tale of 2, 90's

[stonearabiapix]

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I have a 50's vintage, 90mm f/4. and a 70's vintage, 90mm f/2.

 

The f/4 focuses spot on. always.

The f/2, at f/4, (or at any other aperture) is back-focusing two inches.

 

Should I take a file to the focusing cam on the f/2?

Or is there another fix?

 

Thanks.

[elgenper]

......

The f/2, at f/4, (or at any other aperture) is back-focusing two inches.

 

Should I take a file to the focusing cam on the f/2?

.....

 

Well, certainly NOT! That´s not the way a qualified repairperson would use for adjustments (and it´s this kind of person that should be contacted).

 

It´s not a DIY job (if the fault lies with the camera, a skilled and informed owner may be able to adjust the RF with an Allen key).

 

And, with all respect, if you think back focussing might be eliminated by filing the cam, you haven´t quite grasped how it works. Back focus means the lens isn´t extended enough when the RF images coincide, right? Then the focus falls behind the plane where you wanted to put it. Shortening the cam would mean that even less extension would get the RF arm to the same position, and would worsen the back focus.

 

So, honestly, employ someone who knows how to do it properly.

[zombii]

You're in New York. Send it or even take it to Leica NJ in Allendale. They'll do it right and you could take your body as well if you can live without it for a few weeks.

[stonearabiapix]

Sorry Per, i was only joking about taking a file to it, but if what u say is true, maybe i can solder, or add some epoxy to the cam to make it bigger!

Your point being taken, send it in.

But does anyone have an idea why one 90 would focus properly and the other 90 not?

[elgenper]

Sorry Per, i was only joking about taking a file to it, but if what u say is true, maybe i can solder, or add some epoxy to the cam to make it bigger!

Your point being taken, send it in.

But does anyone have an idea why one 90 would focus properly and the other 90 not?

 

OK, kinda suspected some tongue-in-cheek...

 

Still, don´t mess with the cams! In first-order theory, things should work out OK if the end edge of the cam were exactly flat in a plane perpendicular to the lens axis, but there are some approximations in that theory, so esp. for a long, fast lens there has to be small deviations from that plane. These ´undulations´ are computed and precision ground to give correct focus at all distances, and any messing with it would really open a can of worms.... I own a German book from the ´40´es that goes through the maths in all its gruesome detail; good reading if one wants to cure insomnia....

 

As for some lenses not focussing correctly, that´s quite common. If all, or almost all, lenses are off by the same amount, it´s the camera body that´s out of whack. But just one lens off while the others are OK means that lens must be adjusted.

 

Only yesterday i retrieved from service a beautiful, almost mint TeleElmar 4/135 that I bought some months ago, and which was completely off in its focus. Turned out someone had dismantled it and reassembled it incorrectly; one needs a collimator to do a decent job. Now, it´s spot on. An unexpected outlay, but well worth it!

[k-hawinkler]

I don't know anything about collimators but am very interested in learning about them.

How much do they cost? How does one operate them? Where does one buy one?

 

Thanks, K-H.

[elgenper]

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I don't know anything about collimators but am very interested in learning about them.

How much do they cost? How does one operate them? Where does one buy one?

 

Thanks, K-H.

 

Again, I sort of sense several tongues in somebody´s cheek....

 

It is an optical instrument that can be used for exact measurements on lenses, like true focal length, position of the focal plane in relation to the lens mount & c. I´ve never even contemplated buying one, so I don´t know the cost, but for a one-time problem, it´s probably cheaper to buy another lens instead....

 

And, to use a parable, buying a good set of scalpels doesn´t make one a brain surgeon...

[k-hawinkler]

Per,

 

No tongue in cheek intended at all.

 

Thanks for your reply, K-H.

[ramosa]

I have a 50's vintage, 90mm f/4. and a 70's vintage, 90mm f/2.

 

The f/4 focuses spot on. always.

The f/2, at f/4, (or at any other aperture) is back-focusing two inches.

 

Should I take a file to the focusing cam on the f/2?

Or is there another fix?

 

Thanks.

 

I would send it into the shop. But that's just me (and indicative of my low skill in fixing any substantial camera/lens problem.)

[k-hawinkler]

Can anybody say what kind of collimator the qualified technicians use to adjust Leica lenses?

I am retired and not interested at all in setting up a business - but I am very curious.

