Change lenses focal length.

[Rumbo181]

I've heard many times that soviet lenses to be used in Leicas should have their focal length adjusted to match perfectly, but the adjust is minimal. Recently, I've heard about an old Novar lens taken out of its original camera and adapted to be used in digital.

What I don't know wheter they changed its focal length or not. Let's say they didn't and hence, if you use 4/3 should get something similar to 75x2=150 mm, but what if they did it to get let's say 50mm?.

So I wonder, it's possible to get changes of focal length for a lens? let's say something dramatic, from 75mm to 50mm for instance, would it be possible?, if yes, how would you do it?: change distance to focal plane and looking for a new infinity adjustment?, changing distance between lenses?... or it's plainly impossible?

 

[grain elevator]

It's plainly impossible, at least if you want to maintain decent image quality. Sometimes, the front or rear group of a lens will work as a lens on its own, and that is used on so-called convertible lenses, for large format. AFAIK the part-lenses are always longer FLs than the whole lens. But small format lenses are not made for this, and we usually want to be able to use them at wider apertures.

 

[Dali]

What are you talking about? Focal length or lens register? These are 2 different things.

 

[Rumbo181]

Thank you, grain_elevator. Clarified

 

[Petrochemist]

I've heard many times that soviet lenses to be used in Leicas should have their focal length adjusted to match perfectly, but the adjust is minimal. Recently, I've heard about an old Novar lens taken out of its original camera and adapted to be used in digital.

What I don't know wheter they changed its focal length or not. Let's say they didn't and hence, if you use 4/3 should get something similar to 75x2=150 mm, but what if they did it to get let's say 50mm?.

So I wonder, it's possible to get changes of focal length for a lens? let's say something dramatic, from 75mm to 50mm for instance, would it be possible?, if yes, how would you do it?: change distance to focal plane and looking for a new infinity adjustment?, changing distance between lenses?... or it's plainly impossible?

It is possible to change a lenses focal length by adding extra elements or taking some away. Generally neither is likely to give good results photographically (though stacking two complete photographic lenses can work well for macro).
I suspect what your sources were referring to was either the rear flange distance (some soviet lenses are M39 but not LTM) or to have the rangefinder linkage tweaked to match the lenses focal length.

For use on digital the second of these is unnecessary, but the first can be needed, my industar 69 lens needed a small screw that acted as a stop removed (It has a rear flange of 27.5 being designed for the Chaka camera) There are also M39 lenses for Zenit SLRs with a flange distance of 45.2, & at least 4 others that I've not actual come across (Visoflex, Paxette, Kilarscope & Panflex)

 

[Rumbo181]

What are you talking about? Focal length or lens register? These are 2 different things.

Focal length.

In pure theory change of lens register (flange) is easy. it's complicated from the mechanical point of view but conceptually I think it isn't

 

[Rumbo181]

It is possible to change a lenses focal length by adding extra elements or taking some away. Generally neither is likely to give good results photographically (though stacking two complete photographic lenses can work well for macro).
I suspect what your sources were referring to was either the rear flange distance (some soviet lenses are M39 but not LTM) or to have the rangefinder linkage tweaked to match the lenses focal length.

Yes, it's for range finder coupling, if you were focusing "guesstimating" (actually as do with a laser rangefinder) it doesn't matter as all. So not that it's a problem, it's just an introduction to my question.

 

[Rumbo181]

For use on digital the second of these is unnecessary, but the first can be needed, my industar 69 lens needed a small screw that acted as a stop removed (It has a rear flange of 27.5 being designed for the Chaka camera) There are also M39 lenses for Zenit SLRs with a flange distance of 45.2, & at least 4 others that I've not actual come across (Visoflex, Paxette, Kilarscope & Panflex)

Yes, I use my soviet lenses for fun with a micro 4/3 and as long as you have LTM/M4/3 adaptor, everything is right; you focus with the screen. Focal length is the same, but if you compare with a 35mm you have the "effect" of doubling it -with this particular sensor format.

 

[Rumbo181]

Simply, imagine in the theoretical world if you can take a lens from a medium format camera and with ever mechanical work as needed, transform it in a 50mm lens to fit in a 35mm camera. Is not I was planning to butcher any camera, but I was wandering what actually focal distance depends on.

