Can I Calculate The Focusing Distances Between Lens and Sensor?

[ChampionProcrastinator]

I'm trying to work out an adapter for my 1950s-era Baltar movie camera lens that has no focusing unit. On the movie camera from which it came the lens was focused using a helicoid built into the lens mount.

Is there a way to figure out how far or close the lens needs to be from the sensor to focus?

In other words, the lens has to move toward and away from the sensor to focus at different subject distances. How can I figure those numbers out? My plan is to take the focusing unit from a discarded lens and adapt it to my Baltar or alternately to use bellows of some kind.

It's a 90mm lens, the flange focal distance is 46mm, and I'm going to use it on a Nikon mirrorless, if that information is helpful.

 

[sensiblename99]

...It's a 90mm lens, the flange focal distance is 46mm, and I'm going to use it on a Nikon mirrorless, if that information is helpful.

Having a focal length of 90mm suggests that its flange focal length will be close to that. The easiest way to find out how far the lens needs to be from the sensor to achieve infinity focus (ie the closest it will ever need to be to the sensor) is:
simply to point the lens outside your window at a bright subject approx infinity distance (preferably not the sun else you will burn stuff), place a piece of tracing paper (or similar) behind the lens, adjust the distance of the lens until the distant subject is in focus, measure the distance from the base of the lens (the surface which will be mounted to an adapter). This distance is called the flange focal length and tells you how close the lens needs to be to the sensor to allow focus at infinity. It tells you how much space you have for adapters and helicoids.

Given that its a 90mm lens it's very likely that you can put a helicoid between the Nikon body and the lens. This helicoid then allows you to focus the lens at various distances. It's the same process as adapting enlarging, projection or other lenses which don't have a helicoid.






this is an enlarging lens with a helicoid, but the principal would be approximately the same for your lens.

 

[jiberlin]

I'm trying to work out an adapter for my 1950s-era Baltar movie camera lens that has no focusing unit. On the movie camera from which it came the lens was focused using a helicoid built into the lens mount.

Is there a way to figure out how far or close the lens needs to be from the sensor to focus?

In other words, the lens has to move toward and away from the sensor to focus at different subject distances. How can I figure those numbers out? My plan is to take the focusing unit from a discarded lens and adapt it to my Baltar or alternately to use bellows of some kind.

It's a 90mm lens, the flange focal distance is 46mm, and I'm going to use it on a Nikon mirrorless, if that information is helpful.

You already have the essential information. I think the mentioned flange focal distance id for infinity focus. This, together with the flange focal distance for the Nikon Z (16mm) gives the minimum thickness of your adapter. I would suggest to get a thin Nikon Z to M42 adapter and a M42 helicoid. The mount the lens to a M42 macro ring.

The problem with your idea of using an old lens is to get a lens that is short enough but has enough focus travel. If you use a M42 lens together with a normal Nik-Z M42 adapter a 'normal' lens (~50mm) is short enough to mount your Baltar lens, but you would habe only very limited focusing range. If you use a 135mm lens, the focusing range would be good, but the lens is to long to mount your Balta lens.

 

[ChampionProcrastinator]

Ah ha. The lens itself mounts into a clamp and is held in by set screws. Based on the advice from both of you I found this adapter. Is this what you guys are talking about? And then the m42 end would screw into the M42 helicoid?

 

[jiberlin]

To play with some projection lenses i have used a short M42 macro extension ring and 'fitted' the lens with many layers of electric tape wounded around the lens. Not a perfect solution, but good enough for testing, and used M42 extension rings are available for near nothing.