Optics Problem - Expert(ish) optics advice needed

10/04/2013 03:08:57 PM · #1

So, the gray box (Tessar) is what I have, the blue represents the light path exiting the lens.

My desire is to replace the green box with a lens that will cause the light to follow the indicated path.

My biggest desire is to hear that a second lens's front or rear cell will do just what I need.

10/04/2013 03:25:36 PM · #2

So you are looking to take the diverging rays and re-focus them to form a small, real image, perhaps on a sensor of some sort? ;-)
You can experiment using a simple "condensing lens," that is, single-element double-convex lens. The optical performance of such a lens will be crap, lots of aberration of multiple types. The final solution would be an achromatic doublet or apochromatic triplet, but those can be quite expensive. You need to determine the focal length required prior to investing in the final solution.

ETA:
Question: what is the diameter of the rear element on the Tessar?

Message edited by author 2013-10-04 15:30:43.

10/04/2013 03:29:51 PM · #3

Originally posted by kirbic:

Admittedly, a 50mm f/0.4 does sound pretty good doesn't it?

10/04/2013 03:37:29 PM · #4

Now, if I'm right, the rear-cell of a tessar is in fact an achromatic doublet. And that exact thought was what started me down this path.

So, would the rear cell of a tessar be a good experimental unit to try? Seems a likely candidate to me, but I'm not versed enough in light-diagrams to predict what would happen if I set this up:

10/04/2013 03:39:42 PM · #5

Originally posted by kirbic:

ETA:
Question: what is the diameter of the rear element on the Tessar?

About 6 inches, front element is something like 7.

One of the lenses I currently have is a 300mm f/2.5 the other is a 600mm f/5.6.

I have an opportunity right now to score another 600 f/6 tessar, and I'm thinking about doing it. Just wondering if it'd work for this, since it's disassembled, out of shutter, etc, so I'd have to build a housing for it if I wanted to use it as a lens itself.

Building lenses has worked out, so far, to be a bit more expensive than buying L glass, but it's damned fun, and edumactaional tu.

Message edited by author 2013-10-04 15:46:41.

10/04/2013 03:48:40 PM · #6

The rear element of a "Tessar" design lens is a cemented achromatic doublet, true. Now, given that the exit pupil of your primary lens is *huge*, you are going to need a very large lens to capture the entire projected image from the lens. That's a very expensive problem.

ETA: I believe the only way to know if the rear element from another of these old, huge Tessars will serve the purpose is to give it a try.

Message edited by author 2013-10-04 15:50:19.

10/04/2013 03:53:25 PM · #7

Originally posted by kirbic:

The rear element of a "Tessar" design lens is a cemented achromatic doublet, true. Now, given that the exit pupil of your primary lens is *huge*, you are going to need a very large lens to capture the entire projected image from the lens. That's a very expensive problem.

ETA: I believe the only way to know if the rear element from another of these old, huge Tessars will serve the purpose is to give it a try.

:)

Sigh, such are the problems facing those who want to do something different.

I think I'll go for it, at least you've given me some level of confidence that I might expect a usable image to be produced.

Fortunately, I don't need to capture the entire light-cone, I'll be perfectly fine with a center 80-90%.

bhuge

Canon EOS-550D Rebel T2i

Canon EF 50mm f/1.8 II

10/04/2013 03:54:09 PM · #8

My optics knowledge is very limited, but if you use a condensing lens won't it just narrow the light path and in turn destroy any sort of image focus. I thought they were just used to change the size of a uniform light source.

10/04/2013 03:59:48 PM · #9

Originally posted by bhuge:

I think that's true - but to be fair, destroying focus is sorta what I'm after, it's just that I want to also have an area in focus. ;)

I'm betting that a condensing lens wouldn't be completely unusable, but it'd be close enough that my current solution is superior. :D

The obstacle I'm trying to overcome is the fact that right now I need two healthy people to move this camera. That's not good. Impressive perhaps, in an egregious sort of way, but not good.

