TUNING ANAMORPHIC ADAPTER SHARPNESS
You might have seen this sort of thing in forums: "The xxxx adapter is much sharper than yyyy adapter".
Are you sure? Have they both been tuned for maximum sharpness? Did you know that was even possible?
Anamorphic adapters have non-spherical lens elements. They have two groups, a front and a back group. Each of those groups is housed in its own aluminium lens barrel, and these barrels slide one within the other (front element barrel inside rear element barrel) and this sliding is how the adapter is focused for different distances.
While those barrels slide they must remain in the same radial alignment with each other. Two or three brass plates (each screwed to the inner barrel with 2 screws) locate the barrels relative to each other, and these plates slide in elongated slots machined in the outer barrel (see images below).
You can finely adjust this alignment, and it is ABSOLUTELY crucial to sharpness.
(If the lens elements were spherical it wouldn't matter, but they aren't and only work together in one orientation (180 degrees to one another). And since this alignment can be adjusted, and will definitely need to be if it isn't correct to begin with. Remember most of these adapters are 50+ years old. I too need adjusting sometimes!).
I have seen two broad types of adjustment (so far).
(1) The holes in the brass plates through which the screws pass are slightly elongated in the direction going round the barrel. This means if you loosen the screws you have a slight twisting movement between the barrels. This system is easy to tune optically (see below).
(2) In some adapters one set of lens elements are held in place by 3 tiny set screws (eg Proskar 16B). Loosen these and the group of elements can be rotated in the housing. This is actually much harder to tune as once you have everything set up to optically tune it you can no longer get to the lens group to rotate it.
HOW TO TUNE - SIMPLE STEP BY STEP GUIDE (requires a bit of self confidence and practical grit).
This is for adapters that adjust via the brass plates.
(1) remove the focus ring from the anamorphic adapter. This reveals the brass plates and their holding screws.
(2) attach the adapter to your taking lens as you normally do, and attach a high resolution body that has a way to zoom in for fine focusing. Work on a tripod, and focus the two lenses (adapter and taking lens) on a good target until you get the sharpest result wide open (maximum aperture - this is the least forgiving setting).
(3) loosen the screws holding the brass plates. Not too much, just enough to allow any movement to happen.
(4) twist the front of the adapter slightly backwards and forwards watching your focus. You'll see that your image goes rapidly from jarringly out of focus to sharp to jarringly out of focus. You obvious want it at the sharpest point. Moving it into position is a bit of an art and will depend how loose everything is etc. Doing it makes the process a bit more obvious (as is so often the case).
(5) Once you've found the position of maximum sharpness tighten the screws holding the brass plates and check it hasn't got disturbed during tightening (ie you still have perfect sharpness).
(6) reassemble the focus ring onto the adapter.
Elmoscope 1280 - adjustment screws example
Elmoscope II / Kowa 16H adjustment screws example
This anamorphic adapter adjust using rotating elements (front set)
These screws hold the plate in one place only.
KOWA anamorphic 35mm
Sankor 16C - adjustment screws
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KOWA Prominar 16D (one screw removed). The elongation is so slight it's not obvious. This is a hint as to how precise the alignment is intended to be.
AN AFTER THOUGHT: REMOVING PLAY IN THE BRASS GUIDES
The whole point of brass is that it is maleable. If there is any play in the guides allowing rotational movement then sharpness will come and go depending which direction you twist. To correct this a single plate can be taken off, and struck with a hammer or punch to slightly widen it to take up the play. This is a trial and error process!
AN ASIDE: MODIFICATION ALERT!
Close your eyes now if you hate the thought of altering anything.
Below left is my modified adapter which is part of my single-focus build. Each of the three brass plates are now shorter and now fix with only one screw. This is less robust and a bit messy, but was done as part of internal modifications to allow the lens housings to move closer to each other, thereby substantially reducing the closest focusing distance. Compare it to below right, and you can see the difference. Note that the thread length is less on the left (4 less threads) as the front of the outer barrel has been ground down to allow the inner to insert more fully inside it. The threads are now obsolete anyway as the focus is by push-pull.