Pentax FA* 85mm - sticky AF

Started Jun 20, 2022 | Discussions thread
KPM2 Senior Member • Posts: 2,350
Re: last try of a maybe

Hello JawaT

JawaT wrote:

KPM2 wrote:

Hello JawaT

JawaT wrote:

KPM2 wrote:

Hello again JawaT

JawaT wrote:

I am using AF-S.

That is such a nice lens, therefore my last try:

What happens in AF-S and LV AF ?.

This lens is at full open not very sharp and it has also a little focus shift and has at full open beside the main maxima focus point also beside maxima...and that is not so easy to handle for the AF of the camera. Of cause, my FA* 85 focus snappy in AF-S, but, and this is my last maybe: is the front lens or the back lens a little dirty, like a fine film is on it. That could irritate the phase AF, but a contrast AF in LV could live with it.

best regards KPM2

Well, AF was intermittent for a while, but now has stopped working completely. Regardless of whether I am using the viewfinder or Live View, it does not focus when set to Auto Focus.

When the trigger is pressed halfway, I can hear something whirring in the lens, but the lens focusing mechanism does nothing. Manual focusing works beautifully.

about this

I guess I have to get it serviced. Very likely, one or motor in the lens has failed.

The lens has no motor inside for the focus. It look's like that the mechanical clutch mechanism is defect (maybe some screw is loosely) and do not couple to the AF gear.

I only send you this two pictures, for that you can compare the position of the pull and push ring:

manual focus position:

AF position:

The ring must reach this position, else the intern clutch do not couple to the AF gear.

Before you send your lens in, maybe look if your push and pull ring do can reach this AF position fully.

Sometimes I turne a very little and push the ring to get to this AF position. You hear a click sound than.

Thanks for all the suggestions.


good luck and

best regards KPM2

Thanks for the info that the lens does not have a motor for AF.

About this

When I push the lens in Auto Focus position, I can turn the focusing ring slightly in either direction and then I hear it click into place. After that it cannot be turned manually. So it would seem that it has engaged with the clutch. But AF does not work.

so, I think your ring get into it's correct position, but because your AF still do not work, I would let it repair.

You may be right that there is a screw loose somewhere, which is preventing the clutch gear from engaging completely. Do you know how much of the lens I would need to open up to confirm this?

I have no idea how to open the lens like Pentax would do it and without good tools I would not try it out. For example, under the rubber of the ring there are for sure some screws....and so on.

Another possibility is that the electronic contacts are the culprit, and opening up the rear assembly would allow me to clean these out more thoroughly.

When you push the ring into it's AF position, this mechanism close also an electrical contact, so that the camera do show you at once in the Info screen and in the OFV that you are in AF now. And because that seems to be the case with your lens too (because you can hear a little the AF motor of the camera when you try to focus), I don't think you have a contact problem of this contact. When this contact would be not good, your camera would show that you are still in the manual focus mode and when you try to focus, the cameras AF motor would do nothing. Other electrical contact infos are the distance scale and the two end positions. When your lens do have here a contact problem, so that the camera do not know where the position in the focus range is at the moment, it can be that the camera hesitate than to let move it's AF motor and that's why you see the blinking square. This contact you don't see when you open the rear assembly.

I hesitate given how expensive the lens is.

I would not try it too. Maybe, when you let it repair, they even have to send it to Japan.

Any advice?

This makes no sense, when a screw is loosely, but when your lens has a problem with the focus position contacts, I would try out to set the lens to manual focus and would move the focus ring quite often from infinity to the min. focus distance. Such a movement can clean also the focus position contacts.

best regards KPM2

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