Semi-rigorous backfocus and focus shift test

[colorflow]

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My Lux 50 ASPH has been backfocusing, so I decided to run a semi-rigorous focus test with some of my lenses - the Lux 50 ASPH, Cron APO 90, Cron 50 current version, Lux 50 version 2, Cron 28.

 

Photographed a tape measure on the floor, focusing on the 60 inch mark, using a tripot and shutter set on time release. The center of the sensor was approx 60 inches horizontally from the focus point and 58.5 inches from the floor. Thus shooting the tape measure at approximately 45 degrees at a distance of 84 inches.

 

I focused the best I could using 1.25X and shot from 1.4 (2.0) to 5.6 at 1 stop increments. I judged the results subjectively by bracketing the range where the inch markers appear to be in acceptable focus. Again this is purely subjective and relative, but provides interesting (at least to me) insight to the focus issues.

 

The following is what I found:

 

At f1.4 the Lux 50 ASPH is backfocusing by a whopping 5.5 inches with a DOF of approx only +-1.5 inches. The focus then shifts further back as the aperture stops down until at 5.6 the focus error is almost 14 inches while the DOF is +- 12 inches. Therefore the increase in DOF does not make up for the further increase in backfocus. Clearly this lens is out of whack.

 

The Cron 90 is similar but not as severe, and has not been bothersome in real-life situations.

 

It is interesting that the Cron 28, the Cron 50 and the old Lux 50 all start off front focusing at f1.4 and the focus shifts back as the aperture stops down. This allows the increasing DOF to cover the focus error. At f1.4 the old Lux 50 is front focusing by 5.5 inches with a DOF of +- 1.5 inches. But as the aperture stops down, the focus moves into the DOF at 2.8 and beyond. Likewise for the Cron 50. While the Cron 28’s DOF totally covers any noticeable focusing errors in this test.

 

I have actual data in Excel file and , of course, photos, but they are too numerous to post.

 

So my conclusions - the Lux 50 ASPH needs to have focus adjusted. Range finder of M8 is probably ok until proven guilty since both the Cron 50 and old Lux 50 focus acceptably. I have used these two lenses extensively on my film cameras. Still have a couple of other lenses that will put through the same tests.

 

Welcome your critique and suggestions. I plan to repeat this test with my M6 and a few more other lenses which I will put through the same tests as well.

 

Thanks,

Alan

[ho_co]

Alan--

Good test. Very interesting results

 

Just remember that the distances shown on the tape measure are foreshortened because it is at an angle. That is the main advantage claimed by the ubiquitous test chart web-published for--what was it, the Nikon D70?

 

In other words--you do the math, not me! --what the tape measure says is 5.5" may be only 4".

 

On the other hand, using a tape measure may be simpler than using the chart since you're testing so many different lenses.

 

Definitely good that you made the effort. If you calculate the foreshortening for the tape measure at that angle, you can rename the thread to "Almost fully rigorous .... "

 

--HC

[colorflow]

Alan--

Good test. Very interesting results

 

Just remember that the distances shown on the tape measure are foreshortened because it is at an angle. That is the main advantage claimed by the ubiquitous test chart web-published for--what was it, the Nikon D70?

 

In other words--you do the math, not me! --what the tape measure says is 5.5" may be only 4".

 

On the other hand, using a tape measure may be simpler than using the chart since you're testing so many different lenses.

 

Definitely good that you made the effort. If you calculate the foreshortening for the tape measure at that angle, you can rename the thread to "Almost fully rigorous .... "

 

--HC

 

Actually it is the other way around. The tape measure is on the floor while the line of sight of the camera is the diagonal. I already did the math, so the numbers I quoted have already been divided by the cosine of the angle (about 45 degrees).

 

Alan

[ho_co]

Actually it is the other way around. The tape measure is on the floor while the line of sight of the camera is the diagonal. I already did the math, so the numbers I quoted have already been divided by the cosine of the angle (about 45 degrees).

Hmm.

 

How can side C of a triangle be at 45° to side B, and side B not be at 45° to side C?

 

 

But since we're talking about the image plane, the camera's line of sight is determinative, and the tape measure runs through the focus plane at an angle. That's why you did the test.

 

Anyway, no matter how you define it, if you've corrected the tape measure's reading to compensate for the angle, then you've done what I suggested.

 

--HC

[colorflow]

Hmm.

