Seems to me that your lens needs some professional help, like an internal cleaning and lubrication.
Rick > On Jan 29, 2023, at 5:02 AM, Ciprian Craciun <[email protected]> > wrote: > > 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. -- %(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.
