[cyberjunkie]

Hi.
I purchased a Pentax-F 70-210mm zoom for almost nothing.
It has fungal growth inside, in one or more elements close to the front glass. The rear part of the objective seems to be okay.
All Pentax-F lenses are a bit wobbly, i guess because they had to place as little stress as possible on the AF motor in the camera. That opens the door to dust and humid air, which get sucked inside the lens when the zoom/focus rings are operated. It is between the groups which move one toward the other that the growth of fungi is more likely to happen. That's where i want to get to have the elements dismantled and cleaned.
I have dismantled large format lenses in the past, never an AF lens or a modern zoom.
I have most of the tools usually needed for the work, but before attempting to proceed at random, i'd prefer to get some help.
Not asking for a step-by-step guide (that would be perfect! ), just a quick walk-through.
If anybody is aware of any online resource, which refers to a similar Pentax-F lenses, that could be very useful too...
Thanks in advance

cheers

Paolo

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Last edited by cyberjunkie; 08-12-2014 at 07:02 PM.

[Imageman]

I really don't think this is a good idea.


I too have dismantled large format lenses, and I have repaired a large format lens. I have also opened and repaired 35mm lenses. And I have also stripped and reassembled many 35mm lenses.


The lenses I have stripped are I think similar to the ones you have, typically 4 or 6 element prime lenses, with no autofocus no zoom and no automatics.


Zooms typically have not 4 or 6 elements, but 12 or 15 elements and in some cases up to 35 elements, in sophisticated moving carriages than must be precisely aligned and calibrated using an optical collimator so they remain in calibration throughout the zoom range.


On top of this you have automatic diaphragm control with actuators and motors, plus autofocus with actuators and motors.


In my opinion, without the expensive and sophisticated equipment a lens mechanic has at his fingertips. it will be 1000 times harder to successfully calibrate a modern zoom than it would be to assemble and calibrate a simple large format, medium format, or 35mm prime lens, which you can easily strip and rebuild several times until you get it right.


One slip up with a modern auto focus autoexposure 15 element zoom and it will never work satisfactorily again, until of course it is properly calibrated with the right tools.


I personally wouldn't touch one, and I have adjusted precision optical measuring equipment including engineers theodolites and levels.

[cyberjunkie]

That is not 100% sure... it depends on how the lens is made. There could be no need at all to dismantle the single elements, the surface affected by the fungal growth could well be the external one of a sliding group consisting in a set of elements, cemented or air-spaced, encased in a single "tube".
Actually, i bought the lens (little more than 10 euros) cause i deemed that probability as the most likely, and the fungus as an easily cleanable one (usually it is the cobweb one which CAN attack the coating, not the spore-like one).

cheers

Paolo

[Imageman]

Sounds like you have the right approach.


Look out for springs and ball bearings and other gotyas as you go in if you have to attack the rear.


The fact its a front glass where the issue seems to be is good, there might be a way in through the front. The issues should all be in the rear.

[Just1MoreDave]

There are three small slotted screws around the front barrel. You should be able to loosen these, then use a lens spanner on the outer slots around the front element to unscrew the front group of lenses. The front group is three elements. Two are glued together, and sometimes have element separation. My lens looks like this:



When you reassemble the lens, that front group has to be readjusted for infinity focus. Then tighten the slotted screws so it stays in place.

This lens is fairly annoying if you have to disassemble it from the back. Or maybe I should say, I have no idea how they put it together on a production line. It takes me maybe 30 minutes to get all the tiny springs, contacts and insulators in the right holes and working properly. Later lenses have a nice one-piece insulator that lines up all the contacts.

I have a link here to a page with some photos and the hint of more detail, but it doesn't deliver. That may be enough:

Pentax 70-210 Repair

[cyberjunkie]

Thanks, Just1MoreDave!!!

I couldn't ask for more!
Definitely you helped me. A lot!
It's incredible how this forum could prove resourceful, and some people nice and helpful.

I have the lens in Italy.
I bought it on Ebay but i didn't have it shipped to Amsterdam cause i don't have any tool here.
As soon as i go back i will tackle the lens. Hopefully the problem will be at the back of the front group.
There are a few very good (and not so expensive) lenses in the F series. Sadly the way they were made makes them prone to this kind of problems. If the lens is used in a dusty/humid environment, and/or not stored properly, the wobbly construction allows a lot of air/dust to get inside.Zooms and IF primes are pumps, the more wobbly they are, the more they freely aspire air from the outside.
The fact that all the elements at the back are not held together is a surprise. Those poor vietnamese workers! Assembling these lenses should have been a nightmare

cheers

Paolo

[Imageman]

Its usual in assembly of several loose parts to have a jig of some kind, recent assembly techniques have the parts designed so they self locate reducing the need for jigs.


Assembly time is an added manufacturing cost so they take steps to reduce it if possible.

[cyberjunkie]

The (small) risk i took in the end payed off.
Thanks to the advice i got from Dave (btw, thanks a lot, Dave!), the front part of the lens proved easy to dismantle. ALL the three glasses were covered by a net of fungal growth. Cleaning all that mess was not quick, and not as easy as other fungi i cleaned in the past. I had to dismantle the elements once more cause there was some "fog" left. The second cleaning session removed any trace of dirt. Fortunately the fungus didn't have enough time to attack the coating/glass surface, so now the lens looks and behaves like new.

I found a weird thing: the lens works as it should with a K-5 II, but doesn't pass aperture and FL infos to my K-10D. The AF is not working as well!
I didn't test the lens before opening it, but i didn't touch the bayonet flange and its contacts... so i guess it behaves the same way it did before.
While i was writing the previous line i thought it was better to double check: i took out my K-01 and tried the 70-210 with it. Perfectly OK.
The problem shows just with my K10D. Really weird, cause i tried another AF lens and the body focuses and shows the aperture, as it should and as it has always done...

Any clue?

cheers

Paolo

P.S.
If you're on a tight budget, and can't afford the best of the crop, don't be scared to bring back to life some old (or not-so-old) damaged optics.
Do your home work first, practice with a hopeless objective before tackling a valuable one, and use common sense. Even more important: buy cheap so the risk you take will be minimal.
There are a lot of satisfactions along the way, and probably a few disillusions. Just think that what was routinely done by the photographers of the old times can be done today... you just need to learn how to do it!
I recently bought 3 damaged/not working lenses. The Pentax-F zoom is fine, a recent Sigma 30mm EX DC is usable (but i will order the front glass to replace it, if i find how the hell the front of the lens can be opened!), and a wonderful Kino-made 28mm f/2 will have the diaphragm blades cleaned/fixed quite soon.
All of them were payed a fraction of their market value, no great risk involved....