lens adapters and focusing past infinity

[dlove5]

This may be a dumb question, but here it goes:
Is there anything wrong with having a lens focus a bit beyond infinity other than not having quite as close minimum focus?

I know some lenses are designed to focus a little beyond infinity to allow for temperature variation.

I am wanting to use some of my Leica Visoflex lenses on my Pentax SLRs. Eventually I want to use them on a yet to be purchased Pentax dSLR.

I have collected various adapters and extension rings over the years. Currently the best I can do is about a millimeter or so short or long of the proper lens flange to focal plane distance. This creates a situation where either I can focus a bit beyound infinity or I can't quite reach infinity.


Someday I will try to get an adapter has the correct length, but for now I'd like to make to with what I have.

Thanks.

 

[pvdhaar]

Very much depends on the lens..

If you're using lenses that focus entirely by helicoid extension, then there's no difference between extending or having it at a different register and compensating the extension. In short, it's the distance of the optical elements to the film plane that counts..

But if you're using lenses that focus by moving lens elements with respect to each other, you can run into problems. Often the rear element has to have a specific distance from the film plane to achieve decent image quality. But for some lenses like Tessars with front cell focussing, the extension of that single cell is so limited that you can't compensate the register offset enough to get anything in focus at all..

 

[mike goldberg]

I don't understand the technics too fully, however...

On Adapters, I've found that quality SM to M adapters like the Voigtlander or Bower, are well made, thin [1.1mm] and do the job. A bit expensive, they are worth the money.

I've mentioned my J8 bought from, and collimated to the Leica standard by Brian Sweeney. A superb lens, it focuses well at Infinity and close-up. Brian told me in an email earlier this year, that many J8's need a 1/4 turn of the rear "standoff ring" CCW, if they are back focusng.
Good luck...

 

[dlove5]

Peter and Mike,

Thanks for the answers. I should be fine then, because I know most of the lenses I am interested in adapting are Leica Visoflex lenses that use a separate short helicoid focusing mount.

I also will see if I can shim some of the adapters to get closer to the proper film plane to lens flange distance and still get a tight fit.