Testing rangefinder accuracy on M8 body

[harry__florida_]

Here are a couple of images depicting the rangefinder focus accuracy of a recent

M8 body using two late 50mm lenses known to provide accurate focus on a variety

of M film bodies. The lenses are a 2.8 Elmar and a 1.4 Aspheric, both used wide

open. The target is a yardstick placed at an angle to the camera and focus was

on the 18" mark at about 1 meter distance, using a tripod and 1.25x magnifier.

 

I made the test because my images seemed to be sharpest a bit behind the actual

focus point at most distances.

 

My interpretation of the resulting images is that at 1 meter distance, the (or

rather, this) M8 focuses about 2" behind the rangefinder-indicated point of focus.

 

As a side note, it would appear that the depth of field of the Elmar wide open

is close to equal behind and in front of the focus plane whereas with the Asph

wide open the depth of field is mostly behind the focus plane.

 

I wonder if other users have similar test images to contribute? If you use

mainly wide angle lenses and small apertures you might not notice this effect.<div></div>

[ellis_vener_photography]

unless they are testing on the exact same body as you are testing on other people's tests are meaningless. You should see about getting your camera's rangefinder recalibrated if you think it is off.

[harry__florida_]

Other tests would be meaningful in that they would show if few, some, many, most, or all M8's have the same problem.

[dominic_peticca1]

I always thought, as a general rule, that depth of field was roughly 1/3rd in front of, and 2/3rd's behind the plane of focus?

 

"My interpretation of the resulting images is that at 1 meter distance, the (or rather, this) M8 focuses about 2" behind the rangefinder-indicated point of focus."

 

This sounds like a problem. Hope you get that rangefinder adjusted!

[harry__florida_]

I know it is a problem and will return the camera. The point was to find out how common a problem it is.

[ronald_moravec1]

There have been many posts so it must be another bug to be sorted out.

 

I could take a wild guess the engineers forgot the the target point of focus on a film camera is somewhere between the rails and pressure plate because film is not held flat.

 

If they use the same RF calibration with a sensor which is dead flat, they need not compensate.

 

That and/or they compensate for field curvature of the lenses.

 

I will leave this to Leica as only they know what is going wrong.

 

Nobody complains about infinity focus.

[harry__florida_]

How would anybody know if the infinity focus is correct?

[harry__florida_]

My other image disappeared so here it is again, the result with the 50mm 1.4 Aspheric.

[tony31664880056]

Infinity adjustment is both crucial, and fairly easy to accomplish on the M8. Here's a thread which includes photos on how to adjust:

 

http://www.l-camera-forum.com/leica-forum/leica-m8-forum/13933-new-backfocus-thread-solution.html

 

One final note, though. I strongly recommend that you choose a far away point of reference in order to adjust most accurately. In other words, use a star at night, rather than something which is (for example) 100 ft. away, as it will calibrate more accurately.

 

Regards,

 

Tony C.

[harry__florida_]

Hi Tony -

 

As I understand it, the "infinity adjustment" above has to do with setting a lens to the infinity mark and then adjusting the cam for coincidence in the finder of the two images seen of a target at/near infinity.

 

But my concern is the correct focus of the image presented at the film plane or sensor surface in the case of the M8 which of course is not affected by that adjustment. You can turn all the cams all you want and it has no effect on the actual image recorded.

[tony31664880056]

Hi Harry,

 

I won't claim to be an expert, as I'm far from one. However, while I think that understand the distinction you are trying to make, it still seems to me that you will not be able to accurately gauge the performance of your M8, or individual lenses, unless the infinity adjustment is properly made. In other words, if the infinity adjustment is off, then back-focus issues are likely to distort your images relative to the focus screen seen through the viewfinder.

 

Regards,

 

Tony C.

[dan flanders]

Standard focus of a lens is at INFINITY! Close focus is a departure therefrom and the RF is supposed to measure that departure. Ergo, you must assure that the RF indicates infinity when it is indeed focused there. THEN, you can make internal adjustments to the RF innards to adjust for closer distances. This is accomplished by lengthening or shortening the length of the arm of the RF cam follower. You CANNOT make accurate close adjustments INDEPENDENT of infinity!<p> Early Leicas had a hole in the back, through the pressure plate, where a fitting could be applied to collimate the lens to focus at infinity at the image plane, since there was no way to simulate a ground glass at that plane. Later Barnacks omitted this feature when the back-focus was standardized. 'M' system cameras made it possible to access the image plane for testing focusing accuracy at infinity.

[harry__florida_]

Hi Dan-

 

I understand that "standard focus" of a lens by design is at infinity. But I do not understand how a user can determine if his lens is "indeed focused there."

 

I can determine if a lens is focused at one meter by looking at the image on the sensor plane of objects closer and farther away than one meter. See my second picture above. How do I determine focus at infinity without being able to place targets "beyond infinity"?

 

If you have ever had your eyes examined for glasses, you will recall that at the end they do not ask you which proposed prescription is sharper but to say when two adjacent powers of prescription are equally unsharp.

[cardy_chung]

You assumed the lens (or lenses) was perfect, didn't you ? Think again. I have tried 4 Elmar-M (current Elmar) 50/2.8 in the serial number range 3,8XX,XXX and 3,9XXX,XXXX, and NONE of them focused properly. They are all focusing to the rear of the intended point of focus at 1.05m (I used 1.05m to give it a bit of movement back and fro). Then I tried 2 current Summicron 50/2, including a 50th Anniv Summicron, which the great expert Erwin Puts and Leica ambassador claimed to have a much higher tolerance, and that's the only reason that I bought it, hoping that it would focus perfectly. The result ? NO. NONE of the 2 current Summicron 50/2 focus properly. And they are all focusing to the rear, in varying magnitude of approx 2cm to the rear. To add the the list of incorrectly focused lenses, there are ASPH 35/2 (2 of them), 4th Summicron 35/2, ... I used a M3 body as my standard and painstakingly spent properly 20-30 rolls of film on this matter. Results subsequently verified by DAG. When it comes to focusing, use a Rigid Summicron or DR 50/2 and you will be amazed how accurate the focus can be, if Leica does it properly.

