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03-12-2011, 02:02 AM   #1
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Distance between sensor and front of K-x body

I have a problem with my Zivnon 500mm mirror lens, with a screw mount adaptor, and infinity.
If I understand it correctly, the distance between the sensor and the back element of the lens is crucial to reaching infinity?
Can anyone help me with the distance between the sensor and the front (Lens mount) of the camera. And what the distance between the sensor and the back element should be, for the mirror lens to be able to focus correctly? I want to try and see if I can modify the adaptor.
Any help appreciated!

03-12-2011, 03:26 AM - 1 Like   #2
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The distance from the lens mount to the focal plane of Pentax film cameras is 45.46mm. I assume, for compatability reasons, the digis are the same. I have no problems with an ancient Tamron 90mm f/2.5 at infinity focus on my *ist DS body. Can't help you with back element/sensor distance. Maybe you could set up an optical test bench to determine the image distance from the rear flange of the lens at infinity focus.
03-12-2011, 04:22 AM   #3
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Thanks pixelsaurus. I have a Takumar 105, and no problem with infinity, but on the same M42 the Zivnon does not want to screw in completely, as if it is not exactly the same thread. I don't want to measure the distance from the sensor, to afraid of damage to it. thought somebody might know what it should be on the K-x. You mention 45.46mm on film camera's, but is it the same on K-x? Unfortunately I don't have the knowledge or equipment to do accurate tests, it will be more of a trail and error story.
03-12-2011, 05:00 AM - 1 Like   #4
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Registration is 45.46mm for Pentax digital cameras.

If your lens threads are the right diameter, but won't screw in to the m42 adapter it is likely a T-mount which has a different thread pitch than m42.

Here's a T/PK adapter for $5.45 delivered: NEW T Mount Adapter for PENTAX K Metal MX K1000 - eBay (item 220319640074 end time Mar-16-11 20:19:47 PDT) I've had good experiences with the vendor.

03-12-2011, 07:27 AM   #5
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Thanks newarts, will have a look closer to home first.

Thanks to both of you for your input, really appreciated
03-12-2011, 07:48 AM - 1 Like   #6
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You are probably trying to thread the T mount into the camera body. It the same size but a different thread pitch. As mentioned, you most likely need Pentax K /T mount adapter. That should also fix your infinity focus problem. The lens is designed to be used with the adapter.
03-12-2011, 08:34 AM   #7
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Thanks for taking time to help reeftool. Will try it as soon as I get the adapter.
03-12-2011, 02:38 PM - 1 Like   #8
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Those 500mm mirror lenses are almost always t-mounts. Can you unscrew the lens mount from the rest of the lens? If not, then you are almost certainly missing the adapter.

The M42 thread is 42mm x 1.0 thread per mm. The t-mount thread, sans adapter, is 42mm x 0.75 tpm. In other words, it is a fine thread, while the M42 is a coarse thread. If you look closely at the 500mm lens and compare its thread to the Takumar, the difference should be pretty obviousl.

The adapter adds about 3/8 inch to the length of the lens. This is normal.

03-13-2011, 01:00 AM   #9
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Thanks Paul, you were all very helpful. My T-mount adapter is on the way, should get it Tuesday, can't wait to try it. I will let you all know if it worked
03-17-2011, 04:58 AM   #10
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Just some feedback. The T-mount seems to work fine. It is just very difficult to get good focus if you go below 1/800, even on a tripod.
Thanks again for all your input!
Just one test shot I took of a shop across the street from me, about 85m away. Tripod used, ISO 200, 1/800.


Last edited by altopiet; 03-17-2011 at 05:07 AM.
03-17-2011, 10:36 AM   #11
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I trust it is a heavy tripod and not one of those blancmange mounts. :-)
03-17-2011, 09:41 PM   #12
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I got it 30 years ago when everything was still made to last, and it will have to do for now. Saving for some more lenses before I look at new tripod
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