Cobbling together a lens from Pentax 67 parts

[sdivot]

Ok, maybe you optics experts out there can give me some guidance.
I just bought a gutted Pentax 67 135mm lens, with no glass in it (actually the rear element is there, but it can stay or go).
Just for fun I want to make a gloriously imperfect Keith Carter-esque lens from it. And I do not want to pay $400 for the big Lensbaby.
Here's the question, and forgive my ignorance of optics: Since the shell is designed for a 135mm focal length lens, can I order a 135mm positive meniscus lens from Anchor Optics and get close to my goal?
I'm too old and lazy to spend the next few months studying optics theory.
Any help?
BTW, if anyone has suggestions as to a better choice for lens type (instead of the positive meniscus), I'm wide open for suggestions.
Thanks,
Steve

 

[Roger Hicks]

I'm too old and lazy to spend the next few months studying optics theory.

Dear Steve,

Well, getting younger isn't an option.

That leaves one alternative.

Cheers.

R.

 

[sdivot]

Roger,
I'm going to assume your reply was in good spirit, and not an admonishment. If so, cheers right back at you. If not, well...
Sincerely,
Steve

 

[Roger Hicks]

Roger,
I'm going to assume your reply was in good spirit, and not an admonishment. If so, cheers right back at you. If not, well...
Sincerely,
Steve

Dear Steve,

It was.

Trying to rebuild a lens into an existing broken mount is one of those things where a simple answer is not available. A VERY long time of boring and unproductive study would be essential. Sticking a magnifying glass in a focusing bellows is a better idea.

I don't know how old or lazy you are, but I bet I can give you a run for your money on either or both.

Cheers,

R.

 

[sdivot]

Thanks for the clarification Roger. I thought it was in good spirit.
The magnifying glass in a bellows is essentially what I had in mind. Nothing much more than that, at least.
In this case, the helical focusing of the Pentax lens, and the fact that it fits right on my Pentax 67 with no modifications, is what made this piece so appealing.
I'm certainly not trying to make a perfect lens from spare parts. I was just hoping to get some advice or direction on the best piece of glass to use in it for my specific purposes.
Thanks,
Steve

 

[mtjade2007]

Steve, you actually have an excellent idea. A Pentax 67 135mm lens is a macro lense that can extend its barrel quite far out. You have a lot of opportunities to use this barrel, as long as the P67 mount is intact and you can still mount it to a P67.

Try to get an enlarger lens of 135mm focal lenth. Such a lens will be perfect for your project. The result is going to be a macro lens still but with a small aperature. That means it may be darker. but it should be plenty sharp for macro applications. Enlarger lens usually is small so you will need to figure out a way to mount it to the inside of the barrel. I used to see copy machine lenses from surplus stores. I have seen names such as Rodenstock on those lenses. They should very hign quality macro lens for your application. These lenses have aperature that can go down to f45, great for macro. Good luck.

 

[sdivot]

Thanks for the tips for making the macro lens. I was really more interested in building a general purpose lens, however.
I suppose I can simply cannibalize an old loupe similar to the Mark Tucker plungercam. I'm really just looking for something to play around with and make some unusual images.
The helical focusing, and the perfect fit of the Pentax shell on my Pentax body was what appealed to me most in using it to frankenstein together something resembling a lens.
I'll try out a few things and if I come up with anything interesting, I'll share.
Any advice would still be appreciated. Thanks,
Steve

 

[walter23]

I can't see why any lens with a 135mm focal length that fits in the barrel wouldn't work, except for maybe the issue of vignetting / coverage. The design of the multi-element lens might have allowed rays to clear the barrel and fit the 6x7 format, whereas if you stick a lens inside the barrel it might not be positioned properly to hit the edges of the film (because your lens barrel obscures rays heading towards those edges).

That's the major obstacle that I can think of, but it might not be an issue - I'm not sure. At the very least I'd probably figure out the angle of coverage you'd need to hit the full 6x7 frame, figure where a 135mm miniscus lens or whatever would be placed in the barrel to get infinity focus (ie, 135mm from the film), and see if it's physically possible to cover based on that. You could even figure out just how far out you could focus (to focus on closer objects than infinity your lens has to be further from the film plane) without vignette.

The formula is ( 1/ focal length ) = (1 / extension) + (1/object distance)

So if you figure you can get the lens to sit at about 145mm without vignette, your closest focus would be:

1/135mm - 1/145mm = 0.000510855683 mm^-1

1/0.000510855683 mm^-1 = 1957.5 mm

So your closest focus in this case would be about 1.9 meters