 

How can side C of a triangle be at 45° to side B, and side B not be at 45° to side C?

 

 

--HC

 

Let's make the tape measure on the ground side A, the vertical height of the camera side B. and the line of sight side C. So A and B are at 90 degrees to each other, and both are at 45 degrees to side C. In my case, A=60 inches, B=58.5 inches, and C therefore, is 84 inches.

 

But the lesson I take away is all five lenses shift their focus back (longer) as the aperture stops down. So it makes sense to start off with front focusing like the old Lux 50 and the Cron 50 as well as the Cron 28.

 

Alan

[Arne Helme]

The following is what I found:

 

At f1.4 the Lux 50 ASPH is backfocusing by a whopping 5.5 inches with a DOF of approx only +-1.5 inches. The focus then shifts further back as the aperture stops down until at 5.6 the focus error is almost 14 inches while the DOF is +- 12 inches. Therefore the increase in DOF does not make up for the further increase in backfocus. Clearly this lens is out of whack.

 

Your results pretty much resemble the ones I reported here:

 

http://www.leica-camera-user.com/digital-forum/20687-m8-1-4-50mm-asph-backfocus.html

 

My 1.4/50mm ASPH. lens is also backfocusing several inches in the 1-2 meter range wide open.

 

The following article by Erwin Puts on rangefinder accuracy in sensor based camers may also be of interest:

 

M8 Rangefinder Accuracy

 

Though I understand Erwins argument about rangefinder accuracy I have also come to the conclusion that it is the lens (and not me) that is the problem.

 

-- Arne

[colorflow]

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Your results pretty much resemble the ones I reported here:

 

http://www.leica-camera-user.com/digital-forum/20687-m8-1-4-50mm-asph-backfocus.html

 

My 1.4/50mm ASPH. lens is also backfocusing several inches in the 1-2 meter range wide open.

 

 

-- Arne

 

Ame:

My Lux 50 ASPH is also the LHSA version. Wonder if there may be a problem with that?

Did you send it back?

 

Alan

[Arne Helme]

Ame:

My Lux 50 ASPH is also the LHSA version. Wonder if there may be a problem with that?

Did you send it back?

 

Alan

 

I did send it to Solms to have the 6-bit decoder installed and focus adjusted. When it arrived back I could not detect any change in focus behaviour.

 

-- Arne

[misha]

my pre-asph 50 lux is backfocusing by about one inch (no hard science but somewhat consistant at 1.4 & 5.6). the lens came used from b&h. do you guys think this is something b&h can easily (and in a timely manner) correct in-house? another option is to just return it all together..

thanks

m

[misha]

i also noticed that while at f1.4 the backfocus is about 1 inch, but is about 2-3 inches at f4 and f5.6. is that 'normal', and does this ultimately point to a lens problem or camera?

[ho_co]

Michael,

This is probably proper behavior for the lens. There have been a number of threads on the question of focus shift, and there is general agreement that small focus shifts are to be expected with these designs.

 

It's probably not something that B&H could fix; it would require experience and equipment they probably don't have.

 

--HC

[misha]

you are right. i tried the same lens on another m8 and sharpeness was there. so its definately my body, not the lens. do you think this is something that nj can easily (and in a timely fashion) correct?

thanks

m

[colorflow]

you are right. i tried the same lens on another m8 and sharpeness was there. so its definately my body, not the lens. do you think this is something that nj can easily (and in a timely fashion) correct?

thanks

m

 

I would run the tests as I described in this thread (if you haven't done so already) on both M8s before sending the camera in for rangefinder adjustment. I think NJ can do it but the turn around time may be quite long due to their current busy service load.

 

Alan

[ho_co]

you are right. i tried the same lens on another m8 and sharpeness was there. so its definately my body, not the lens. do you think this is something that nj can easily (and in a timely fashion) correct?

Misha--

Any experienced Leica repair tech can adjust the rangefinder. It should not take much time at all.

 

Since the camera is under warranty, New Jersey would be your best bet. You could call ahead to ask about turn-around. They will do it as quickly as they can.

 

If you need it quicker than they can promise, you can use another service center, but they'll charge.

 

--HC

[misha]

gotcha, thanks.

 

i live in new york, hence thinking fo going there myself this week.

thanks for the advice.

the other camera was new for sale, so i only had a couple of minutes to fiddle with it. in any case, it was clear to both me and the sales agent that his m8 was focusing much better. that being said, nj!