true. Like the assumption that the lens has an adjustable infinity stop. It does not. Or you may be assuming that there is no manufacturing variance in the focal length of these lenses - there is. Or you may not have considered the fact that the distance between front strut and film plane is shorter than the actual focal length of the lens by design - to accommodate a variety of lenses (trinar vs solinar for example) and to accommodate manufacturing variances.

No matter what, the lens equations still dictates that...

1/focal_length = 1/image_distance + 1/object_distance.

I reiterate, the distance between front strut and film plane is FIXED (!) and a tiny bit shorter than the nominal focal length of the lens. This appears to be by design - to accommodate various lenses and compensate for manufacturing variance. Thus the need for the shims. To set the lens at the proper image distance when the object distance is infinity. When the object distance is infinite, and the focal length of the lens is fixed at its longest (at the infinity stop), one must adjust the image distance to achieve focus. This is what the shims are for...

 

[AgX]

Brad, you misunderstood what I wrote.

One can mount both, a lens with front-element focusing or a complete lens in a helicoid in a standard that got a fixed distance to the film plane

Furthermore in both cases one could do the calibration at the helicoids if the infinity arrest is adjustable.
The difference between both systems is, that adjusting with the full barrel in helicoid is basically no problem, whereas adjusting with just the front element can bring the lens out of its correction state.

 

[BradS]

Brad, you misunderstood what I wrote.

One can mount both, a lens with front-element focusing or a complete lens in a helicoid in a standard that got a fixed distance to the film plane

Furthermore in both cases one could do the calibration at the helicoids if the infinity arrest is adjustable.
The difference between both systems is, that adjusting with the full barrel in helicoid is basically no problem, whereas adjusting with just the front element can bring the lens out of its correction state.

Yes. Of course. But this is a statement of what is theoretically possible. It does not address the OP's specific question.