Leica Reducing focal length, Jupiter-8

helinophoto · Apr 12, 2016

Hi (not sure if this is the correct place to post this, please move the post to the appropriate location if it isn't).

-

I know Brian has explained how to reduce the focal length of the Jupiter-3's and 8's in various forums in the past, and I have had the pleasure of using his informative answers in my own adjustment (shimming) cleaning and lubrication-jobs on my Jupiter-8's.

Though, one of my lenses seem to be "long in the tooth", that is, on the long end of the nominal focal length. It's spot on close up and increasingly front focusing the further away you get form the target at F2.
- Only a stopping down to around F8 will make it ok from 5 meters and further away.

So I am trying to reduce the focal length, however.

The question I have is this:

Before removing the rear lens group, I always put some wallpaper-tape (paper tape) which covers the whole threaded area of the optics fixture + the rear optics group. Then i cut the tape with a knife, so that I am able to screw out the rear optics group.

The optics group then has a small piece of the tape and the rest is left on the optics fixture

(I find this better than marking the alignment with a sharpie).

When I then start sanding the rear part of the optics fixture, I can clean the area regularly and screw the rear lens group back on and observe how much the tape-pieces have moved to each other.

Two questions:
- I've seen the formulas on calculating the focal lengths, but I don't have the equipment necessary to perform anything else than "put a matte-screen where the film is and check focus with a magnifier". How to you measure a lens actual focal length..?
I do own a pair of calipers, and even though it actually, digitally, can measure down to 0.01mm, it's not practically possible to measure up the lens with it. (each measurement gives different figures).

- When I sand (I use wet-paper to sand, which creates much less metal filing-mess) I have sanded so much that I know that the rear group now screws one additional round (360 degrees) before it stops, compared to when I first put the tape on.
So the rear group is "one round closer" to the aperture and front lens group.

Still, the actual focal length _distance_ reduction in mm is hard to tell.
Does anyone know how much actual distance closer the rear group moves in a 360 degree turn?

After sanding so the rear group now comes "one turn closer" to the aperture blades, i also needed to sand the original shim down from 1.4 mm to 1.10 mm, so the lens is spot on at close focusing limit.

But I still see that the lens is still horrible front focusing, when comparing to previous test-shots, it seems that the focus error has hardly moved at all.

So I really don't know if I have sanded to little, or if the lens is faulty somehow.
- Since I really don't know how much I have reduced the actual focal length with, just "one turn" of the rear group.

The distance-scale on the lens itself seem to agree pretty well with the 1 meter, 5 meter 10 meter shoot distances.

I can see if I am able to attach some actual photo-tests a little later.

Tips?

Brian · Apr 12, 2016

http://aperturepriority.co.nz//wp-content/uploads/2013/02/Jupiter-3-Focal-Length-Adjustment.pdf

Jason Howe hosts my instructions for modifying the focal length- The "Lens maker's Formula" gives the distance to set two optics apart to produce a given focal length. I worked backwards from it, figure the J-8 is a 52.4mm focal length, the front group is about 125mm and the rear group is about 50mm. You want it to be closer to 51.6mm. The calculation gives 1.85mm to bring rear group closer to the front. Remember that you have to reduce the main shim to compensate because the rear element is moving farther from the image plane. The thickness of that shim is likely to limit the about of movement, but the net result ends up being much closer to the Leica standard. On the rear element: figure 1 full turn is 0.5mm. I use a scribe mark to watch rotation.

Jupiter-8_Closefocus

1957 Jupiter-8, rebuilt

Late J-8. I need to document converting one to close-focus. Basically use 1 stop screw and misthread the helical so that it acts as close and far stop. Re-index the focus ring and ring with the DOF marks.

helinophoto · Apr 12, 2016

Wow, thanks for that great information and links (I need to read that schema quite slowly ^^ )

Good to know the average distance of a 360 degree turn of the rear group (triplet?), it seem to correlate somewhat with the reduction needed on the main shim then? (or it may be a coincidence in this case?).

I do have some left of the shim, so I'll keep on grinding and checking until it's gone

If the lens is still off by then, I guess I will hold on to it, shoot it at f5.6-f8, or just buy another one, to see if that is better

it's the lens with no knob on it, on my re-grease write-up here: helino-photo: januar 2016

The knobbed lens also need to be filed a little I think, but I don't think it's as much off as the former one. (Love that older knobbed lens and after re-greasing, it's a real joy to use).

