mx with focus problem

[alan_wayman]

<p>Hi all<br>

just received a slightly battered but apparently mx from that auction site. I was excited! until I realised that when I attached a 50mm 1.7 prime lens the focus changed as I pressed the shutter. So if I focus at 3 meters, after the mirror returns I have to refocus, and sometimes this changes to around 2 meters.<br>

So I thought this might be the mirror taking a while to settle down after each shot. However, when I attach the lens the camera arrived with, no problem. Focus stays rock solid. This lens is a Tokina 28-70mm F3.8 zoom. The only difference i can see between the mounts of the two lenses is that the aperture lever on the Tokina is a few mm longer than the Pentax prime. This prime lens has been working perfectly with my other k-mount cameras.<br>

I can't understand what's happening. Ideas anyone???<br>

Alan</p>

[dan_daniel2]

<p>Strange. The aperture mechanism and the focus mechanism are pretty isolated systems. In K, M, and A versions, you won't even have a drive screw connecting the focus mechanism to the body in any fashion.</p>

<p>Off the camera, play around with the lever and different aperture settings. It should move pretty freely. Also just confirm that the aperture stays in the same alignment as you focus in and out. See if the lens stays aligned. If it rotates with the focus ring, a stop of some sort has broken loose, but maybe this is impossible (the inside isn't clear to me).</p>

<p>At least the 50 f1.7 isn't a pricey lens. If it is the only one that causes the problem, just set it aside.</p>

[john_shriver]

<p>While the stop-down lever and the mirror operating levers are inter-related on the side of the mirror box, I don't think misbehavior by the lens' stop-down lever should mess up the mirror operation. There's some independence there due to the DOF preview.<br>

The mirror is always free-wheeling, staying down only by the pressure of a spring. You'll note that you can easily raise it with the lens removed. It lands on a pair of stops. They are spring loaded, probably to dampen the landing of the mirror. Still, you should be able to feel that it is properly landing on those stops, either move it gently with a finger, or poke down very gently with a cotton swab.<br>

Also, do make sure the focusing screen is properly and stably installed.<br>

The MX is a wonderful camera, if there is something wrong it it, invest in a CLA by <a href="http://www.pentaxs.com/">Eric Hendrickson</a> .</p>

[alan_wayman]

<p>Hi Dan and John,<br>

thanks for your ideas. I'm pretty sure this is a camera body problem because all my lenses work perfectly on my other bodies. In the new year I'll get a quote for a CLA and whatever else needs to be done.<br>

Thanks again.</p>