Q. for tech. gurus: adjusting FSU lens for Leica

[Spyderman]

Hi guys,

this is a question for the more experienced technical gurus (Kim, Brian, ...)

I'd like to adjust a J-8 for Leica.

AFAIK and according to Dante Stella the FSU lenses are built based on the Contax standard focal length 52.3mm but Leica uses 51.6mm as standard focal length. Because of this, many users of this combination encounter the focussing error caused by this incompatibility.

I'd like to adjust the physical focal length of the J-8 lens to work on Leica close up and wide open. Somewhere I read that it can be done by decreasing the distance between the front and the rear lens group...

So my question:
Is it even possible? 😕
About how much would I need to decrease the distance? (I might do it the "trial and error" way, but some apriori info would help 🙄 )

 

[Xmas]

Ondrej

If you are lucky your J8 may be ok, you need to try a focus target at the end of one of your cassettes, at the distance you want eye-lashes to be sharp.
You need to be sure that your Leicia rgfdr and register if ok, on a known good lens, at the same distance, a friend with a screw 'cron etc. and the same target.

I think you would need to add a shim, in theory, and with the thick lens Jupiter between the rear two groups as the front cell focus was only resonable with thin lens triplets. Sums are possible but will hurt my head...

"Dont." - O. Wildes advice on those about to be married

Noel

 

[Spyderman]

Thanks for advice Brian. The lens we're talking about belongs to a fresh RFF member from Bratislava who asked me to to help him with his J-8. He also has some Leica glass for his M3, so he said he's willing to sacrifice the J-8 for this complicated operation.

I thought that after removing the rear group I might file down the front part - the one with diaphragm. The tube where the rear group screws seems long enough. After this, shimming the lens for infinity focus is easy 😉

 

[Nickfed]

The first thing to consider is - is this really necessary?

If you just feel like doing some pointless tinkering, who am I to stop you? But, I submit you should read Oscar Wilde's advice above, or at least don't until you have done some basic maths (probably the same maths they did at the FED factory in the first place). If you put Zeiss on a Leica and focus on 1m, the error will be about 25mm. This assumes that all other factors are following the theory. This percentage error will follow you linearly throughout the focussing range.

If an error of 25mm at 1m really bothers your friend, I submit he is using the wrong camera and an SLR might better serve his purpose.

 

[Spyderman]

Nickfed: I don't know what physics says about this, but from more sources I know the focussing error is about 5 cm at close focus. It's not pointless tinkering - it's adjusting J-8 for Leica. I believe that if I am succesful, many people here would follow me and adjust their J-8s. J-8s are great lenses, and many people would like to use them on their Leica...
After all I didn't ask WHY NOT do it. I asked HOW is it possible. And I really don't know why I'm trying to convince you...

 

[lubitel]

May be I am just lucky, but I have used J-3, J-8, and J-9 on my M2 and have not had any problems at all focusing wide open and close. May be some of the lenses have problems others dont?

 

[rxmd]

A well-calibrated RF has a more precise focus at this range, for this focal length, and at this F-Stop than does an SLR.

Just as a matter of interest: what is the mathematical basis for this kind of estimate of focusing accuracy on SLRs? Is this taking account all the various "RF-esque" focusing aids for SLRs like split-image indicators and microprisms?

Philipp

 

[Nickfed]

Nickfed: I don't know what physics says about this, but from more sources I know the focussing error is about 5 cm at close focus. . After all I didn't ask WHY NOT do it. I asked HOW is it possible.