 

[xkaes]

As Petrochemist has mentioned, it's possible to change the focal length of a lens -- kinda, sorta -- by adding extra elements. That's what any 2X tele-converter does, but it doesn't really change the focal length of the lens itself -- it just adjusts the FL of the lens while it's attached. Tele-converters increase the focal length, and "close-up filters" can be used to do the exact opposite:

https://www.subclub.org/fujinon/close-up.htm

 

[wiltw]

There indeed exist supplemental optics which can increase FL, or (as in recent times have shown) decrease FL (effectively making its max aperture a larger f/stop). And, as exhibited in the few 'convertible' large format lenses) change FL without supplemental optics! But without one those three methods, the FL of a given lens is fixed...if it is a (whatever) '6 element in 4 groups' design, what it is is whatever it is.

 

[ic-racer]

Accessory diopter lenses will change the focal length of the lens.

 

[Petrochemist]

As Petrochemist has mentioned, it's possible to change the focal length of a lens -- kinda, sorta -- by adding extra elements. That's what any 2X tele-converter does, but it doesn't really change the focal length of the lens itself -- it just adjusts the FL of the lens while it's attached. Tele-converters increase the focal length, and "close-up filters" can be used to do the exact opposite:

https://www.subclub.org/fujinon/close-up.htm

There are a total of four options with added elements:

Tele converters are designed to increase the effective focal length of the new combination without changing its focus distance, Focal reducers (such as the speed booster) are designed to reduce the focal length of the combination without change in focus distance.
Then positive diopter lenses 'close up filters' reduce the focal length & cause a shift in focus distance.
Less common are negative diopter lenses which can be used to increase focal length with a shift in focus distance. These used to be used with bellows cameras.

In addition changing the spacing between elements can change the focal length (this is what zoom lenses do) & I guess there is also the more drastic approach of re grinding lens elements too, but I doubt any of us would have any luck with that!

There is a homemade lens group on flickr for more info on the drastic approaches. The group has some excellent resources for optics geeks

Homemade Lens

Welcome to the Homemade Lens Group. This group is for photographers who make their own lenses or who modify commercial lenses. This includes... DIY optics (lenses made from optical lens elements, magnifying glasses, lenses recycled from telescopes, binoculars, etc; lenses made from...

www.flickr.com

 

[Dali]

Simply, imagine in the theoretical world if you can take a lens from a medium format camera and with ever mechanical work as needed, transform it in a 50mm lens to fit in a 35mm camera. Is not I was planning to butcher any camera, but I was wandering what actually focal distance depends on.

A 50mm lens is a 50mm lens, regardless the format. So, to take a 50mm medium format lens and adapt it to a 35mm camera, you need a mount adapter.

Now, if you talk angle of view and no more focal length, it is a totally different story.

 

[r_a_feldman]

I've heard many times that soviet lenses to be used in Leicas should have their focal length adjusted to match perfectly, but the adjust is minimal.

What that refers to can be one of two problems:

1. The focusing helicoid needs to be adjusted so that the lens can focus to infinity correctly on the Leica. It does not mean changing the optical focal length of the lens. The distance for a true LTM lens is 28.8mm, but some Russian lenses actually use a different distance.

2. Some Russian LTM lenses use a different thread pitch in the helicoid, so that while the lens might focus properly at one distance (e.g., 1 meter), it will be off at another (e.g., 20 meters).

 

[OAPOli]

I think the issue with the Soviet LTM camera and lenses is that the RF standard is 52.4mm vs 51.6mm for Leica. If you put a Soviet lens on a Leica or vice versa, the focus won't track accurately via the RF. This can be adjusted via the RF cam follower IF you only use one type of lens. But if you alternate them, one can alter the spacing between the front and rear group the lens with a shim, which changes the focal length slightly.

 

[Rumbo181]

There is a homemade lens group on flickr for more info on the drastic approaches. The group has some excellent resources for optics geeks

Homemade Lens

Welcome to the Homemade Lens Group. This group is for photographers who make their own lenses or who modify commercial lenses. This includes... DIY optics (lenses made from optical lens elements, magnifying glasses, lenses recycled from telescopes, binoculars, etc; lenses made from...

www.flickr.com

Thank you @Petrochemist , really great information!

Besides, thank you all. I think I have now a -hopefully- clear understanding of the matter.