Jupiter 3 Lens Shimming Instructions

Optimizing the Jupiter-3 for close-up and wide-open

By Brian Sweeny

use on the Leica

Jupiter-3 Focus Calibration. The Jupiter-3 is a faithful recreation of the wartime Carl
Zeiss Jena 5cm F1.5 Sonnar “T”. The original KMZ Jupiter-3’s used German
components, and some can be found with Zeiss serial numbers stamped into the fixtures.
Jupiter-3’s made in the early 1950s used German glass. Around 1954, supplies of the
German glass were running short and a new computation was made using Russian glass.
Manufacture transferred from KMZ to ZOMZ in 1956. There were some changes made
to the optical fixtures: the shape of the rear triplet and fixture were changed, three screws
were used to hold the helical into the mount, and color of the coating changed.

The 52.4mm focal length of the Contax mount Zeiss Sonnar was used for both the Kiev
and 39mm thread mount versions of the Jupiter-3. There have been long standing
arguments concerning this, but I tend to trust the data sheet from the manufacturer. The
optical fixture of the Jupiter-3 screws directly into the inner helical of the focus mount.
The inner helical is a solid piece of metal and also serves as the RF cam of the lens. The
motion of the optics is 1:1 with the inner helical. When used on a Leica which is
calibrated for a nominal 51.6mm, this will lead to focus errors. A simple adjustment to the
main shim is usually all that is required to compensate for this difference.

There are cases where Jupiter-3’s work just fine on a Leica camera. Looking at a
manufacturer’s data sheet, it is easy to see why.

The focal length is given as 52.4 +/- 1%. That gives a range of about 51.9mm to 52.9mm.
Those on the short end of the range will work well on a Leica. The average will work
well once the main shim is adjusted, and those on the upper end will require the focal
length to be reduced and then the main shim to be adjusted.

This procedure is to adjust the main shim, which should work well for most lenses. This
is a “do as I say, not as I do” procedure and all you will need is your camera and a tape
measure/ruler. I use a through the lens viewer from an old camera repair shop, found on
Ebay for $15, ie I cheat… But I do use the Leica M9 to double-check that the $15 viewer
did the job.

Calculations for computing the shim are given in millimeters, so a metric ruler is the most

Focal Length (Millimeters)

Distance to Object (Meters)

Back Focus (Millimeters)

52.4

1

55.29759392

52.4

1.035

55.19438225

direct to use. Pick the “sweet spot” for exact focus. 1meter works best for using a

Jupiter-3 wide-open and close-up. This trades close-up performance for distance work.
The lens will focus slightly short of infinity. For a lens of the median focal length, F2.8 is
best used for infinity. Sonnar focus shift is towards infinity.

For this case, the RF of the camera was focused precisely on the 1m mark. I lined the
metric ruler up with “16cm” straight across from the 1m (39.4”) mark on the tape
measure. The actual focus is between “19cm” and “20cm”, meaning the lens is back­focusing by about 35mm.

This is where the formula for focal length gets applied.

1/f= 1/d+1/b, where f is the focal length of the lens, d is distance to the subject, and b is
the back-focus to the formed image. In “Excel”; solved for back-focus, it is:

“1/((1/(A2))-(1/(B2*1000)))”

where cell A2 is the focal length of the lens in millimeters, and cell B2 is the distance to
the subject in meters.

The RF focused at 1m requires that the back focus of the lens be 55.298mm. The lens
was actually at 55.194mm. All of this assumes the lens is 52.4mm in focal length, the
center of the range. So the difference given is a good starting point for adjusting the lens.
In this case, thickening the shim by (55.298-55.194), about 0.1mm is suggested. Try
layers of kitchen aluminum foil for making shims; the thickness is about 0.1mm. Use the
shim(s) of the lens as a template for cutting the new shims. Or buy lots of parts lenses.
One day, I will contact a machine shop to make them. As an alternative, you can use
paper for shims. Try different thicknesses; loose-leaf paper measured 0.07mm and ink-jet

printer paper measured 0.1mm. Screw the optics module back into the focus mount with
the new shims. With a thicker shim, the optics will not screw as far into the mount. A
thinner shim; it will screw in farther. Make sure to hand-tighten the optics snugly into the
mount. The aperture ring will not come up straight; you will need to “re-index” it after
the focus is satisfactory.

