Re: Lens aperture slow to respond, thus overexposed pictures
Thank you all that have responded! I'll try to see if there is stil a Pentax service in my country (Romania), although I highly doubt it... If you say that disassembling the lens on my own is out of question, and if I don't reach (an affordable) service here, I'll have to use it as it is... I often use it at f/5.6, and from playing a bit with the lens (actuating manually the aperture lever), it seems it became a bit "quicker" to respond... :) Thanks, Ciprian. -- %(real_name)s Pentax-Discuss Mail List To unsubscribe send an email to pdml-le...@pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
Lens aperture slow to respond, thus overexposed pictures
Hi Friends, had a similar problem with a Pentax "A" 35-105 mm f3.5 Lens - bought Like New for US $ 65 + Tax at Kaufman's Camera in the Bay Area CA. I reckoned a CLA would cost more than buying another one, say from KEH com - $ 149 then. I then took the DIY route after reading up some resources on the Internet and Pentax Forums worldwide. I decided on a rear entry as this was easier and less complex then entering the Lens from the front. I knew both sides should be cleaned to remove the gumming up of the blades, but reasoned the front facing side may just loosen up a bit too. I used lots of Johnson's Q-Tips dipped in WD-40 - gently and very little WD-40 at a time avoiding drips and fibres coming off from the Q-tips. A lot of brown gooey stuff came out till there was no more discoloration of the Q-tips. Put back the rear elements. Had a perfect working lens - checked and compared with a lot of test shots I had filed earlier. Regards and Good Luck. Bipin from Thornhill Ontario. -- %(real_name)s Pentax-Discuss Mail List To unsubscribe send an email to pdml-le...@pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
Re: Lens aperture slow to respond, thus overexposed pictures
On 1/29/2023 4:02 AM, Ciprian Craciun 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)... Lazy aperture syndrome. It's almost always caused by a small amount of whatever lubricant they use in the mechanism migrating onto the aperture blades making them sticky. I've never torn down a lens and put it back together with any success, but if it's what I think it is, the lens needs to be disassembled, the blades need to be removed and cleaned, then the lens needs to be reassembled and recollimated. bill -- %(real_name)s Pentax-Discuss Mail List To unsubscribe send an email to pdml-le...@pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
Re: Lens aperture slow to respond, thus overexposed pictures
Yes, the aperture mechanism is dirty/gummed up: the lens needs a CLA (clean, lubricate, adjust) service. The auto-aperture actuating lever should snap back instantly, somewhere around 10-20 milliseconds max. Half a second is almost literally forever in terms of these mechanisms. G > On Jan 29, 2023, at 2:02 AM, Ciprian Craciun > wrote: > > ...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). > > ... > (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. > ... -- %(real_name)s Pentax-Discuss Mail List To unsubscribe send an email to pdml-le...@pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
Re: Lens aperture slow to respond, thus overexposed pictures
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 > 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 pdml-le...@pdml.net > 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 pdml-le...@pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
Lens aperture slow to respond, thus overexposed pictures
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 pdml-le...@pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
