Help for dismantling & re-adjusting a Takumar 28mm f/3.5

[bc_the_path]

Hello,


I have a Takumar 28mm f/3.5 in seemingly good condition.
I bought it about two years ago from the usual place(!) and it never produced an acceptable photo.
All the objects that were seemingly "in focus" on the viewfinder were a bit blurry in the end...
Recently, after installing a new focus screen with prism(s) into my camera, I have tried it again.
To my surprise, I have noticed that it was unable to focus on anything, far or close.
I guess it had been improperly serviced before and (at least) its rear element is not close enough to the sensor. Hence its inability to focus.


I am not thinking of giving it to a pro for a CLA; too expensive for this lens.

Here are my questions:
- Do you think a technically above average guy like myself can dismantle it, correct the fault and re-assemble it?
- I have directions for 50mm f/1.4 Takumar (from The Internet). Are they similar in construction?
- Should it be first opened from the front or the rear?
- Is there any other way than trial and error to bring the rear element to the correct place (like a mark or guide)?
Thank you in advance for your attention.

Bulent

(Please forgive me for cross posting this to another forum as well. Takumars are also used by many non-Pentaxians...

[Just1MoreDave]

The lens should be similar in construction to the 50/1.4, so the instructions you've seen should work. In general, Takumars are disassembled from the front and very little if anything can be done from the rear. However, on some lenses you can access the rear elements. I think on this lens, you can see the spanner slots for those elements and get to them without front disassembly.

Try to find an optical diagram for this lens, because it sounds like an element was installed backward - or possibly missing. A lens out of adjustment for infinity focus will still focus, infinity is just in the wrong place.

[GerryL]

You can google it online how to dismantle lenses.
There is even one for the Takumar.
Most of the takumars the front ring detaches and you should use a friction tool on that.
One I have read uses the rubber end of a crutch (used for limps and leg amputees).
You can also buy these rubber friction tools specifically for disassembling lenses.
There is also a lens spanner tool that you have to use on some of the lenses.
You can buy this off ebay or amazon for $15 plus shipping I think or you can make yourself one once you see how it looks like.
Here's one site but don't drill holes on the front trim ring like he did..use a friction tool and the front ring will unscrew: Disassembly and Cleaning of the F1.4 50mm Super Takumar Lens

Here's the friction tools and dis-assembly method of lenses: Manual Focus Lenses :: View topic - Trying the rubber cups for lens disassembly

I hope this helps!

[GerryL]

Here is a lens spanner you might need: Amazon.com: Lens Spanner Wrench For Camera or Enlarger: Camera & Photo

Another variant of the lens spanner is like this one (got the picture from ebay I think):

[Just1MoreDave]

Here's a photo of the Super-Takumar 28mm f3.5. Other versions have different mount details but the rest is the same. I suspect the outermost spanner slots unscrew a light shield. The innermost one unscrews a metal ring holding the rearmost element in. The middle slots might unscrew the entire rear group or a middle element within that group. That's all you can access from the back. If these rings are difficult to unscrew at all, try a drop of acetone on the threads. Sometimes they are glued in place with lacquer.

I'm not sure why you think the rear elements are in the wrong place. The photo shows their position with the lens near infinity, almost as far into the mount as they'll go. With the lens focused to its minimum distance, the front and rear groups all move forward, away from the mount. I don't have this lens anymore to tell you the exact distance forward.

[bc_the_path]

Many thanks to you all.
You have provided a wealth of information and images.

I will definitely give it a go soon. I will inform you all by reviving this thread at a later time.

I could not find a diagram for the exact lens. Bojidar's site has one for the K-mount; I just guess they should be similar. At least, I can count the elements!

You are certainly right; there may be one missing or (hopefully) installed wrongly.

Just1MoreDave,
The number seen on the Auto/Manual lever on mine is next to your photographed Takumar!!!
Yours 43871, mine 43872.
The central part of the rear view is exactly the same and the elements do protrude into the body.
However, my lens also have a protruding stop-down indicator!
It also has a tiny push-pin to manually release the auto/manual switch.

I have not noticed these before!
I guess this is a dedicated lens for a certain body (Spotmatic?).

Your contributions are very much appreciated.

[Nesster]

Before dismantling, let's double check the symptoms!

When you say you are unable to focus on anything - does the lens actually move as though something's going on, or does the focus ring turn and nothing else? You might have a loose ring.

Also, the helicoid that actually achieves the focusing action may be slipping at some point along the chain. You can check, but I believe these are accessed via the front of the lens rather than the back.

My first suspicion is that something along the focusing mechanism is disconnected or slipping, rather than a mis-assembled lens.

[bc_the_path]

I have just discovered (while re-testing the lens following Nesster's advice) that the source of my problem was the improper adapter I have attached to the lens (and left on it).
That was one of those cheap adapters that do not allow infinity focus.
With the proper adapter, the situation is different and there is no problem at all!
The entire movement of all the assembly is about 3-4 millimeters from 30cm to infinity and the thickness of the adapter becomes a most serious problem because of this. The lens is not only unable to focus on infinity but on almost anything in front of it!
I guess my original idea that the rear element of the lens was not close enough to the sensor was correct, after all
I wish I could take credit for that

I am sorry to have bothered you with a non-existent problem and I thank you again for your sincere attention.


Bulent

[Nesster]

Well, I'm glad you figured it out without surgery

Too bad the bad adapter isn't thick enough to serve as an extension tube ...