Sweet spot on unit-focus lenses (such as Agfa Solinar)

[BHuij]

I'm building an MP120TC camera, and plan to use a 6x6 back for it. I already have an Agfa Solinar 85mm f/4.5 ready to go, and I have taken some nice, sharp pictures with this lens before.

My question is this: These Solinar lenses (as well as similar Apotars and Agnars) had front-cell focusing on a helicoid because they were in a fixed position on the Isolette cameras they were designed for. My understanding is that there's a non-trivial optical tradeoff to using that approach instead of unit focusing by moving the entire lens group forward and backward in relation to the film plane.

Since I will now have the luxury of using unit focusing on this technical camera, where should I set the front cell to? Is it going to be sharpest at infinity? Minimum focus? Somewhere in between?

 

[Nicholas Lindan]

If the red dots - snapshot settings - on an Apotar are any guide it looks like f9 with the focus set for 20ft results in 12ft->Inf depth of field. The red focus marks are at 10ft/DOF 7->15 ft, and at 30ft/DOF 14ft->a bit past Inf.

20ft is, I believe, the distance most fixed focus cameras were set to.

If it were me, I would pick 20ft as a reasonable setting. But I wouldn't lose any sleep over it.

 

[BHuij]

Yeah I have a shot from when I was trying to use this lens on a Goodman Zone. It's one of the few shots that turned out really well despite the issues with that camera. I don't even know what the cell would have been set to when I shot it. Probably infinity. But it came out very sharp and I had no concerns about it.

Probably overthinking the whole thing

 

[Dustin McAmera]

If you're planning to unit-focus the thing, it doesn't make any sense to set the front-element at anything less than infinity. Otherwise, if you want infinity focus, you will be using the unit-focusing to correct for the front-element focus. I don't know if that even works; certainly Agfa didn't design the lens expecting it to be used like that.

 

[reddesert]

If you're planning to unit-focus the thing, it doesn't make any sense to set the front-element at anything less than infinity. Otherwise, if you want infinity focus, you will be using the unit-focusing to correct for the front-element focus. I don't know if that even works; certainly Agfa didn't design the lens expecting it to be used like that.

Front element/group focusing de-facto changes the focal length of the combined lens. This means, I think, that the front element has to physically move less distance to focus from infinity to 3 meters, than the distance a unit focusing lens would have to move for that amount of focus travel. That makes it mechanically easier to build into a folding camera, in addition to not having to move the shutter.

However, building your own camera, you can hold the front element in a fixed position, fixing the focal length, and use unit focusing on the lens as if it had no moving groups. The question is at what position the manufacturer optimized the lens (front element focusing introduces some aberration, I think spherical, away from the optimized position). This question has been asked before and here's one reference that says about 40x the focal length: https://www.photo.net/forums/topic/55825-front-cell-focusing-optimal-distance-setting/

It probably doesn't matter an enormous amount as long as you're between say 10x the focal length and infinity. Performance of a lens computed for infinity (front element focus or not) will typically be good between ~10x the focal length and infinity without any special measures, but closer than ~10x the focal length is likely to require more care from the designer. (I think this rule of thumb can be found in optical design books, but am not near any at the moment.)

 

[ic-racer]

It won't make any difference if the 6x6cm camera does not have adjustable centering detents and you don't have a way to align it to fractions of a millimeter.

A typical 6x6cm camera would be spec'd around 0.02 to 0.05mm tolerance for the film plane to lens board.


Image below from https://www.optomeca.fr

 

[Dan Fromm]

Hmm. 120 film isn't that expensive. Have you considered the lens how the marked focus distance affects image quality? Pick several apertures that you're likely to use, pick a couple of marked focus distances -- closest, farthest, several in between -- and after you've built your camera try test shots at near, middle, and far distances. Look at the negatives with a magnifier. Then decide which distance setting gives the best results at which distances.

Why futz around with fuzzy theory when you can measure?