Building a steinheil periskop out of close up`s

[ruilourosa]

Hello

I am planning to build a 500mm lens out of two close up +1 positive meniscus, and maybe interchange the elements to build different focal distances... could someone tell me the distance that the two elments should be from eachother, or from the stop? i am planning to build the lens out of 77mm +1 close up lenses using rings (removing the glass from skylights and uv´s) so the spacing is easier. I have found that this way is a lot easy than alan´s green do it yourself barrel...

so spacing... is there any formula? and for a rapid rectilinear (achromats instead of positive meniscus)? or a double gauss design (two positive meniscus instead of one in each side of the stop)?

i am chasing a relatively cheap relatively bright long lens for collodion... and maybe something else...


Thanks


Rui Lourosa

[Drew Bedo]

First read about Petzval.
Petzval: http://en.wikipedia.org/wiki/Joseph_Petzval#Optics

Here is something on optics design with a few equations
.General information: http://en.wikipedia.org/wiki/Lens_(optics)#Compound_lenses

Let us know what you do.

[desertrat]

From a little recreational reading I did in lens design, the spacing between elements in a periscopic lens controls field flatness among other things. Meniscus elements with fairly steep curvatures give a flat field with fairly close spacing. Elements with shallow curvatures need to be spaced further apart. You might be able to improvise a temporary barrel that lets you try different spacings, and check for field flatness and overall image quality on the GG. A fairly small stop should be used for testing, f22 to f32, otherwise image quality at the edges and corners will be so poor there will be no way to estimate field flatness.

If you build yourself a DIY spherometer (instructions can be found on the web) you can measure the radii of curvature and plug the numbers into a freeware lens design program like OSLO lite or EDU (I forget what the current free version is called). Rough measurements can be taken with a depth micrometer and some machinist's parallels.

[ruilourosa]

well... i have some success already, specially in building a landscape lens (an achromat version of wollaston) but i want to build a better corrected lens... a rapid rectilinear should be fine... but i really needed an expert to answer me some questions... because i do not have a lot of means to do a lot of testing...

petzval is a bit tricky to build, i will keep myself out of ordering order made lenses! i have some achromat close ups and i need to do some building... whaterhouse stops etc...


i will try to keep an update


thanks


Rui Lourpsa

[goamules]

Hello

I am planning to build a 500mm lens ... is there any formula? and for a rapid rectilinear (achromats instead of positive meniscus)? or a double gauss design (two positive meniscus instead of one in each side of the stop)?

i am chasing a relatively cheap relatively bright long lens for collodion... and maybe something else...
...

Oh, you're chasing something alright! "Is there a formula?" Yep, lots and lots of formulas. Lens designers had to understand all this, from the 1700s until today.

[ruilourosa]

I´m refering to a formula that associates the Steinheil Periskop lens or the Rapid Rectilinear to a distance to the diafragm, given the focal distances associated.

i have already 3 minolta achromats of +0,94 ; +2 and +3.8 diopters that means a 1063mm, a 500mm and 263mm of focal distance, that would work well as simple landscape lenses but i plan to add 2 lenses to the equation (2 achromatic pairs) and i would like the element-to-aperture best distance (since they are of different focal distances probably the aperture distance may differ...)

maybe this is an empirical work... and i don´t have an optical bench...

any suggestions?


thanks


Rui

[Mark Sawyer]

well... i have some success already, specially in building a landscape lens (an achromat version of wollaston) but i want to build a better corrected lens... a rapid rectilinear should be fine... but i really needed an expert to answer me some questions... because i do not have a lot of means to do a lot of testing...


Two achromatic doublets don't make a well-corrected Rapid Rectilinear, and two close-up menisci won't make a Steinheil Periskop. The optical engineers who made those lenses put a lot of calculations into the proper correction, not just spacing, but types of glass, the curve of each lens, etc.

You can make a periscopic lens fairly easily, but expect lots of aberrations. Spacing isn't a singular formula, it's a balancing act to find the best compromise between field flatness, coma, spherical aberration, distortion, and other aberrations, and the lens won't be an Achromat or an anastigmat.

Playing with all the aberrations can be educational and fun, and the look from a "flawed" lens design can be beautiful. But coming up with something well-corrected, especially at a relatively long focal length and reasonably fast aperture, is beyond anyone without a strong background in lens design.