Incidentally I broke a J3 in January, was a really great specimen, but glued shut. Not even 4 days of a acetone bath (which I made sure didn't flood the glass, but did remove all the lens-markings), heating up or freezing, or even gloved hands helped, I made sure to tell the seller that gluing these lenses efficiently ruins them.
- Final attempt with two pipe-wrenches (I knew it would break the lens) was just on spite, to see how hard the lens was really glued, also failed, I wonder what glue they have in Russia!

From my inspection of that J3, it seemed that there were no apparent holes with screws to hold the lens completely together (or they may have just been lacking). The two screws I did find, had to be drilled out, because they were also glued.

Kind of put me off buying these J3's from eBay, as they are quite a bit more expensive than the J8's.

Thanks for taking the time to answer me =)

Brian · Apr 12, 2016

Sad to hear about the J-3! I've removed the glass from a couple before, then let soak for over a week in acetone. The most common thing I see is gluing the optical barrel into the inner helical. I take out the glass, take off the focus ring, remove the helical, separate the inner/outer and let that part soak. I've busted a front element getting one apart, fortunately had a good spare.

So with the J-8- that's what I do, move the rear group (doublet on J-8, triplet on J-3) in close as possible, keeping an eye on the main shim. I've seen a couple with shims greater than 2mm, but must seem to be about 1mm.

Hap · Apr 12, 2016

Brian said:
http://aperturepriority.co.nz//wp-content/uploads/2013/02/Jupiter-3-Focal-Length-Adjustment.pdf

Jason Howe hosts my instructions for modifying the focal length- The "Lens maker's Formula" gives the distance to set two optics apart to produce a given focal length. I worked backwards from it, figure the J-8 is a 52.4mm focal length, the front group is about 125mm and the rear group is about 50mm. You want it to be closer to 51.6mm. The calculation gives 1.85mm to bring rear group closer to the front. Remember that you have to reduce the main shim to compensate because the rear element is moving farther from the image plane. The thickness of that shim is likely to limit the about of movement, but the net result ends up being much closer to the Leica standard. On the rear element: figure 1 full turn is 0.5mm. I use a scribe mark to watch rotation.

Jupiter-8_Closefocus

1957 Jupiter-8, rebuilt

Late J-8. I need to document converting one to close-focus. Basically use 1 stop screw and misthread the helical so that it acts as close and far stop. Re-index the focus ring and ring with the DOF marks.

Well.....I would be happy to donate my new J8, KMZ black, extremely nice condition to the cause.......should you require it.

helinophoto · Apr 28, 2016

Just a small update here:

I have two J8's, one form 1955 and one from 1963.

I started grinding and adjusting the 1963 lens, but that one is now grinded to the max. The rear-element cannot be screwed any more in towards the aperture-blades.
Lens is still off (wide open), about 0.3-0.5 meter front-fcusing at 5 meters, which isn't bad at all, seeing i began with 1-2 meters.

Observation:
I found that if I pushed the focus point, on close focus, as far away as possible, while still retaining the focus, the lens would be a bit more civilized on further distances.
Thus, my target for adjusting the shim, was three 250ml cans of red-bull (they are nice focusing targets), placed right next to each-other at an angle.

On close focusing at around 1 meter, I set the focus to the center can, in such a way that the center and the far can were the most in focus. (very important that you set it up as a slightly "latent" back focusing lens).
The can closest to the camera should be "clearly" out of focus. This is a real pain to get right, but once adjusted, I saw that the lens will be in focus at f2.4-f2.8 all the time. (the 1963 lens isn't all that great wide open anyway).

The 1955 lens however was a nice piece of breeze ^^
I grinned it so that I was able to screw the rear element in an extra 3/4 of a round, adjusted the shim as notes mentioned in "observation" and lo and behold, the lens is pretty much spot on (except infinity) at f2

- This lens is a much better performer optically, so it's great that it can actually be used wide open.

Here are a few shots, mostly to indicate usability of the 1955 in a practical, every day setting. They are shot with my Leica M6, using an adapter.

I see the shots are way to small to gauge exact focus and actual sharpness, but I think they illustrate pretty well that the focus is nailed in just about all the distances I plan to use it for (I rarely shoot at infinity and if I do, I cannot fathom why I should shoot wide open in such a situation).

Focus on the handle of the bike furthers away, shot between the other bikes.