Pentax 6x7 45mm F4 Lens - Focus issue

[graemea]

Hi all,

i've recently acquired a Pentax 6x7 to add to my collection. I have a 75mm F4.5 and a 135mm F4 Macro lens for it. I've been offered a 45mm F4 but it doesn't focus - well not strictly correct, it does but only at about 0.6m.

The focus ring turn and stops at infinity and at the closest focus distance, but this doesn't appear to have any effect on the actual lens length or the focus. It looks like that the focus ring isn't attached to the lens elements.

I've worked on a few cameras and lenses over the years, but this has me stumped. You'd expect to see a screw under the rubber focus band, but there is none and initial web investigations have shown others stuck with the same issue. Removing the lens front "beauty ring" doesn't reveal anything either. Does anyone know how this works, of perhaps if there's a technician somewhere who can fix it?

Thanks

Graeme Arlidge

 

[Lachlan Young]

It has (to the best of my knowledge)a double helicoid focusing system - something seems to have come adrift on your example internally. Anyone who is skilled at P67 repairs should know what needs to be done.

 

[graemea]

Hi Lachlan, yes but where do I find such a person?

 

[OAPOli]

I haven't worked on the 45mm but I believe most P67 lenses are similar. 1) Remove the name ring with a rubber tool; 2) Remove the bayonet/filter ring (4 screws); 3) underneath you'll find the three screws holding the focus ring to the helicoid. May be those are loose. Once they are attached, focus to infinity using ground glass and a distant subject (or an autocollimator). Then loosen again and fix the ring to infinity.

 

[F4U]

If you don't understand how the double helicoid assembly works, you're in for a world of hurt trying to get it back right again. There's as many combinations as a Rubik cube. I would not touch that lens, free or not.

 

[abruzzi]

I'd check with Eric Henderson here:

Home-Pentaxs

pentaxs.com

He is an expert in all things Pentax.

 

[graemea]

Thanks for the replies and the info about Eric Henderson.

Tennessee is a long way from New Zealand however.

I might try aopoli's advice and have another look at it.

 

[jonny88]

It might just be a design quirk where the focus ring isn’t engaging with the lens elements the way you’d expect. Maybe try giving it a gentle nudge or check if it’s misaligned, but if nothing changes, having a tech take a look would be the best call

 

[MattKing]

Tennessee is a long way from New Zealand however.

@graemea ,
It would likely help if you changed your location information to indicate that you are located in the New Zealand version of "Hamilton".
Otherwise, people might think you are posting from "Steeltown"
(Hamilton, Ontario, Canada)

 

[abruzzi]

Thanks for the replies and the info about Eric Henderson.

Tennessee is a long way from New Zealand however.

That is definitely is, but I did send my Kodak Retina to NZ for a CLA from the main expert (sent from New Mexico, USA.) If you can't find anyone more local, I do reccomend him.

(to @MattKing 's point, I think there is a Hamilton, NJ, MA, and MI, and who knows there may be many others...)

 

[itsdoable]

The Pentax-67 4/45 has floating elements, which typically means the rear group moves a different distance than the front group when focusing. So the adjustment is not just infinity focus, but optimum sharpness at infinity as well. Check for set screw where the focus scale is, you have to rotate the ring to expose them. If there are no set screws there or under the rubber of the focus ring, they may be set from under the front beauty ring. I worked on one a long time ago (back when I had a full Pentax 67 kit), and I cannot remember if the 2 groups were linked, or if they had to be independently set.

I had a Zeiss 1.4/35 with floating elements that had to be sent back to Germany to get the infinity set and calibrated when the Canadian service center couldn't- back then I did not have access to an optical bench, or I would have tried to do it myself first. ... and it probably would have gone back to Germany anyways!

 

[graemea]

Hi

I sent it to a repair shop in Auckland for an appraisal and they came back with a repair price of rather more than the lens is worth - about $450 USD. Since they talked about "focus motors" I was not impressed. I've had the lens returned to me and I don't believe it was inspected in any way.

I'm certain the issue will be quite simple, but I've taken on board the comments about double helicoids. I don't have the right size rubber rings, so I'm buying those and will update the forum with what I find when I look further into it.

Thanks everyone for the input.

 

[choiliefan]

I'd go with the first suggestion by unscrewing the front lens surround that's engraved with the lens' nomenclature and inspect the underlying screws for looseness.