A couple of oldies made with a cobbled-together periscopic lens, (my old IWSWGon!), a 12" f/3.9:

[ruilourosa]

well, that´s not particulary true, most designs from the past were purely empirical (petzval were the first to calculate extensively) and good replicas of those lenses can be easely made... i´m not trying to make an amazing lens, just A long lens for a process that is mainly blue sensitive and is not for enlarging...

alan greene in his book makes some lenses and they work moderately well for what they are intended...

just searching for some empirical and practical experience, lens taxonomy is another business, and lens makers and designs apart, i´m trying to make something out of simple things...

this search started when i started to look for a 1000mm or so lens to make big collodion´s in a trailor... i know that there are some lenses with this focal leghts around... but where? and at what price?... this way i will do it


THanks


RUi

[goamules]

well, that´s not particulary true, most designs from the past were purely empirical (petzval were the first to calculate extensively) and good replicas of those lenses can be easely made....

Yep, and all designs after the Petzval were by careful computation. That was 1841. The photographic designs before that were a handful of slow meniscus and the failed Chevaliar. And telescope and microscope optics. Petzval proved it took scientific minds to create good lenses. All other lenses were the work of mathematicians and skilled technicians. No backyard mechanic invented a decent lens since the 1830s....and they were only decent anyway, not good.

But like you say, you are just trying to replicate a known design, not invent one. You should be able to do it once you understand the refractive indices, how thickness affects them, how null out aberrations. I've read that symmetrical designs are good because they cancel out most aberrations, if stopped down quite a bit.

[wombat2go]

You should get a lens design app.

I use OpticalRayTracer and it is too simplistic but simplistic is sometime desirable.

I used it to model a very old periskop prescription from a link given by Dan. I put a post here or Apug, I forget.
It immediately showed the artifacts ( like Mark's chair, I suppose)

Now I am using it to model a double gauss.

I wish you happy ray tracing!

[Drew Bedo]

This is not a trivial project.

If it was easy to design and build a lens that is sharp, well corrected and "fast" in the equivalent of a home workshop, the history of lens development and photography in general would have been very different—from 1840 to the present.

[Amedeus]

well, that´s not particulary true, most designs from the past were purely empirical (petzval were the first to calculate extensively) and good replicas of those lenses can be easely made... i´m not trying to make an amazing lens, just A long lens for a process that is mainly blue sensitive and is not for enlarging...

As Garrett already stated, most, if not all, lenses after Petzval were computated. I have a decent background in optics and have access to optical modeling programs + custom lens building. What you're trying to do is not straightforward, by no means. Yes, you can put to meniscus lenses or two achromats together and get something that resembles a periscope or rectilinear but since you're relying on available lenses you don't have control over either curvature nor refractive indexes. This said, you can combine them and get "acceptable" results (insert your own definition here) if you follow few empirical guidelines.

For starters, use lenses with identical focal length and diameter. If you're chasing a 1000 mm design, then get two 2000 mm focal length lenses. They're pretty hard to find in large diameters. Once you have the lenses, space them between 1 to 2 diameters apart, play within this range. Put the stop in the center and play with the distance between the lenses while observing the different aberrations at play. This is purely empirical. Different lenses, different compromises. When stopped down though, you can expect results like Mark was showing. The more you stop down the better the performance and the longer your exposure will need to be. The latter is not something you want to hear for wet plate ... but you know that already. You will have to improvise a simple optical bench. It doesn't have to be complicated ...




just searching for some empirical and practical experience, lens taxonomy is another business, and lens makers and designs apart, i´m trying to make something out of simple things...

this search started when i started to look for a 1000mm or so lens to make big collodion´s in a trailor... i know that there are some lenses with this focal leghts around... but where? and at what price?... this way i will do it

I understand the need for simple things because when you start looking for the 1,000mm and longer lenses to make large wet plates in a trailer ... well, that's another endeavor. There are indeed few lenses around that will do what you want to do but they all come at a price and you'll have to be patient ... very patient. There are a decent amount of 1200mm lenses around but there's hardly anything that is longer ... I know quite a few people looking for > 1500mm lenses to do large wet plates ;) ... I'm one of them and the message is: be patient and be willing to invest one way or the other. Making large wet plates in a mobil set-up is a relatively expensive habit ... you might find the cost of the lens one of the smaller problems.

Good luck in your search and evaluating lenses, I'm down the path quite a distance now ... ;)

[ruilourosa]

well... i´m not getting such bad results judging by the projections on a ground glass... corners are soft... but nothing to worry about... 2 leica 0,75 achromats are doing a nice job towards a periskop tipe lens, aperture at 11 to 16 seems ok by now... one of my worries was the lack of contrast but so far so good.


cheers and thanks for your help!!!


Rui Lourosa