Learning more about that subject would keep me entertained for awhile.

 

Any articles or books on the subject?

Where would I start reading?

 

K-H.

[ho_co]

So far as I know, lenses are collimated when produced. I doubt that many optical technicians have or have any interest in having one.

 

When a lens suffers a severe knock, it may need a rebuild, at which time a collimator will likely again come into play.

 

K-H, call Leica Solms and ask what kind of collimators they use.

 

 

 

But the point is, T, if you've got a cylinder misfiring, do you think you should just disassemble the engine and squirt some putty into the cylinder?

 

If your eye doesn't focus correctly, do you just apply pressure to it till it works right?

 

Send the lens to Sherry or Dan or Leica.

[k-hawinkler]

Howard,

 

Thanks. I do send in my lenses to a qualified technician. That's not the point.

(and BTW, he just let me know he couldn't focus one of my lenses on his collimator.

So he must have one).

 

I thought one of the reasons for the existence of this forum is education - my education.

For me education entails a little more than just the basics.

 

Thanks for the pointer. I will ask Leica then.

 

Best, K-H.

 

PS: The most illuminating article I have found so far is: Principles of Autocollimation

[ho_co]

Good! K-H, I'm sure you noticed that I addressed the second part of my post to T, the opening poster. I know you use good technicians, and now you've let me know I'm wrong about who has collimators! My error. Glad to be corrected!

 

(BTW, I put "someone disassembled it and reassembled it incorrectly" in the same box as "I dropped it thirty feet onto a granite floor." I know those may not be technically the same thing, but that equivalence is in my personal tuning.)

 

Google "optical collimator" for more accurate information on the topic than mine!

 

 

I thought autocollimators were for cars?

[elgenper]

...

(BTW, I put "someone disassembled it and reassembled it incorrectly" in the same box as "I dropped it thirty feet onto a granite floor." I know those may not be technically the same thing, but that equivalence is in my personal tuning.)

....

 

Not quite so dramatic... There´s a helicoid in there with 16 ´entries´ (if that´s the correct English word for it), and only one of them is the right one; the others put the optical head at a different distance than intended at the infinity stop (and all the way through). No markings, either...

 

My technician told me he had to try one entry after the other, and check focus on his collimator each time. He hit home on the thirteenth try, he said...

 

And now, I have a GREAT lens!

 

BTW, what a collimator does in this context is primarily to present an image (of a thin line) at virtual infinity, to find the secondary image projected by the camera lens in question, and to measure very exactly the distance from the lens flange (say) to that image.

[luigi bertolotti]

Sorry Per, i was only joking about taking a file to it, but if what u say is true, maybe i can solder, or add some epoxy to the cam to make it bigger!

Your point being taken, send it in.

But does anyone have an idea why one 90 would focus properly and the other 90 not?[/quote]

 

The old Elmar 90 has a very simple RF coupling mechanism, no dissimilar from Elmars 50 and siblings. On the contrary all long focus lenses with "wide" max apertures (see, for instance, the Elmarit 135, and of course your Summicron 90) have a rather more complex RF coupling, with more capabilities of fine adjustments... this, naturally, has pluses and minuses... more points that can go out of specs: but leitz lenses were anytime very well engineered, and, luckily, good labs can still operate the right way on "over 50" lenses.

 

BTW : it happened, but was pure luck, that with my Summicron 90 of 1977 that suffered the same issue (differently from my Elmar 90 3 elements...) I had a good result simply attaching a strip of scotch to the coupling cam... its thickness was, by chance, right for the job... but isn't by sure a rule, and, btw, I still have some issue if focus to infinity at f2...

[stonearabiapix]

Luigi,

I just have to tell you the scotch-tape worked very nicely on my 90 as well!

T.

[jaapv]

Can anybody say what kind of collimator the qualified technicians use to adjust Leica lenses?

I am retired and not interested at all in setting up a business - but I am very curious.

 

 

I can tell you - a target at one meter, one at three meters, one a ten meters and a church tower at 2 kms. Their little box of spanners and screwdrivers and a collection of shims. That's all - oh - and years of experience and gaining skill......

[luigi bertolotti]

Luigi,

I just have to tell you the scotch-tape worked very nicely on my 90 as well!

T.

 

:)Fine... the Summicrons of 70's clearly share a love for do-it-yourself...