I'd love to design a smaller solution that was portable.

Focusing the thing, of course, would be the next problem.

10/04/2013 04:19:27 PM · #10

Originally posted by Cory:

Focusing the thing, of course, would be the next problem.

Should not be your biggest problem... at all. Ass-u-me-ing that the image formed by your secondary lens is much smaller, the projection distance (back-focus distance) from the secondary lens will also be much smaller. Adjusting this distance will change the focus of the system.

10/04/2013 04:23:34 PM · #11

Here's some more food for thought. You can approximate the primary Tessar as a "simple" lens of known focal length. You also know the f-number, so you can figure out the "aperture" or diameter of the equivalent simple lens. Now, model this lens using some free optical design software, and add the desired element(s) behind it. You should be able to figure out focal length and spacing you need.

Canon 12x IS USM (S3 IS built-in Lens) -- 36-432mm (35mm EQ)

10/04/2013 05:03:24 PM · #12

Originally posted by kirbic:

Originally posted by Cory:

Focusing the thing, of course, would be the next problem.

Should not be your biggest problem... at all. Ass-u-me-ing that the image formed by your secondary lens is much smaller, the projection distance (back-focus distance) from the secondary lens will also be much smaller. Adjusting this distance will change the focus of the system.

Which, of course means that now instead of moving centimeters to fine-focus, I'll need to move microns, and any play in the system or misalignment becomes much more problematic.

What fun! :)

GeneralE

Canon PowerShot S3 IS

10/04/2013 05:16:18 PM · #13

Not that it's immediately relevant, but this seems like as good an opportunity as any to point out that the site EvilMadScientist.com is NOT one of Cory's projects ... :-)

pamb

Canon EOS-5D Mark III

Canon EF 70-200mm f/2.8L IS USM II

10/04/2013 05:36:01 PM · #14

Originally posted by GeneralE:

Not that it's immediately relevant, but this seems like as good an opportunity as any to point out that the site EvilMadScientist.com is NOT one of Cory's projects ... :-)

Are you sure about that?

DrAchoo

Panasonic Lumix DMC-LX5

Canon EF 24-105mm f/4.0L IS

10/04/2013 06:14:44 PM · #15

Is there anything Fritz doesn't know?

Bear_Music

Samsung Galaxy S24 Ultra

Sony 24-70mm F4 Vario-Tessar T* FE OSS

10/04/2013 07:21:11 PM · #16

Originally posted by DrAchoo:

Is there anything Fritz doesn't know?

Well, I'd wager he knows about as much about poetry as I do about engineering... :-)

DrAchoo

Panasonic Lumix DMC-LX5

Canon EF 24-105mm f/4.0L IS

10/04/2013 07:27:28 PM · #17

Well, lucky for Fritz' ego there aren't a lot of people on DPC asking for help on their iambic pentameter...

Can Fritz help me here?
The light flies impreciesely
like haiku of 18

Bear_Music

Samsung Galaxy S24 Ultra

Sony 24-70mm F4 Vario-Tessar T* FE OSS

10/04/2013 07:32:21 PM · #18

Originally posted by DrAchoo:

Well, lucky for Fritz' ego there aren't a lot of people on DPC asking for help on their iambic pentameter...

Can Fritz help me here?
The light flies impreciesely
like haiku of 18

Let Bear help you out:
When light flies imprecisely,
Haiku IS 18!

DrAchoo

Panasonic Lumix DMC-LX5

Canon EF 24-105mm f/4.0L IS

10/04/2013 07:39:13 PM · #19

:)

But that lacks the deeper subtlety of actually having the problem it describes.

Bear_Music

Samsung Galaxy S24 Ultra

Sony 24-70mm F4 Vario-Tessar T* FE OSS

10/04/2013 07:44:29 PM · #20

Originally posted by DrAchoo:

:)

But that lacks the deeper subtlety of actually having the problem it describes.

Or adds the even deeper subtlety that, in MY version, the light does not fly imprecisely :-)