Hello all! I'm a long time lurker, but in the last few years I've not focused seriously on photography, thus if what I'm asking about might have already been discussed please point me in the right direction.
So, I have the following problem with a Pentax D-FA 100mm macro lens: the aperture system on the lens is sluggish to respond and thus all images are overexposed, especially on high f-stop numbers (i.e. 5.6, 8, 11 and onward). Has anyone encountered this problem? How has one solved it? I'm especially interested in do-it-yourself approaches, as I doubt I can send the lens to be serviced (without costing more than the lens itself)... Here is how I've confirmed the issue is with the lens indeed and not the camera. (A) First of all empirically, I've detached the lens and removed the caps, then looked through it which by default is fully closed (f/20+). Then I've manually pushed the aperture lever wide open and suddenly let it go (just like the camera I presume does). The aperture blades don't snap back to the fully closed position, but slowly crawl to that position. Here "slowly crawl" is compared with how other lenses react to a similar experiment; but in real time it takes perhaps half a second or a bit less. (B) I've taken raw photos (in manual mode at constant ISO and exposure time, focused at infinity) of a close white surface (my monitor) at various aperture values (f/2.8 through f/16 in one f-stop increments) making sure the resulting image is not over- or underexposed (by choosing a proper, but then constant, exposure time that works for all tested aperture values). Then using the `rawshack` tool I've identified the mean value of one of the green channels. Then I've computed the ratio between two successive aperture levels (starting with f/2.8) and the resulting values are 1.61, 1.96, 1.27, 1.04, 1.05. The theoretical values should be close to two. I've confirmed this with a 35mm lens (in a similar fashion) whose values are around two, with the exception of f/2.8 that with the 100mm lens seems to be somewhat off. (I've tripled confirmed this by taking photos without a lens, only with the semi-transparent body cap, but varying the exposure time in one f-stop increments.) Thanks all for the feedback, Ciprian. -- %(real_name)s Pentax-Discuss Mail List To unsubscribe send an email to [email protected] to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
