I have a Tamron Adaptall 500mm f8 mirror lens, which of course is fixed aperture. However there is an indexing ring with the two slots to match the two tongues on the adaptall mount. My old Rolleiflex SL35 mount didn't have the tongues, but having recently switched to Nikon, I find that the ring seems to rotate freely (within limits) and will not hold a fixed aperture setting when mounted on the camera - ie instead of sitting at f8 it moves around. Will this cause incorrect exposure? (I guess it will). Second - how is the ring meant to be fixed in position? Is there perhaps a tiny screw missing? Thanks for any advice.

-- Paul Fisher (pdfisher1@iprimus.com.au), March 31, 2001

Answers

Dear Paul,

I think that the setting or the angle in which that particular ring on the adaptall gizmo sits has no influence on anything with the camera. I have used this lens both with an F70 and an F100; the pictures all came out well, the exposure was right on. Once when cleaning the lens i noticed that the readout could be shifted. So I worried, just like you now do; I did not see any way to make it always sit in the right position f 8, then realized that it is just on a dummy, unconnected part and has no effect, at least with those two Nikons.

Good luck! And let me know if not worrying about it does any good. I.e., check your pictures only. Frank

-- Frank Uhlig (uhligfd@auburn.edu), April 02, 2001.

Frank, thanks for the comforting words, but I'm not sure I believe them ;-) I'm pretty sure the F70 turns off the meter when used with non-AF lenses, same as my F60. (Don't know about the F100) So your well exposed pics were a tribute to your own photographic skill, not the meter in the camera.

On my FG, turning the ring by hand with the camera set on aperture priority causes the indicated shutter speed to rise and fall through a range of several stops. However, I also recall with the FG that it does its final shutter speed adjustment after stopping down the lens, so exposure will be correct, but not necessarily at the indicated speed. So maybe I will stop worrying, at least until I get a different camera.

-- Paul Fisher (pdfisher1@iprimus.com.au), April 02, 2001.

Modern Nikon cameras couple the aperture on a ring around the lens mount, and the Tamron 'Adaptall' mount cannot disconnect from this ring. If the aperture indication on the lens is able to rotate, then the camera will select a different shutter speed to match.
The Tamron lens should have a catch to prevent the Adaptall mount from going beyond the minimum aperture, and it sounds as if this is broken or not engaged properly.
Try demounting the Adaptall ring, and then fitting it back into place on the lens to see if it'll lock. You may have a faulty adapter.

-- Pete Andrews (p.l.andrews@bham.ac.uk), April 03, 2001.

Thanks Pete,

The adaptall mount is OK, so the problem is with the lens. I suspect there is a tiny screw missing from the index ring.

-- Paul Fisher (pdfisher1@iprimus.com.au), April 03, 2001.

Paul: I have a Tamron SP 500. The adapter is for a Pentax K mount which has two sliding rings on the lens side of the mount. One ring has the f settings, the other has nothing but they both have a tab on them that fit into slots on opposite sides of the lens body when the adapter is mated to the lens. f8 lines up with the red dot when everything is locked in place and then nothing moves. The only thing I could see going wrong with mine is if that tab on the f stop ring was missing. Harv.

-- Harv Jenkins (hjenkins@prcn.org), July 07, 2001.

Thanks for the responses - I took the lens to a tech and ascertained that there was a tiny screw missing from the aperture ring. Also found that the ring can be set in a number of positions (ie f/8 f/5.6 etc)so it is essential to get things lined up properly before the screw goes in.

-- Paul Fisher (pdfisher1@iprimus.com.au), July 07, 2001.