Single element lens design questions...

[Scott --]

Okay, I'm ordering glass from Surplus Shed. Designing a single-element lens, to be used on 8x10 with no aperture. Need some help deciding what, though. Magnifying glasses are double convex, but Surplus shed has positive (and negative) meniscus lenses, achromatic, etc, etc. What's a good choice? And is there any way to predict coverage? Anything else I should be considering, too?

Thanks for any help.
Scott

[Wimpler]

Convex - concave (meniscus) lenses have better spherical aberration properties then double convex.

Achromatic lenses are corrected for chromatic aberration.

The lenses illuminate quite a big angle. 'Coverage' depends on what area you want to be reasonably sharp and reasonbly evenly illuminated. If you are considering this type of lenses you probably don't expect a high quality image. About 50-60 degrees would probably work quite fine.

If it were me, I'd order a convex-concave positive miniscus of 300-400 mm focal length.

Make sure to put the concave side toward the front and put the aperture in front of the lens.

[Steven Tribe]

Now I don't wish to promote e**y. But there is a very interesting Germany lister where the key search "achromat" would find him. He describes his surplus products very well as regards diameter, focal length, clarity and colour and coating. I think you can guess where his lenses come from (Z,S) and they often have slight edge damage. His listing is different in a big way so you will know when you have found him.

[Ernest Purdum]

An achromat would be very desirable. Non-achromats focus different colors onto different planes. In most cases, the image from an achromat would be preferable.

You have some control over coverage and softness when dealing with a simple lens. If you stick your aperture well out in front, you have a narrow angle lens. If the tube is short, you have a wide angle lens, but you may not like the results very much. I would suggest trying to find a lens with a focal length on the long side for your format. As an example, 210mm or a little more if you use 4" X 5". Washer stops slid into the lens tube,and held in place by a snap ring, are the easiest answer to the
aperture requirement. You'll want several so as to find out what softness you desire.

[Steven Tribe]

Yes he is still there - he has the same surname as the "boss" of cafe Central Perk!
Current offerings in "our" sizes are. D42mm - EFL285mm, 34mm - 285mm, 33mm - 400mm. Very simple German.

[Dr Klaus Schmitt]

Yes he sells some Zeiss (Jena) optics, but his ads are full of - anyway. The optics are good and that is what counts. Search for "HQO".

[Scott --]

Thanks for the input, guys. Let me ask you this: I'm thinking a positive meniscus lens; there's a 308mm FL lens, 66.5mm diameter. Gives me a little faster than an f/5 lens. I'm not planning on using an aperture - wide open only. Now, I'm pondering on barrel design for this lens. I want to use a Packard with it, but other than that one restriction, I have no idea how long a barrel it needs, or how inset the lens should be.

Ideas?

[Mark Sawyer]

The "barrel" will function mainly as a lens hood, but will also vignette if it goes too long. I'd say an inch to an inch-and-a-half extension front and rear for 8x10. (Regarding movements, keep in mind you probably want to keep the optical axis pointed at the center of the film, as the lens will likely be visibly sharpest in the middle of its image circle.)

[Richard Rankin]

I just bought a box of stuff that has some 'real' lens barrels in it. If you get an idea of what you diameter/length need, I'll check and see if any of it might suit you. That way you might have an aperture or an existing opening for waterhouse stops at least.

Richard

[Ernest Purdum]

If you rig up a temporary cardboard or plastic tube with a telescoping extension, you can see how long a tube you can use without vignetting. Does your groundglass have clipped corners? If not, remove it for the moment. Look through the lens from each corner and make sure you can't see the tube.

[Dr Klaus Schmitt]

It can deliver quite interesting results...

http://www.pbase.com/kds315/image/127464020/medium.jpg

http://www.pbase.com/kds315/image/127464019/medium.jpg

(both shot on digital in Nancy France / Place Stanislas )

[papah]

Beautiful, Klaus!

Details on the lens you used?

