Autocollimation- Photographic Application
What is a collimating telescope?
A collimating telescope consists of a target fixed at exactly one focal length away from a lens. In this position, the target appears to be located at infinity focus when viewed through the lens. If the target is transparent and illuminated from the back, the collimating telescope becomes a handy source for checking infinity focus of cameras. Though simple and easy to build, the collimating telescope can be refined into a much more useful and powerful instrument- the autocollimator.
What is an autocollimator?
Before we describe the full blown autocollimator, it will help to think about the simplest autocollimation process. Imagine the above described telescope. A light ray from a single point on the target travels to the lens and is bent such that it travels away from the telescope parallel to its axis. In other words, the telescope sends out a parallel beam for any point on the target.
Now, imagine a flat mirror in front of the telescope. The distance doesn't matter. If the mirror is square to the telescope, the parallel beam described above is reflected right back into the telescope. The lens will focus the rays right back to the original point they came from.
Here's the important part. If you look through the target with a magnifier, you'll see the returned image of the target. If the telescope is focused at exactly infinity, the return image will be focused at the exact plane of the original target. If it isn't, you can rack the lens in and out slightly until it is.
If the flat mirror is tilted slightly, the image will stay in focus, but will move in relation to the original target. Thus, the telescope is a very sensitive device for measuring angles. Most commercial applications like machine tool alignment take advantage of this fact, and most published information (and the web) describe only this aspect of the tool. That's not how we're going to use it.
It's difficult to illuminate the target and look through it with a high power eyepiece at the same time. To get around this problem, the autocollimator was born. This is simply the collimation telescope with a beamsplitter or prism installed, giving two identical paths to the lens. Now the illumination system can be focused through the target, and the eyepiece can view the return image without interference from the illumination system. The only downside is that the instrument has to have the two paths carefully adjusted to be equal.
View autocollimator PDF file here.
What good is this for camera adjustment?
Here's the use that's rarely described. You can think of your camera as another collimator when focused at infinity. Any parallel bundle of light rays will focus to a single point on the film. Placed in front of the autocollimator, the target (appearing as if it were located at infinity) will be focused through the camera lens and will form a perfectly sharp image of itself on the film.
Here's the clever and fantastic part. That image on the film is visible (generally at high magnification) in the eyepiece of the autocollimator. Any fuzziness of the image is easily seen, and the infinity setting of the camera can be changed until the image is sharp. The big advantage is that the focus is being checked directly on the film, wherever that happens to be, under actual picture taking conditions. Think of the autocollimator as two instruments combined into one- an accurate infinity source, and a powerful microscope for examining the film plane.
Note that the exact alignment of the camera in front of or under the autocollimator isn't nearly as critical as was the flat mirror due to the focusing action of the camera lens.
Is it only good for infinity?
No. Most autocollimators have adjustable lenses and can be set to different working distances, including infinity. In reality, infinity is the most useful distance, and the autocollimator is easily checked for infinity focus using the flat mirror. Thus, the instrument is typically left at that setting unless some special test requires otherwise.
OK, what can't it do?
Rangefinder alignment and binoculars. If you had a big enough autocollimator, these would be no problem, but the typical autocollimator lens is one or two inches in diameter. You simply can't see the target through both windows of the rangefinder or through both binocular eyepieces at once.
Notes and special cases
A workable procedure for rangefinders is to set the taking lens at infinity using the autocollimator, then adjust the rangefinder itself on a distant target. A crescent moon in the evening is ideal, as it has lots of contrast and the "points" make it easy to see when the images are perfectly aligned. It's also reasonably far away!
Another special case concerns SLRs and TLRs. Checking focus on the film is no problem, but getting a return image off the focusing screen can be a problem.
TLRs are generally no problem, though there may be several false images depending on what surface the actual focusing is done on. The texture of the screen will be easily visible through the instrument, and that's where you want to focus!
There are so many focusing screens for SLRs that no general statement can be made. You'll get a great return image off the central split rangefinder, but have no ability to see if it's in focus. The microprism area is almost as bad, though there may be a faint image that can be focused. What you really need is a plain ground area, usually present around the microprism area. Unless it isn't Tilt and offset the camera under the autocollimtor until you find an area that can be focused. If all else fails, you can sometimes go to the exact center of the split image rangefinder and focus on the fine line that bisects it.
Note that a fast lens makes the process as accurate as possible, but only if the autocollimator aperture is of equal or larger size. Remember to fully open the aperture of the camera lens!
Finally, there is one other application for the autocollimator that is similar to the usual machine tool alignment use. You can check the parallelism of the film plane to the lens mount by placing a piece of glass or mirror in the film plane (or just using the pressure plate) and aligning the camera to the autocollimator (no taking lens). You'll need a plate with three screws to adjust the angle exactly. Then, without moving the camera, place a mirror on the lens mount. the deviation will be indicated by how far off center the image of the target moves.
The most common and useful visual target is the Sieman's Star. This is a round target with pie-like slices alternating in opaque and clear (dark and light). They all converge in the center, so any focus error is immediately obvious as a circular blur of the central area. The smaller the blur, the better the focus. There are usually 24 or 36 slices. You can easily draw this up in CAD, print it out on a good printer, then photo reduce it to make a suitable target. Email me for a suitable file if you need one. They are also available commercially in chrome on glass, but are quite expensive.