PDA

View Full Version : Adapter thickness vs. focusing accuracy: probably more than you want to know!


jlw
04-19-2005, 20:28
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 (http://www.dantestella.com/technical/compat.html) , 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 focus.

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 amount.

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!

pfogle
04-20-2005, 02:03
jlw - sorry I don't know your name...

what are the columns in table 2?

As I wrote here (http://www.rangefinderforum.com/forums/showthread.php?p=68405#post68405)
I got some improvement with rubbing down the adapter for the 21.

Also, with the Jupiter 8, which was back-focusing, I glued a thin paper ring onto the rim of the rangefinder cam, and like you say, it fixed the focus problem, but being a 50, it put the rangefinder setting out, so now the lens appears to focus past infinity. Still, I only use it wide open for portraits, so as long as it's correct at about 1-2 meters, I don't really care.

cheers
Phil

jlw
04-20-2005, 04:42
Sorry about Table 2 -- originally it was right below Table 1 on the spreadsheet, and I forgot to copy the column headings. They are the same as on Table 1: Infinity, 50m, 30m, etc. (There are no numbers in the infinity column because the % of lens extension vs. cam extension always should be 100% at infinity.)

Incidentally, you might think that the % of cam extension vs. lens extension should be constant across the range of distances (and so did I.) In fact, in the table it changes slightly. I believe this is because the simplified equation I used doesn't compensate for the change in effective focal length as you focus closer. Kingslake also provided a fully corrected equation -- but it requires that you measure distance from the first principal plane of the lens rather than from the film plane, which was common practice when he wrote the book but not what's customary today.

Todd.Hanz
04-20-2005, 05:17
Wow! good work. I always wondered how my VC 28mm Ultron LTM focuses the same with an adapter as an M mount lens without the extra spacer. How do you tell if your lens is front focusing or vice versa?

Todd

Jim Watts
04-20-2005, 05:33
Wow! good work. I always wondered how my VC 28mm Ultron LTM focuses the same with an adapter as an M mount lens without the extra spacer. How do you tell if your lens is front focusing or vice versa?

Todd

Todd,
Photograph a ruler at 45 degrees to the camera noting the focus distance (full aperture will show the greatest errors). Compare rangefinder focused point with the resulting image. Some precuations are advisable to check result is valid:
1. Use a tripod to retain focus distance.
2. Repeat a few times de-focusing and re-focusing the lens to check that front focusing is consistant and not down to the inability of you or the rangefinder base length to achieve focus.
3. Use a focus point of about 1 meter as its common for lenses not to couple to the rangefinder properly close to their absolute minimum focus.

Remember twice the distance behind as in front of the focussed point should be sharp.

Jim Watts
04-20-2005, 06:10
Although all this offers an explanation and (better still) a possible solution with regard to lenses using adapters it doesn't explain the same problem with direct M mount lenses. I have speculated in other threads that this could be down to the shape or angle of the lens cam but I can see little difference between the shape of the Cam on my 35mm Cron (which shows front focus) and my 50mm Cron (which is spot on). The error may be more visible of course because as jlw says "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 focus". Although Chris (Driggett) reported front (not back) focusing with his 90mm Cron.

I took a careful look at the rangefinder cam follower on my R-D1 in comparison to that on my M4. They appear to be pretty much identical. The wheel is 2.7mm deep in each case and they are both 4mm from the inner edge of the lens mount so they should engage the cam in the same place but when I look at the (slight) wear lines on the wheel with the M4 they are dead centre and on the R-D1 about 2/3rds down, which would seem to show the cam actually meets the wheel at a different spot. I'm not sure why, does the angle of the follower change when under pressure from the cam? and how much of the cam needs to engage the follower wheel to achieve accurate focus?

pfogle
04-20-2005, 10:21
I've started a thread on fixing the front focus by grinding down the adapter here (http://www.rangefinderforum.com/forums/showthread.php?t=5799)

cheers
Phil