lens focus inaccuracy

[k_iwao]

<p>I recently tested a lens for sharpness by taking several pics of a newspaper positioned flat against a wall, and it seemed ok but I noticed that the word in the center moved position slightly in every photograph. In other word the lens had focused in a slightly different place - .5mm variation approx. The camera was on a tripod, 2sec timer, refocus every time. Would this slight variation be normal??? Also since buying the lens 4 months ago the focus ring has developed quite a bit of play - over 1mm. Any comments would be much appreciated.<br>

Mike</p>

[pbarnes]

<p>If the word in the center has moved, then the camera has moved between shots. If the focus had shifted, but not the camera, then the word in the center would be more or less sharp but would not have moved. Were you manually focussing or did you touch the camera for any other reason between shots? That could have been enough to move the camera slightly. If not, the taking of the shot, with the reflex mirror snapping up and down, could be enough to move the camera slightly if the tripod head and the tripod itself were not locked down tightly.</p>

[k_iwao]

<p>Thanks for your quick response. I used autofocus. Everything was tight but not bolted to the floor. It didn't focus perfectly every time but I would not expect it to and I suppose half a mm at 4 feet is very small at 100 percent magnification; so yes maybe the mirror slap between shots is enough to cause a little movement even though every thing was tightened up. </p>

<p>Thanks again for your response.</p>

[michael_elenko]

<p>Michael,<br>

Some specifics about the camera body and lens would help. Some Pentax DSLR bodies automatically lock the mirror with a 2-sec self-timer. Intuitively, it seems like your lens has some internal looseness.</p>

<p>ME</p>

[james_lai]

<p>Which camera are you using? If it has SR, turn that off and try again -- SR is not recommended for tripod use.</p>

[andrewg_ny]

<p>You say that the manual focus ring has close to 1mm of play. If that is the case, then the different apparent position of the ring may vary even though the optics may be in exactly the same place--one possible determining factor might be the direction (near-to-far vs. far-to-near) of the last autofocus operation. If the results are in good focus, this sounds plenty reasonable. Michael E. says 'internal looseness'--if results are OK, then it would sound mostly good with the looseness being just at the external focus ring level. Depending on the lens, this wouldn't be very surprising at all. Does the lens offer quick-shift focusing?</p>

<p>I would be surprised if a lens went from 0mm play to 1mm play in just a few months--most likely you didn't notice it earlier.</p>

[k_iwao]

<p>Thanks Michael<br>

Specifics - K20d. fa77 ltd. You are right about the mirror and I think the K20d locks up with the 2 second timer so it should not have moved but maybe the slight change in position and the occasional slightly out of focus image, 8 out of 40, could be attributed to play in the autofocus gear?? Maybe??? Perhaps I should visit Pentax and get them to check it.<br>

Thanks again<br>

Mike</p>

[k_iwao]

<p>Thanks Michael<br /> Specifics - K20d. fa77 ltd. You are right about the mirror and I think the K20d locks up with the 2 second timer so it should not have moved but maybe the slight change in position and the occasional slightly out of focus image, 8 out of 40, could be attributed to play in the autofocus gear?? Maybe??? Perhaps I should visit Pentax and get them to check it.<br /> Thanks again<br /> Mike<br>

Sorry I seem to have posted this twice</p>

[michael_elenko]

<p>Michael,<br>

The SR is also deactivated with the 2-sec delay, so you can rule that out too.</p>

<p>With a lens of that quality I wouldn't expect any issues. I'd take it to a reputable camera repair shop (research the best in your area) for evaluation.</p>

<p>One more question: do other lenses exhibit a similar problem?</p>

<p>ME</p>

[laur1]

<p>Some basic questions and comments:</p>

<p>What tripod was used?</p>

<p>If it wasn't sturdy, it might not be stable enough.</p>

<p>What was the tripod sitting on?</p>

<p>If it was sitting on a rug it may not be very stable, if it's sitting on hardfloor it might slide as a result of vibrations from touching the camera or just from walking around on the floor.</p>

<p>How was the shutter triggered?</p>

<p>If you pressed the button, it might move the camera with the tripod - you should use a remote shutter mechanism - I always use the IR remote when doing tests on a tripod. And I refrain from moving so that my walking doesn't transmit vibrations through the floor.</p>