brainwagon

I like it when my life’s experience and interests toss me an opportunity, and out of the blue last week I received an invitation to help with a class a colleague is trying to put together to help people build their own cameras, and he wondered if I could give an hour or so introduction into camera lens design. It’s really odd that I know anything about camera lens design, but when I was really into building telescopes, I acquired and read a fair number of books on optics and cameras, and in my job it’s proven occasionally useful. (I even managed to be a coinventor of a light field motion picture camera.) But really, it’s always been pretty much just a hobby.

Which isn’t to say it isn’t fun, and interesting, and an opportunity to build some cool stuff.

The history of camera lens design is actually pretty nifty, and goes back over two centuries, even predating the invention of film (early lenses were used in camera obscura). I remember reading (and subsequently forgetting a great deal) of the history of the camera lens in a borrowed copy of Kingslake’s History of the Photographic Lens (a great work, I should someday purchase a copy of my own). But I do have copy’s of Conrady’s Applied Optics and Optical Design. This book was written in 1922, and detailed the mathematical design methods used to design a variety of optical instruments. In particular, I recalled a particular design consisting of a “stop” in front of a simple, concave forward positive meniscus lens. I couldn’t recall the name, but a few minutes of Googling reminded me that it was called the Wollaston landscape lens.The lens is, well, just lens and a stop, but can yield surprisingly good images. The simplicity also makes it a great lens for experimenting with simple primitive cameras. The lens is typically mounted in a barrel that accepts cards with different size holes for the stop, about 15% of the focal length in front of the meniscus. When the lens is stopped down to about f/16, the performance can be quite good over fields of about 45 degrees or so. Conrady’s book covers the design of such a lens, and tells you exactly how to optimize the system, but frankly it probably doesn’t matter that much. I’ll probably review that material, but I doubt doing any math is called for in this class. I suspect we’ll just select some roughly appropriate lenses from Surplus Shed and have at it.

A former Pixarian and colleague, Craig Kolb (along with Don Mitchell and Pat Hanrahan), did a really nice paper back in his Stanford days entitled A Realistic Camera Model for Computer Graphics which showed how you could simulate more complex camera lenses which have many subtle effects not usually captured by the simple pinhole projection model used by most rendering software. I can’t remember if I reviewed the paper for SIGGRAPH, or if I just talked to him about it, but I always thought it would be cool to try to simulate one of these simple camera lenses and show how the “defects” of these simple lenses could be appropriately simulated in CG. He never did it, and neither did I. It still remains on my list of projects to do.

One good thing about looking at these old designs is that a lot of information can be had from materials which you can get for free online. A bit of googling revealed this nifty little book (digitized by Google and available on Google Play) which has a lot of good information about the Wollaston landscape lens, and other simple lenses of the day. It’s conveniently out of copyright, so free to all.

Bolas and Brown’s The Lens

Hopefully more on this to come.

Addendum: Bolas and Brown’s book is not without it’s problems: I saw this diagram while reading it, and realized that it’s not accurate. Off axis parallel rays should be focused, well, off axis, this diagram shows them coming to focus on the optical axis. Whoops!