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Tolerances of optical elements in lens


Ethan Brake
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I've been mulling what it would take to make a lens from scratch, particularly a projector lens. With proper machining equipment, I believe I could construct every part of the lens except the actual glass elements themselves. As far as designing the optics, instead of ordering custom glass elements, I could take advantage of the low cost of mass produced elements from companies like Edmund Optics. However, I'm wondering just what kind of tolerances in the glass thickness would be acceptable. For example, in an old episode of "How It's Made", they show the construction of a Canon EF 500mm F4 L IS USM lens, a $9000 lens, and they mention that the glass elements have a tolerance of +/- 0.001mm. That kind of a lens has far more precision than I would need. The cheapest elements from Edmund Optics however have a tolerance of +/- 0.1mm., something I would guess is way too low, but I'm not sure.

This is for projecting Super16 film in a modified projector. Really, I don't think I would need better precision than what you would find in a $100 DSLR kit lens. My question is: What kinds of tolerances would be acceptable, and/or what kind of precision is used in mass produced kit lenses.

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I'm curious why you'd want to go to so much trouble when there are plenty of cheap projection lenses around that would cover S16?

Do you have any experience in lens design? 

I suspect there are a lot of tolerances involved, not just lens thickness, but also things like diameter, surface, refractive index, centration, wedge, and spacing. You need to think about how to mount optics with curved surfaces, how to secure them and how to measure optical dimensions that you can't measure physically. It could be a fun project if you don't have terribly high expectations.

Here's a very basic introduction to tolerances in optical systems:

https://wp.optics.arizona.edu/optomech/wp-content/uploads/sites/53/2016/08/10-Specifying-optical-components.pdf

But you probably want to read up on lens design a bit if you're serious.

I know from servicing lenses that certain elements - ones with strong curvatures for instance - require more care than others. A fast wide angle lens needs much tighter tolerances than a slow normal lens.

Edmund Optics have a help line that you could call to get some advice.

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With stock lenses, such as those from Edmund, you are not concerned with the tolerances of the elements themselves.  All you are doing is machining the barrels, which is not difficult for someone with the tools and know-how, but I get the impression that you don't really have that.  For a kitchen table project, though, using toilet paper tubes and scotch tape or plastic pipe fittings, you can still get usable results.

One of the easiest and most useful configurations to construct is a symmetrical lens.  This is nothing more than two identical achromats arranged back-to-back with a stop in between.  Performance will be best when stopped down to f/8 or less.

Even singlet lenses start performing surprisingly well when you stop them down enough.

Lenses like these were the mainstay of photography for half a century!  

So, yes, stock lenses can be useful, but since you have no control over the types of glass being used or the curvatures of each surface, it would be exceedingly difficult, if not impossible, to match the designs found in modern, fast lenses.  

Like Dom said, there are lots of good, inexpensive lenses already out there.

 

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Using optical design software I can, in theory, design a perfect lens for my application. But in practice, I have no experience whatsoever in actually constructing a lens, and dealing with the manufacturing imperfections of real world glass. Although it's something I've never done before, I do have plenty of experience in using cad software to design parts and then machining them out (I have a relative who owns a machine shop), so making a good barrel wouldn't be too difficult. I'm just wondering, if I can precisely control all other aspects of the design and construction, would stock lenses be good enough to produce a quality projection of a super16 print.

As far as existing s16 lenses go, the main reason I'm looking at this route (other than fun and learning) is that I need a much larger lens that allows for plenty of lens shifting, as this is for projection at varying and possibly large venues that might have the projector at audience level, or high up in a projection booth. In my experience most larger lenses are much more expensive, and this route also allows me to possibly make several lenses designed for different throw distances.

I know I have plenty more research to do in order to undertake this, just trying to get a feel for it.

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You said, "I'm just wondering, if I can precisely control all other aspects of the design and construction, would stock lenses be good enough to produce a quality projection of a super16 print. "

Stock lenses can give you very usable results, but because of their generic design, they are almost always going to be slower in speed than lenses specifically made for the application.  So, for projection,  that means less light output.  You either need to use a more powerful lamp to make up the loss or accept a smaller image that maintains the overall illumination.

 

 

Edited by dan kessler
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A few intricacies of projection optics are

  • you want to know the geometry of the light cone or fascicle which depends mainly on the mirror’s form,
  • shifting demands as stated,
  • focal length determined by image size on film, screen size, and distance,
  • overall performance with an image format.

The closer you are at a circular or square picture the simpler can your system be. Many Petzval lenses were in use with the classical 3-to-4 aspect cinema but had to make way for the CinemaScope system and crop sizes. If colour fringes aren’t predominant drawbacks, you may well use triplets that offer the following freedoms: 3 glass types, 6 curves, 3 distances within the system, and ways to build a stepped lens, I mean the front lens larger than the second and the third element or all elements larger than usual.

Longitudinal position tolerances are generally at around 1 ‰ of diameter like ± 0.0005" with a one-inch dia. element. This is only a rule of thumb, things can become drastically more critical when you’re looking into five, six or seven elements designs.

Before you build your own lens you might like to browse through second-hand markets. Meopta had an f/1.0 projection lens of 50 mm FL, those are quite cheap today. The lens is large enough to allow for some shifting. Try ebay.de

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