Bought a faulty Pentax-DA 11-18?

Started 7 months ago | Questions thread
KPM2 Senior Member • Posts: 1,806
Re: Bought a faulty Pentax-DA 11-18?


Carlos U wrote:

Besides the changes in light levels, if you look carefully, the foreground is softer in the lighter pictures, or a bit sharper in the darker ones (look in to the granite block on the right and the grill on the left, specially between photos #4 and #5). I asume all pictures where focused at the same distance.

This could be a sign of a sticky aperture. According to EXIF all pictures where taken at f5.6. Looks like on the three that are brighter the aperture was wider.

well spotted ! On the other hand, why this happens only at 18mm I have no idea, because the apertures have nothing to do with the focal length.

To the OP: you can try out easier test pictures: a close up, where the change of the DOF get very quick visible. When the illumination of the scene do can not change (maybe done at home with room light) and you use always the same setting and the same focus and camera position (maybe use a tripod or put the camera on a desk or something), than your aperture blades do close not always equal, when the pictures have different brightness and different DOF's.

On the other hand, for to do such test's, I have a lens, where such sticky apertures can happens only, when the lens get cold. Therefore, when you send back the lens and they try it out in a warm room....they may find nothing and send it back to you. Therefore try it out if a cold temperature of the lens cause it and mention this to them.

best regards KPM2

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