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10-22-2020, 03:44 PM   #1
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FA 100-300 only autofocuses in closest focal length

Hi all,
The other day I bought a FA 100-300 for practically pocket change from a local and reputable camera store. He said someone brought it in with an old film body and they hadn't been able to sell it since due to age, and the fact that most people here don't care to shoot modern Pentax. For $25 I bought it, thinking oh, it's clear of fungus, it'll be a great thing to have in the bag just in case.
As soon as I bought it, I threw it on my K-S2 to make sure everything worked- it hunted some when it focused but that's expected of these older lenses, other than that it was great. No problems at any length except for my own shaky hands ruining the shot a few times.
Today it went out with me to a wildlife reserve, along with my DA 40 XS and the SMC-F 35-70. I probably got about 25 shots in with it before the AF just....stopped working. I panicked, thinking it was the body (my K-x did this with all lenses and the cost to fix the electronics outweighs the worth of the camera) and immediately switched to the 40mm to make sure the AF still worked. Yup no problems there, quick as ever and sharp as a tack. Same story with the 35-70. I came home and tested all my other AF zooms and they're all working fine with it, too.
Okay, so back to the 100-300...whenever the focal length is at its shortest, the camera registers it as AF and will focus, but the second you move beyond 100mm, it switches back into MF. Same if it's mounted while extended beyond 100, it'll register it as a manual lens and ask for focal length. I turned the camera on and off with it mounted, took it off and cleaned the contacts of both and no dice... what should I try next? I know it's not a particularly expensive or hard to find lens and for $25, it's okay if I can't get it working but... I quite liked the few shots I did manage to get before it kicked itself into my MF lineup.

10-22-2020, 04:09 PM   #2
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Check the mount. You've cleaned it, but I'm curious if the mount is loose, disengaging data pins and asking for focal length. Attach it to your camera and wiggle the lens. How much movement occurs between the lens and the camera?
10-22-2020, 04:44 PM   #3
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QuoteOriginally posted by eixel2631 Quote
The other day I bought a FA 100-300
Hi @eixel2631,

There are two versions of the FA 100-300mm. One of them, 100-300mm f/4.5-5.6, has a 'power zoom' label marked on the barrel at the focal length window. I'm not familiar with it, but recalled that some users have had trouble with the zoom.

For example, see the first post at the Pentax Forums review page: SMC Pentax-FA 100-300mm F4.5-5.6 Reviews - FA Zoom Lenses - Pentax Lens Reviews & Lens Database

Not sure if your issue is related.

Here's a link for the operating manual. The Power Zoom feature is described on page 20. http://allphotolenses.com/public/files/pdfs/42752d9911da625cbe22dd3361c6f3b1.pdf

- Craig

Last edited by c.a.m; 10-22-2020 at 04:54 PM.
10-22-2020, 05:06 PM   #4
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QuoteOriginally posted by c.a.m Quote
Hi @eixel2631,

There are two versions of the FA 100-300mm. One of them, 100-300mm f/4.5-5.6, has a 'power zoom' label marked on the barrel at the focal length window. I'm not familiar with it, but recalled that some users have had trouble with the zoom.

For example, see the first post at the Pentax Forums review page: SMC Pentax-FA 100-300mm F4.5-5.6 Reviews - FA Zoom Lenses - Pentax Lens Reviews & Lens Database

Not sure if your issue is related.

Here's a link for the operating manual. The Power Zoom feature is described on page 20. http://allphotolenses.com/public/files/pdfs/42752d9911da625cbe22dd3361c6f3b1.pdf

- Craig
this one is a 4.7-5.8 100-300 without the window, not the power zoom, unfortunately.

10-22-2020, 05:15 PM   #5
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QuoteOriginally posted by eixel2631 Quote
this one is a 4.7-5.8 100-300 without the window, not the power zoom, unfortunately.
Okay, thanks. 50/50 chance, I thought!


If you don't mind, I'll leave my post intact here, in case someone in the future has a similar problem with the FA lens.

- Craig
10-22-2020, 05:20 PM   #6
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QuoteOriginally posted by builttospill Quote
Check the mount. You've cleaned it, but I'm curious if the mount is loose, disengaging data pins and asking for focal length. Attach it to your camera and wiggle the lens. How much movement occurs between the lens and the camera?
It doesn't feel like there's any play in the mount on the lens when turned or gently wiggled out (it's plastic) and the mount on the camera feels solid too. There is a small 'click' sound when the lens comes back to 100mm and AF engages but I don't know if that's a function of the camera or not.
10-23-2020, 05:57 AM   #7
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My 80-320 died without any warning losing af and aperture automation. When I looked into it there were a few articles that explained that a delicate ribbon on the lens of a small chip was likely damaged. I read about some who had more intermittent issues. Those consumer fa lenses aren't super durable.

10-23-2020, 12:49 PM   #8
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Update: I took it off the camera this morning, left the rear cap off and played around with moving the zoom in and out a bit really slowly to see what happens. Here's what I noticed-
At 100mm, the contacts look like this:

https://flic.kr/p/2jYpuTT

However, if you extend the zoom barrel at all, the last one "recedes" and looks like this:

https://flic.kr/p/2jYqpTm

I'm assuming something to do with AF gearing is knocked out of place causing this. I don't have the right tools to dig in there and see otherwise I would take the mount off and try to DIY a fix. I don't know if paying for a fix would cost more than it's worth, tbh, but maybe it's something really simple after all.
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