How can a lens do this?

[dan_tripp]

<p>I have a Tamron 28-75 f/2.8 lens that I have been using for about 3 years. About 6 months ago the rubber grip became loose. About 1 month ago I started noticing that my pictures are over exposed and I have to set the EV to -.03 to -.07, but now things are getting out of control. Here is a series of pictures I took the other day.</p>

<p>I took a few dozen pictures and they were fine, but then I took another picture and they start getting worse.</p><div></div>

[dan_tripp]

<p>Most of the pictures looked like the one above, then the started over exposing like this:</p><div></div>

[dan_tripp]

<p>From here on out the pictures get worse. Then I look in the view finder and have to drop the exposure to -0.7 to -1 and sometimes it doesn't even help. Then I switch lenses and everything works fine.</p>

[dan_tripp]

<p>Here is the pic.</p><div></div>

[johnw436]

<p>I get exactly the same thing on my istDS when shooting in any mode other than Manual. That is with the kit lens.</p>

<p>Ever since I got my K20d I haven't used the DS. I just took for granted it was the camera body and since I always shoot in Manual it wasn't an issue. I would chimp the scene until I liked the histogram and then I was all set. In any other mode I would do the same thing as you: set the EV Comp and then it would begin to drift.</p>

<p>Maybe I have a lens/body issue on the DS. Now you've got me wondering. Time to experiment.</p>

[personalphotos]

<p>Have a look at the aperture blades. If you see any residue on them, the blades have oil on them and are probably sticking. Check the blades to see if they move almost instantly from closed to wide open.What is happening is the lens stays in the wide open state all the time except the instant you hit the shutter button. If the blades are sticky, you may have the lens set at f8 but it only manages to close down to f5.6 or some other than idea setting. Another thing you can do to check is take shome shots wide open. If the exposures are fine, then it's an aperture issue. If not, then possibly the electrical contacts are dirty.</p>

<p>Clean them with proper electrical contact cleaner. Do not use a pencil eraser. Then test the lens out again.</p>

<p>This sticky blade issue will happen to a lens that has bee stored poorly. For example leaving a lens in a hot car for an extended period of time. The lubricants that keep the zoom working smoothly will run onto the blades and make then make them stick. It can be cleaned and adjusted at a proper service shop.</p>

[michael_elenko]

<p>What Peter said. I had the same thing happen with a really nice Pentax A 24mm f2.8. It got progressively worse over the months. It can cost about $80US for a cleaning. Have them reattach the rubber grippy stuff too.</p>

<p>ME</p>

[jgredline]

<p>My beloved sears 28mm lens does that and I love it because it allows me to shoot a full stop less in the ISO dept. (800 instead of 1600). Having said that, Tamron does have a great 6 year warranty. There service dept is also quite good. Shoot me an email and I will forward you the repair form and the the techs email that fixed my lenses. So far I have had my 28-75F/2.8 and my 28-300 super zoom repaired and they are better than they have ever been.</p>

[michael_kuhne]

<p>That's a great warranty, and apparently Tamron backs it up!! </p>

<p>Would it not be a great thing if Pentax had such for their lenses, especially the expensive SDM lenses!</p>

[johan_ingles_le_nobel]

<p>Do you know anyone else nearby with a Pentax or know the local camera shop well? See if you can mount the lens on another body - if it does the same then you can at least positively narrow it down to the lens. After that, it's a cleaning job as Peter said.</p>

[jpo3136b]

<p>". . . About 6 months ago the rubber grip became loose. "<br>

I've got lenses here which are from the 1960s, and they don't have parts which have come loose. Anytime anything starts working loose or falling off, that's bad news. There are no parts which are supposed to fall off. Time for repairs.</p>

[ejchem101]

<p>If you look at your photos... the one that is exposed at 2.8 is fine. The ones with problems were at 5.6. I'm guessing it didn't close (as was pointed out earlier). I would just take a series of photos, start from 2.8 and work your way up. If it continually gets worse, it's definately the blades.</p>

[andrewg_ny]

<p>I had a similar problem with an otherwise pretty nice Vivitar Series 1 28-90; when it first arrived (e-bay purchase) stopped down shots tended towards overexposure; I noticed that the aperture blades were a little sluggish. After more use it improved somewhat so I ended up hanging onto the lens (I may regret this). Pretty sure that a more permanent fix would involve opening it up and cleaning the blades but I imagine this is a difficult job that if done by a professional would easily cost more than the lens is worth.</p>

<p>A theory that I haven't checked yet. When using MLU (2-second self-timer) does the aperture stop down immediately or only at the moment of exposure. I suspect the latter--in which case this idea wouldn't help at all--but if the former, it might be a way of checking this.</p>

[orly_andico]

<p>Either the iris is sluggish closing down, or it's not closing down at all. This happened to a Sigma lens I had (for Canon) where the EMD ribbon cable died.</p>

<p>But Pentax lenses have a mechanical iris linkage, so, take the lens off the camera. It should snap to the smallest aperture right away. Then exercise the linkage with your finger (the lever at the back). Open up the aperture to 2.8 then release your finger. It should snap shut very quickly. If not.. it's a cleaning job.</p>

<p>One other possibility is that the linkage got bent somehow and is now dragging on the lens mount, so it's slow. The finger trick will quickly verify if this is so. And if it is, you can safely bend the linkage back with a needle-nose plier.</p>

[zak_nyberg]

<p>I had what I thought was this same problem with my K2000. I thought it was my Sigma 70-300, as it was only doing it with that lens attached. Sent the lens back and they said it was fine. Then it started happening with other non-Pentax lenses. Then my kit lenses. Send the camera back to Pentax and they replaced the main board. Got it back and no dice. Sent it in again to Pentax and they replaced the aperture control unit, now all my lenses are operating as they should. I would take above posters advice and find another body to test it on, I would have saved 4 weeks of hassle if I had not sent in a perfectly working lens for repair...</p>

<p>Good Luck!</p>

[peter_engel1]

<p>Looking at your Exif The first picture is at 1/160 The second at 1/60 The third at 1/40 My question is this the lens or the camera ?</p>