Either way, the comment still stands. And knowing a little of the physics won't do you any harm in tmes like these either, for I submit you would then find the focussing error is 26.75mm. And while you're at it, a quiet bit of measuring of the forward movement of the lens wouldn't go astray. Who knows what surprises may show up? And what do you do if you find one? Re-cut the thread? 🙄

 

[Kim Coxon]

Lets remember the rules and keep it friendly. 😉

Kim

Either way, the comment still stands. And knowing a little of the physics won't do you any harm in tmes like these either, for I submit you would then find the focussing error is 26.75mm. And while you're at it, a quiet bit of measuring of the forward movement of the lens wouldn't go astray. Who knows what surprises may show up? And what do you do if you find one? Re-cut the thread? 🙄

 

[Spyderman]

Sorry Nickfed. I know I SHOULD study the physics... 😉

 

[Nickfed]

Sorry Nickfed. I know I SHOULD study the physics... 😉

You probably did but have forgotten it. We are only talking the most basic high school stuff--------> 1/u + 1/v = 1/f

where u=object distance, v= image distance, and f = focal length

Derived from Snell's Law, I think, c1620

 

[Spyderman]

Thanks. I have also a nice book about optics and principles of cameras, but I always skip the part with complicated formulas, and instead jump to pictures and exploded diagrams of M3s and OM-1s 😀

 

[Nickfed]

Aha! Then you will probably find the above formula large and clear on page 1!

It's the thing you really need to know. Everything else is merely what you would like to know......

 

[ZorkiKat]

Ondrej

Lubitel has stated it- many Jupiters actually work right. 🙂 My J-9, J-8, and J-3 focus properly on my Leica M3 and Leica IIIc/IIIf/III cameras. It's probably more of individual quality issues that make certain specimens of these lenses focus erroneously on Leica and similar cameras. I don't think it should be taken as a rule to assume that all Jupiters would not focus properly on Leica unless they've been modified.

I do have a couple of J-9 which won't focus right with the Leica. Then again, these lenses won't focus right even with Zorki or FED. I've had more serious focusing issues with Industar-22 when used with cameras other than what they were found with. These problems could be directly attributed to the way these lenses were made- their optic blocks were not seated at the 'right' place so that they would have the right working distance to focus correctly on the Leica's focal plane. Such issues could be easily resolved (with the right tools, though procedure is lengthy and repetitive) by simply adding shims - or even removing and replacing them- to achieve the right seating position for the lenses in the barrel.

One version of the FED-50 (I-10) lens cannot be easily fixed. The camming and lens movement are not standard. The lengths at which the lens cam at the rear is extended at various distances do not conform to what is necessary for focusing with the Leica.

These comments are based on the limited experience I had with my lenses.🙂

Jay

 

[wyk_penguin]

Just as a matter of interest: what is the mathematical basis for this kind of estimate of focusing accuracy on SLRs? Is this taking account all the various "RF-esque" focusing aids for SLRs like split-image indicators and microprisms?

Philipp

I believe the assumption was that you used a split image screen in your SLR. So you would go on calculation the apparent size of the split image and so on.

 

[Kim Coxon]

Whilst the formula that Brian is referring to gives the theoretical maximum accuracy of the finder, it does not take into account the practical aspects. With an RF, the accuracy is based purely on the focal length, EBL and mag of the finder. It does not matter if you are using a 50/3.5 or a 50/1.2, the picture you use will be the same. Also any errors will depend not only on the camera RF but also the lens's focussing accuracy. Unfortunately, any errors will be much more pronounced with the faster lens. With an SLR, depth of field becomes an issue. With a slow lens, it is far more difficult to see the correct focus than with a faster lens. My Pentax 50/1.2 is a breeeze to focus but the 2.8 macro is more difficult at moderate ranges. Also as long as the body is correct, any errors in the lens will be compensated for as well. I know some people have had minor problems with the Canon 50/1.2. The J8 and Indys tend not to cause a problem because of the more moderate aperture and the simpler mech.

Kim

 

[mike goldberg]

Hi All,
Earlier this year, Brian Sweeney told me that many J8 Alignment problems can be resolved by:

"backing off the rear standoff ring one quarter of a turn."

Still and all, I bought a J8 from him, specifically collimated for the Leica standard, and it is superb at close focus as well as at Infinity.

See photo Link:

http://www.flickr.com/photos/mikegoldberg/439631251/

Good luck,