This is about as good as I can get an F1.5 lens on my Leica M8 and M9. You will need to
reshoot the ruler, adjust the thickness of the shim accordingly. Reduce the shim slightly if
it is now front-focusing, and thicken it a little if it is still back-focusing. The amount of
residual error is an indicator of focus deviation from 52.4mm.

The shim is the aluminum ring that controls the stand-off between the optics and the RF
Cam of the inner helical. This, and exact focal length of the lens, controls agreement with
the camera’s RF. To achieve critical focus at F1.5 an accuracy of 0.02mm is required for
the stand-off. The retaining ring that the aluminum shim sits on can be used as a variable
shim: consider this “Plan B” for getting exact focus.

The ring is held in place by two set screws; with taps underneath to hold into position.
The threads are 0.5mm per turn. A 15 degree turn of the ring is about 0.02mm. The trick
is to get the ring to hold position while you test the focus. I mark the position with a
silver sharpie. Once the correct position is determined, you need to make new taps for the
set screws. I use a hand-drill with a triple-zero tap; the same as will be required to re­index the aperture ring. The head of the screws must be flush with the ring in order that it
does not destroy the threads of the focus mount as you screw the optics module into
place. The screws must be put in evenly and the ring must be well centered: if the ring is
off-center the optics module will not screw back into place. This “just works” with a
Jupiter-3, the aperture ring itself has enough threads to keep it from wobbling when the
retaining ring is backed off.

Once you are happy with the new focus, try the lens at infinity. Typically, the Jupiter-3
will not quite reach infinity when used wide-open. This is due to the focal length being
longer than the Leica standard. A Sonnar suffers from focus-shift; when stopped down
actual focus shifts towards infinity. This works in your favor. If infinity is unusable, time
to shorten the focal length and then start over adjusting the main shim. Next lesson.

Chances are, with the thickness of the shim changes, the Aperture ring no longer lines up
with the index mark. Assuming the focus is good at this point: time to re-index the
aperture ring. Make sure the lens is screwed in tightly. Mark the filter ring of the optics

module where it aligns with the index of the mount. After all of this, you need to take the
optics module out of the focus mount. After you finish, you need to get it back to the
exact position.

The aperture ring is held in with three small set screws. Take them out. On lenses made
before ~1964, the aperture ring is threaded. Some early ones have “wings”, like the 1950
KMZ shown above. Sometime in 1951, J-3’s had their wings clipped. Later lenses depend
on the three set screws to hold in position, and on straight. Some force might be required
to turn it to the new position. It is easiest to open the aperture blades to F1.5, hold the
aperture mechanism in place, and turn the chrome aperture ring to the new index mark.
Use tape to hold the chrome ring in place. The most difficult part of working on the
Jupiter-3 is about to hit- you have to make small tap holes to put the set screws back into
place. The screws must sit almost flush with the aperture ring, or else they will hang up
on the focus mount. You’ll wonder why you cannot screw the lens back into place, or the
focus grinds when the lens is in one piece. Well, I did… The screws were not in deep
enough and they scraped against the focus mount. The best way to make tap holes is to
use a hand drill with a triple-zero drill bit. The metal is fairly soft, it is aluminum. The
idea is to make a hole for the end of the set screws to fit into.

The tape is to keep the aperture ring from moving as you tap out the holes. Do them
sequentially; some trial and error might be required. Once the holes are done, put the set
screws back into place. If they are not flush, remove and drill deeper. At this point, put
the optics module back into the focus mount and make a final check. Your Jupiter-3 is
now optimized for wide-open and close-up work. Make a test with infinity; try at F1.5,
F2, F2.8, and F4. The results indicate where on the focal length range your lens falls.
Lenses made before 1964 use a separate optical fixture for the rear triplet. It is possible to
change the focal length on these Jupiter-3’s: moving the rear triplet in reduces the focal
length, moving it out increases the length. Next write-up.