 

It is now 2007. Not 1957. Leica cannot afford to make lenses, and more importantly, QC them like 1957. If you run a company and you're on the brink of going broke, what do you do ? You cut the invisible expenses - like Quality Control. On the bright side, a friend of mine has the 75/2 Summicron and that was perfect. To be fair to Leica, we do not buy the equipment as photographers in 1957 did. Paying several hundred US$ for a camera and a lens back then was a lot more money than what Leica is charging now, if you take into account the worth of a dollar.

[harry__florida_]

Hello Cardy-

 

I greatly appreciated the comments from one who has actually done some similar testing. I agree fully with your comments on 57 vs 07.

 

My assumption of the lenses used being good was based on good results using them on various M film cameras.

 

It was interesting that so many bad lenses seemed to show the same problem, a true focus about 2cm to the rear at 1 meter. Was your M3 quite accurately calibrated, flange to film plane? Using the old formula 1/f = 1/u + 1/v as an approximation, it would seem that an error of about 50 microns or 0.05mm would account for that 2cm error.

[john_shriver]

The collimation adjustment of the M8 is more sensitive than on the film M's, because the "depth" of the light sensitive area is much less than for film. Focus somewhere in the emulsion on film, and you're OK. Not so on the Kodak sensor, especially with the micro-lenses.

 

To check collimation of an M8 correctly, you need a "standard lens" that is focused at infinity. Leica must make one. I know that Topcon did for their cameras, having that service manual.

 

As I've noted before, there's about seven things that have to be correct, both on the lens and on the camera, before you get accurate focus. All you need on an SLR is one thing: the focusing screen being the same optical distance as the film.

[ant_nio_ferreira]

And you're forgetting focus shift when stopping down! ALL my leica lenses above 50mm present some focussing errors (they all focus behind the film plane) the 90 summicron does focus properly 2.8 down due to fopcus shift, and my noctilux focus perfectly only wide open, then the focus shift makes taking sharp pictures very difficult until depth of field takes over, from 5.6 down. What a PITA! On the other Hand all my wide angles work perfectly!

[harry__florida_]

I went back and retested my 50mm Aspheric on two M film bodies (new MP and newish Zeiss Icon) and at one meter and f1.4 (same conditions as used with the M8) the 18" mark on my yardstick on which I focused is perfectly sharp, using a microscope on the negative.

 

My question now is has anybody ever used a 50 Aspheric lens at f1.4 to do a focus test at one meter similar to mine on a M8 body?

[cardy_chung]

Harry,

 

My reference M3 was CLA'd by DAG about 18 months ago. Whatever method DAG used to calibrate I did not ask Don in detail. That M3 has since paired with many lenses (incl. Rigid 50/2, DR 50/2, Fat Tele-Elmarit, 3 of my Summilux 50/1.4 (non-ASPH), etc) and the results are 100% spot-on, and most importantly, the results are repeatable and predictable. But it also showed many lenses were hopelessly out.

 

I have since sent some of the lenses that I reckon are spot-on to DAG, and Don confirmed that those lenses are indeed spot-on.

 

I now test EVERY lens and EVERY body that comes into my possession. I compiled a variance table, documenting the deviation, if any, of a lens or body at 1.05m and 0.75m (if applicable). And the results are so consistent that if I use a body that is, say 1.5cm to the FRONT, and a lens that is 1.5cm to the REAR, and I put the 2 together, the result will be spot on.

 

I use ASA100 or slower B&W film for the testing. I set up a measuring tape at 45-degree, and very importantly, I use a scotch tape marked with a fine black marker to make an arrow (to increase the contrast for focusing). I then look at the actual point of sharpness, using the 1/3 & 2/3 DOF rule. Since the tape is at 45-degree, I apply the simple math of X2 + Y2 = Z2 to work out the actual deviation on a straight line.

[tom_murray2]

"And the results are so consistent that if I use a body that is, say 1.5cm to the FRONT, and a lens that is 1.5cm to the REAR, and I put the 2 together, the result will be spot on."

 

If Leica made watches people'd wear one that runs 5 min slow on the same wrist with one that runs 5 min fast and tell time by averaging them, and scoff at everyone who wears one Seiko that's dead nuts on.

[dan flanders]

HARRY, in Florida: I don't know about the M8, but with the earlier M's you can stick a piece of frosted tape across the film gate to test for focus at infiniity. For Barnacks you just might have to use a depth micrometer to determine the back-focus distance. If the image plane isn't accessible in the M8 I suppose the mike would be useful there too. In any event I would assume that the RF follower arm was set at the proper radius at the factory. If not then it isn't a DIY adjustment!

[yeffe]

"If Leica made watches people'd wear one that runs 5 min slow on the same wrist with one

that runs 5 min fast and tell time by averaging them"

 

Tests on my M6 show a consistent 3/8" (in front of the target) focusing error with all my

lenses wide open at their closest focusing distances. I assume the error is my own eye's

limitation as I really can't swear that I'm able to crisply image the coincident split images. I

shoot low-light portraits and expose an average of five frames per attempted photo.

 

I'm convinced that anyone who really wants to become an expert at taking impromptu

photos on the fly get as much physical conditioning and eyesight correction as they can.

Also, maybe that famed FBI course where a wild man bursts into the classroom, creates a

ruckus,then leaves. Students are then asked for a detailed description of the man's

appearance and behavior. Usually, no one agrees on either.