Exposure Issue with New Lens

[jean_b.]

<p>Hi folks, got my new (used) DA 21mm f3.2 from KEH this week and took it for a test drive today to make sure that there weren't any bugs. Also wanted to confirm that the performance of the lens was as predicted by photozone.de, which it appears to (i.e., soft in the corners at f3.2 and f4, but awesome overall at smaller apertures). However, I did notice something disconcerting: the exposure seems to shift with changes in aperture. I don't yet know how to post a series of images in a single post, so bear with me as I upload the following examples one at a time.</p><div></div>

[jean_b.]

<p>Next one.</p><div></div>

[jean_b.]

<p>Final one.</p><div></div>

[bob_sunley]

<p>So what's the problem? ? ? You change the aperture to a smaller value by one stop and the shutter speed moves the other way in lock step.</p>

 

[jean_b.]

<p>At least on my monitor, when I scroll through these images, they get progressively darker as I go from f5.6 to f8.0 to f11. Histograms agree with what my eyes tell me. Though I did not show you the others, f16 is darker still, and f22 gets brighter. Is this normal? I have not seen this with other lens that I own. I am about to leave on a business trip, so if this sounds like something with the lens I need to return is ASAP. Please advise. Note that white balance was accidentally set to Tungsten when I took the photos, but I converted the raw files to daylight and resized to smaller JPEGs in the Pentax software. Original Tungsten photos show the same phenomenon, in camera and on screen.</p>

[bob_sunley]

<p>Well when I scroll thru the images on this page, I see three apparently identical images with no differences with firefox. I did save the three images, there are small differences in brightness and the brightness decreases as the aperture gets smaller. If the aperture was sticking the problem is the aperture doesn't stop down enough and the image at smaller apertures is brighter than it should be. Without knowing how the internal linkage works in that lens, it appears to be stopping down more than it should until it reaches f22, which I assume is the smallest aperture setting and then it can't go past the end stop and the image appears brighter than the one shot at f16.</p>

<p> </p>

[bob_sunley]

<p>I'd suggest returning the lens.</p>

<p>You could examine the actuating lever if it has one and see if it looks bent when compared to your other lenses.</p>

 

[pbarnes]

<p>Jean,<br>

I can see a slight darkening of each image from f5.6 to f11, possibly just the result of the shutter speeds the camera chose for each exposure. Not a problem I would worry about - I adjust the exposure in Lightroom or Photoshop anyway on each image I want to use . And I do like my images sharp so to avoid diffraction have never used my 21mm Ltd at f16 and would not think about using f22. If I need slower shutter speeds than I can get at f11 I use a polarizer or neutral density filter.</p>

[markus maurer]

<p>Jean, I agree with Peter. I see the darkening as well here and all photos seem to be a bit underexposed. Did you use a polarizer? Do you get equal results with the kit zoom or another lens?</p>

[jean_b.]

<p>Thanks for the responses so far. No polarizer or filter was used when these photos were taken. I often take a series of exposures at different apertures (when possible), since I don't always know in the field what I want, and I can take my pick in the comfort of my home. I took a series with my FA 35mm f2.0 a while ago with this same camera (a K-7) and I see no change in exposure there. BTW: after I uploaded the photos and looked at my posting I too found it difficult to see the difference in exposures. It's when I'm clicking through them on my computer, when one image replaces the other, that I can tell the difference. I could be that what I'm seeing is too minor to be relevant, but I thought I would ask here to see what others thought. I too am unlikely to use f16 or f22, but the sequence shown is in the sharpness sweet spot for this lens, so I want to make sure it's OK there.</p>

<p>Oh, and I forgot to mention: it was love at first sight when I pulled this lens out of its box. What a great complement to the camera!</p>

[john_shriver]

<p>There are two calibration points for the aperture mechanisms in Pentax-A and higher lenses. If they are not done right, the apertures won't be accurate when the camera pushes the "stop down" lever to the f/8 (for example) position.<br>

I'm sure KEH's repair center has the appropriate documents and jigs to fix this.</p>

[andrewg_ny]

<p>I see the difference but it's pretty subtle. Might be within spec...How much do you need to brighten the darkest image to make it look like the lightest one...range probably within 1/3 EV?</p>

<p>I've seen some cases where lenses appear a bit underexposed at wider apertures due to vignetting but that doesn't appear to be the case here as it actually gets a bit <em>darker</em> as it stops down. I wonder if this isn't just a characteristic of the design.</p>

<p>If the issue concerns you enough, my suggestion is to try a few other lenses and see whether this is typical or if only this lens exhibits this behavior. KEH is pretty good about exchanges, I expect they would allow you to try a different one too.</p>

[r.t. dowling]

<p>For what it's worth, I was able to spot the brightness difference right away. It's not a drastic difference, but it's certainly noticeable on my monitor/browser combo.</p>

<p>If your other lenses don't exhibit this same behavior, then that pretty much rules out a camera issue, so I'd suggest contacting KEH.</p>