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collimation question


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#1 Robert Glenn

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Posted 29 January 2006 - 07:06 PM

I have a question about lens collimation. If you focus your viewfinder on the groundglass, then you can focus your lens properly for the film plane, correct? Where does the collimation come in to play? It would seem that you could adjust for any abberations if your viewfinder is right.
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#2 Charles MacDonald

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Posted 29 January 2006 - 08:15 PM

I have a question about lens collimation. If you focus your viewfinder on the groundglass, then you can focus your lens properly for the film plane, correct? Where does the collimation come in to play?


IF your Viewfinder is properly set in relation ot the film plane, (including the postion of any interviening Mirrors and prisms (including the mirror on the shutter of most movie cameras.)

Normaly when the talk about Collimation they are setting the lens to focus coretly at infinity and also to stop at the inifity setting- although I can't imagine any competent camera tech how would not check the viewfinder at the same time.

If you lens will not focus at infinity all the focus footage/meterage markings will be out.
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#3 Boris Belay

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Posted 29 January 2006 - 08:19 PM

I have a question about lens collimation. If you focus your viewfinder on the groundglass, then you can focus your lens properly for the film plane, correct? Where does the collimation come in to play? It would seem that you could adjust for any abberations if your viewfinder is right.

IF YOUR VIEWFINDER IS RIGHT... indeed, and then again, perhaps, maybe... The point of lens collimation is to text a lens' focus without anything else coming into play (well, besides the collimating system, but it's supposed to be right, obviously). In other words, excluding any possible problem with your camera lens mount, the gate setting, and any part of your reflex viewing system (a complex system in its own right).
If you're confident about your camera (and its reflex viewfinder), then you can have a decent idea of your lens's accuracy through the viewfinder. A test roll will give you an even better idea. And a collimation of your lens on your camera (light reflected off the filming gate) will be the ultimate test and most precise way to set it up (accuracy of collimating systems is extremely high).
Another way to think about it is that before you start checking things, you need to have at least one thing that you know to be right. Have a test of the lens mount, flange distance, viewfinder accuracy on your camera, and you can check your lenses to a degree, or collimate your lenses and you can check your camera's (or at least viewfinder's) accuracy.
Then again, if you've shot with that combination of camera and lens and the results are good, keep shooting !
-B
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#4 Tim Carroll

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Posted 29 January 2006 - 08:33 PM

It is really scary how many cameras are set up where everything is just to spec for that camera. That is one of the big problems with buying used cameras. I have talked to a certain company in NYC that sells used cameras and lenses and they set the lenses to work with the out of spec flange focal distances on cameras, instead of setting the camera right first, and then setting the lens right. So if you buy just the camera, or just the lens, and try to use either with equipment that is set right, you are screwed.

I don't understand that mentality. I firmly believe what Boris said about having to start somewhere with something that you know is in spec. For me it is the FFD. Next it is the ground glass. When I know these two are in spec, then I get the lens set, because even if the lens is off, if my FFD is dead on and my ground glass is dead on, I can work around the out of collimation lens. But it is best to have all three set dead on. And if someone tries to sell you a lens that they will set up to work fine on your out of spec camera, tell them to forget it, and get the camera set to spec first.

My 2 cents,
-Tim
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#5 Charles MacDonald

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Posted 29 January 2006 - 09:09 PM

It is really scary how many cameras are set up where everything is just to spec for that camera. ......
And if someone tries to sell you a lens that they will set up to work fine on your out of spec camera, tell them to forget it, and get the camera set to spec first.


I have the manual for an old 1930's Keystone Home movie camera. The lens mount on that is a stamped metal plate. The users guide sugests splurging on the optional 3 inch telephoto, but to send your camera along so that the lens can be set to the camera! This is a plain c mount with no reflex finder of course. Yet the optional normal lens was a 1 inch f1.9, the botom of the line standard lens did not need to be adjusted for focus.
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