This note describes the problems which arise when trying to attach standard camera lenses to CCD imagers and spme solutions. Other ideas for convenient attachment of optics, cameras and imagers to telescopes are discussed as well. Photographs of some of the adapters described here are in other articles on this web site.
A table of film to flange distances for many cameras is given at this page. Camera Film/Flange Distances
There are several problems that must be solved. One is obtaining a suitable camera flange to fit the lens mount. Modern lenses generally are of bayonet mount type and thus require an adapter ring which mates to the lens. This ring is like the flange on the camera. Additionally, there must generally be a mechanism that allows the lens to be stopped down. If the lens is to be used at full aperture, the stop down mechanism is not required. Some lenses can be stopped down manually with or without the stop down mechanism. In the case of the Canon FD lenses, which I use, such a mechanism is required.
Another problem is to adapt the camera flange to a T-thread adapter which fits the CCD imager and still keep the total thickness of the adapter short enough so that infinity focus of the lens will allow it to focus on the CCD chip. This requires a very short adapter in most cases.
A knowledge of the camera flange to film (focal) plane distance is necessary and a way to make a male T-thread on the back of the adapter is required. This is because CCD imagers generally have on their front surface a female T-thread. Unfortunately, the imaging chip in the CCD imager is some distance behind the front surface of the T-thread. In the 416XT this distance is 15.5 mm. In the ST-7 it is 23 mm. In the 216XT it is only about 10 mm since the 216XT has no shutter mechanism. This means that the adapter can only be a thickness equal to the camera flange to film distance minus that required for the CCD imager.
I have made a number of adapters for Canon lenses. These are among the most difficult since the Canon (FD type lenses) flange to film distance is only 42.1 mm. This is among the slimmest of all modern 35 mm SLR cameras. (See list attached at the end of this note.) It is so short in fact that even a Nikon lens to Canon camera adapter can be made. This adapter is only 4.4 mm thick and has both the male Canon bayonet and the female Nikon bayonet within its structure. This is a real tour de force in adapter ring design. I have one so I can put a Nikon 500 mm f 5 mirror lens on a Canon FD camera.
Most adapters for lenses for other cameras are not quite as difficult as those for the Canon because the flange to film plane distances are larger and thus the lenses shorter. The easiest lens to adapt is a T-mount lens. The standard T-mount to film plane distance is 55 mm. Thus all that is required is a straight through female T to male T tube. T- thread lenses are now quite rare because the T-mount cameras are obsolete. There were never many really high quality lenses of this type made that I know of. They were mostly cheap and of marginal quality. But you might want to search out camera stores and be lucky enough to find one of the few good ones made. Do not underestimate the quality of the lens required for CCD imagers. A 10 micron pixel CCD can resolve 100 lines per mm. So you want a lens significantly better than this resolution and longer focus lenses that do this well even on axis were not all that common until the last 15 years or so.
Usually, it is desired to place a good quality 35 mm lens on the CCD imager. I will describe below a method to do this which produces, in my opinion, a professional quality adapter. It is so difficult and time consuming to produce the bayonet mount for any good lens, such as Canon or Nikon, that this part, the bayonet mount itself, must be purchased. The preferable product to purchase is an adapter which has a quality bayonet mount and the mechanism for stopping down the lens manually. The rear of the adapter would ideally have a T thread and the problem would be solved. Unfortunately, such adapters are not made to the best of my knowledge. I have not been able to find them. I have more recently heard that some CCD imager manufacturers do supply such adapters for a limited number of lenses. The reason is that all camera flange to film distances are much less than the T-thread distance of 55 mm. Of course, the 55 mm was chosen so that T-thread lenses could be adapted to any SLR.
However, it is quite possible to obtain adapters for specific lenses which adapt the lens to a standard C-mount which is used on many movie and video cameras. This is the adapter type to purchase and if it is made right, the C-thread will be on a removable element. Such is the case for an adapter made by NPCM and available through almost any good camera dealer. They will most likely have to order it for you.
This adapter has a removable C-thread piece on the back leaving a flanged hole 38.2 mm in diameter and 3.9 mm deep with three set screws in its rim. Into this flanged hole a turned piece can be attached which carries the male T-thread required to mate with the CCD imager female thread.
This piece needs to have a thread which is 42 mm diameter by 0.75 mm and 5.0 mm thick. (the T thread) And then a step down to 38.2 mm for a distance of 3.8 mm. This small stepped (donut shaped), threaded ring is inserted in the flanged hole and tightened with the three set screws. And there you have it. A perfect adapter for the selected bayonet lens to T-thread. This small part can be made of brass which is very easy to machine.
I believe the above described adapter is designed and made in about the easiest way possible. Anyone with a small lathe can make it easily. I have made several for myself and friends. I use another adapter with the original C-thread to attach Canon lenses to my video cameras. One can also make the bayonet camera mount the basis for several adapters for CCD, video or standard cameras to the telescope by using the Schmidt to T-thread tube with a standard T-thread to camera adapter ring. These are quite common. Then any of several receptors, camera, CCD, video camera or other can be quickly bayoneted to the telescope. Of course all the receptors should be made par-focal.
For example, I have made up several par-focal eyepieces by mounting an eyepiece in an appropriate length tube and adding to the front a camera bayonet flange. Then one can focus visually, bayonet off the eyepiece and bayonet on the receptor. This is especially convenient with the Canon bayonet mount since it is of the breach mount type. That is, it has a bayonet flange which is tightened with a rotating ring similar to the breach of a cannon. But other bayonet mounts are also very good. I made most of the attachments described when I got tired of having to work with Schmidt style rings in the dark.
There are many photographs of these adapters included in other articles on this web site.
Here are some flange to film plane distances.
|Leica (screw)||28.8 mm||Leica (M bayonet)||27.8 mm|
|Canon (screw)||28.8 mm||Canon (FD and earlier)||42.1 mm|
|Nikon||46.5 mm||Minolta||43.5 mm|
|Pentax K||45.5 mm||Exacta||44.7 mm|
|Alpa Bayonet||37.8 mm||Contarex||46.0 mm|
|Contax RTS||45.5 mm||Ikarex BM||44.7 mm|
|Konica Autoreflex||40.5 mm||Miranda||31.5 mm|
|Olympus OM||46.0 mm||Praktica/Pentax*||45.5 mm|
|Petri Bayonet||45.5 mm||Rico Bayonet||45.5 mm|
|Rollei 35||44.7 mm||Topcon DM||44.7 mm|
|Voigtlander||44.7 mm||Yashika FR, FX||45.5 mm|
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