Spacing Lens Elements

[illumiquest]

I found a 252mm Bausch and Lomb Aero Tessar at my local shop and would like to give a whirl. Problem is it's just the two element groups and I have no idea what the correct space between them should be. Is there some way of figuring this out?

Thanks!

 

[steven_e007]

I have a similar dilema. I have a Ross wide angle 5" aerial photograhy lens that was unscrewed - as two identical elements - from the front and back of a huge shutter / iris assembly. I wanted to mount these in a simple tube to restore the lens to a useable condition, but also don't know what the spacing should be - or how to work it out.

 

[keithwms]

If you have the curvatures of the elements and their approximate indices of refraction, the optimal spacing can be computed. Otherwise you could simply do some systematic experiments...

Here are some simple calculators...

http://www-ferp.ucsd.edu/LASERLAB/TUTOR/lensdesign.shtml

 

[keithwms]

P.S. This might help as well...

http://www.lightmachinery.com/paraxial-ray-trace.php

....but you should be able to find a tessar specific calculator somewhere.

 

[keithwms]

P.P.S. Here it is, the software I was trying to remember...

http://www.lambdares.com/education/oslo_edu/

I think you can plug in your lens data and calculate the optimal tessar formula.

 

[bsdunek]

Most of the lenses were spaced by the shutters. Spacing for Copal shutters is:
dimensions in mm Copal 0 Copal 1 Copal 3
front to rear cells flange 20.0 +-0.025 20.0+-0.025 28.6+-0.025
distance

Information on between the lens shutters for large cameras can be found at: http://www.largeformatphotography.info/shutters.html
And for Prontor shutters: http://www.skgrimes.com/prontor/index.htm


Maybe this will help.

 

[illumiquest]

I have no idea how to figure out the curvature of each element...

Would it work to point the lens at the sun (infinity) set the lens 252mm from the ground and then move the front group back and forth until the sun is a pinpoint? I've put the lens on a shutter (with duct tape since there's no shutter which is going to fit it) and it shoots but does a pretty lousy job.

 

[keithwms]

Sounds like your best option is to decide for which subject distance you want to optimize it, and move the front group around, seeking the best image on ground glass. Perhaps you could use a cardboard tube or some PVC to cradle the elements. (I have some fancy lens mounts in the lab, we use them to set up things on an optical breadboard, but that's a pricey way to go about it; if you have a physics dept nearby, you can probably find somebody there with a breadboard system and they can help out)

Idea! If you use PVC or similar, note that you can get PVC which has mating screw threads, which might make a nice ghetto helicoidal adjustment system You really need some sort of cradle mechanism because having the lenses slightly off axis or tilted will very quickly degrade performance. (OTOH it might degrade in an interesting way!)

The "correct" answer depends what focal length and focal distance you want to optimize for. You might say the only "correct" choice is what duplicates how the lens was produced.

Probably the best thing would be to get your hands on a duplicate lens and inspect that... you could at least measure the distance from the front and rear surfaces with calipers.

Are you planning to cement it back together?

 

[illumiquest]

Well I'm talking about the lens groups, there's the front and back, both are mounted in their holders. I just need to figure out what the spacing between them was supposed to be. I'll see if I can find some pvc which would hold them and try that.

 

[Joe VanCleave]

I was doing this imperically on my adapted optics box camera, testing out various lens spacings. What ended up working well for me was to do it at night, outdoors, with a full or partial moon in the sky for distant focusing, and a closer, well-lit subject, like through a window of your house into a well-lit room. The ground glass is very easy to see at night without a dark cloth, and you can go back and forth, between near and far subjects (the window and the moon) until you've found the optimal lens element or group spacing.

~Joe