We've been having several discussions here about the effect that out-of-spec screw-to-bayo adapters might have on focusing accuracy, and about whether deliberately thinning out an adapter could help correct an LTM lens that has a "front-focus" problem on the R-D 1.
This evening I put in a little quality time with a spreadsheet and the formulas in Rudolf Kingslake's book "Lenses in Photography," and have come up with some tables that MIGHT help show what's happening here.
First things first: The R-D 1's rangefinder, like all Leica-type rangefinders, is designed to have a 1:1 coupling ratio with a "50mm" lens. (Actually, the magic focal length is 51.6mm, according to Dante Stella's excellent article
, so that's the number I used in the tables -- but here, I'm just gonna say 50mm to keep from getting unwieldy.)
What that means is that as the lens barrel moves forward to focus closer, the coupling arm on the rangefinder moves forward by the same
amount, and the rangefinder is set up to convert that amount of movement to the correct distance reading.
Table 1 shows the amount of focusing extension required, in mm, to shift from infinity to various distances, for both the magic 51.6mm lens and various other common focal lengths. (The formula I used leaves out a few small variables, so if you do your own calcs using a different formula, don't be surprised if you get slightly different numbers.)
Since a 50mm lens and its RF cam move in and out at a 1:1 ratio, an adapter that's out of spec won't affect focusing accuracy -- the positions of the lens and cam will be "off," but they'll be off by the same amount.
But if you're using a different lens, it needs a different amount of focusing extension than a 50; wides require less and teles require more, as you've seen in Table 1.
Since the rangefinder coupling is still set up for a 50mm lens, designers have to provide a coupling cam that "translates" the lens' focusing movement into the amount that a 50mm would move to focus at the same distance.
Table 2 shows the 'gear ratios' that other lenses require to translate their focusing movement to match a 50mm lens.
You can see from these tables how an out-of-spec adapter affects the focusing accuracy of a non-50mm
lens. If we use an adapter that's too thick, we move both the lens and the coupling cam forward by the same amount. But since a wide-angle lens needs less extension to focus closer, that amount will produce a bigger change of lens
focus than it will of rangefinder
The result is that a wide-angle lens on a too-thick adapter will "front-focus" -- when the rangefinder indicates correct focusing at, say, 1 meter, the lens actually is focused at a closer distance.
The effect is just the opposite for a tele lens: it needs more
focusing extension than a 50 to reach an equivalent distance, so a too-thick adapter will make it "rear-focus."
And the results swap themselves for an adapter that's too thin: it makes a wide-angle lens rear-focus, and a tele lens front-focus. Again, the reason is the difference between the amount of extension the lens needs, and the amount the rangefinder expects.
The upside is that you can take advantage of this disparity to partially compensate
for a wide-angle lens that naturally front-focuses, by using a slightly thinner-than-spec adapter.
The reduced thickness will throw off rangefinder coupling accuracy by a small
amount... but it will correct the lens by a larger
Table 3 shows how much thickness you'd need to add or subtract to the adapter to correct various amounts of front-focus or rear-focus, based on an intended focusing distance of 1 meter (1000mm.)
For example, suppose that you mount a 21mm lens and focus via rangefinder at a distance of 1000mm. But your lens has a front-focus problem, so the sharpest distance in the picture is actually 988mm.
The table shows that you can correct that by removing 0.006mm thickness from your adapter (good luck!) This will move the lens closer to the camera body, shifting its focus point backward by the required amount.
The reduced thickness also will move the RF coupling cam back by the same amount, and that will throw off the rangefinder's accuracy. BUT, since your 0.006mm change has a much smaller effect on the "standard" 51.6mm focal length, what you lose in RF focusing accuracy will be less than you gained in lens focusing accuracy.
You can see this in the table by looking past the middle column, at the distances greater than 1 meter. You've moved the RF cam back by 0.006 mm, and you can see from the table that instead of indicating correct focus at 1 meter, the rangefinder now will indicate correct focus at just under 1.002 meters. But on a percentage basis, that loss of accuracy is much less than you've gained in lens focus accuracy -- so you've come out ahead!
If you're really
ambitious, you could try to adjust the RF cam back out by 0.006mm, which would eliminate the disparity and should allow the lens and RF to agree perfectly. But such tiny numbers call for a very delicate adjustment... probably a job for a professional technician. After all, if you screw up a generic screw-to-bayo adapter, you aren't out all that much money; make the same goof on the lens itself, and you've got a much bigger problem!