[Dr Klaus Schmitt]

Simple quartz singlet, mounted into old enlarger lens barrel I had laying around.

works also nicely for closeups:

http://www.pbase.com/kds315/image/127464603/large.jpg

http://www.pbase.com/kds315/image/127784848/large.jpg

[desertrat]

Those are great shots! BTW, the biggest problem with surplus lenses, even meniscus lenses, is that the curvatures aren't strong enough to flatten the field. Therefore, most of these lenses suffer badly from curvature of field. This is where most of the softness at the edge of the field comes from, far more so than the other aberrations. The Wollaston meniscus is what you want. The lens curvatures are optimized to give a (fairly) flat field for a normal focal lens. This is what most of the old box cameras used. The lens curvatures follow a certain ratio to give the desired focal length, but I haven't been able to find out what that ratio is yet. My google-fu isn't so great.

You can download for free a lens design program that's used by amateur telescope makers called OSLO.edu:

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

Warning, this is an extremely complicated program, with a steep learning curve. In a couple of years of my spare time I may succeed in learning how to use it properly. ;)

[desertrat]

OK, I was able to find some design information for the Wollaston landscape lens at the following link:

http://books.google.com/books?id=acrmuZpdSFEC&pg=RA11-PA17&lpg=RA11-PA17&dq=wollaston+%2B+lens+%2B+meniscus+%2B+ratio+%2B+radius&source=bl&ots=naA07h7R_j&sig=2Y8xaNlYwVdoRWuCUSv_fjcSMVc&hl=en&ei=b69-TMapG5C-sQO41eCiCw&sa=X&oi=book_result&ct=result&resnum=4&ved=0CCAQ6AEwAw

Scroll down a couple of pages for a discussion of the design and the lens prescription. The lens is tiny, having a focal length of one inch, but it should be able to be scaled up to any focal length by multiplying the radii and thickness by the same constant. I suggest multiplying by 13 to 15. This will give a slightly longer focal length than is customary for 8X10, but the edges of the field will probably look a little better.

[desertrat]

I made a mistake on the focal length of the Wollaston lens in the previous post. It's not one inch, it's one in whatever system of measurements you want to use. I caught this re-reading the article and checking some other sources I have on optics. This lens' focal length can be one inch, centimeter, foot, meter, furlong, etc. That means you should be able to scale it to any focal length you want.:o

[Dr Klaus Schmitt]

Well, I was happy about that curvature, as I achieved what I wanted, to have the outer parts of my images appear (much) more unsharp that the inner parts ... depends what one wants and what for. I have experimented to use a concave element in front or behind which acts as a field flattener - works quite well. Piazzy Smyth once invented that.

[desertrat]

Now that I think about it, the OP probably wanted the same effect. I'm not aware of any Wollaston formula lenses available to purchase, so one would have to make the lens.

Some amateur astronomers grind and polish their own objective mirrors, and a few of them make their own lenses for special purposes. This isn't a beginner's project, but I understand the equipment required can be built in a home workshop. A small lathe is usually needed to make some of the measuring instruments, like the spherometer. Ihave seen plans for a spherometer that doesn't require a lathe to build.

[papah]

I can just see it now...

Aspheric hand lens grinding at home, in pursuit of the perfect "Pinkham & Smith" look.

The analog equivalent of writing Photoshop plug-ins to mimic pictorialist soft-focus.

Count me in! :)

[Maris Rusis]

Single element meniscus lenses are widely available: spectacle lenses! My optometrist sells me 75mm diameter spectacle blanks in a variety of powers for $20ea. The latest on my 8x10 camera is a 2.5D (400mm F.L.) in CR39 plastic that works a treat.

[Steven Tribe]

This is interesting Maris! I had thought that:

1. Plastic lenses are about to dominate the market. How does CR Plastic take to the usual methods of glass working and polishing?
2. The great majority of optometrists/opticians "order" perscriptions from central supply organisations? Like Dentists and crowns!

I would love to hear more about optical properties of modern plastic lenses, too?

[Dr Klaus Schmitt]

Yes, works pretty well, tried that too (as my optometrist is just a few steps from my door)