Overcoming the Back Focus issues of the Leica 135mm M mount lens…


Yes, HOW could such a high quality Leica Lens be so problematic….

 

Leica 135mm

Leica 135mm

Leica 135mm

Leica 135mm

In todays world of modern digital camera systems, the Leica M 240 really stands out above the others.  One of the reason for this is the duality of an electronic viewfinder with a robust peaking focus system coupled with a real range finder focus as well!  This makes this camera IDEAL for mounting any lens that can be manually focused with reasonable expectations of good focus!

But what happens if you have some of the older Leica systems like the M9, Monochrom, M8 or a film body like the M7?  Should we not be able to expect the same focus quality of the M 240 using the built in rangefinder?

Well for ALL of my Leica and Voigtlander glass this has been a truism until I met the famously hard to focus 135mm lens.  Now understand that this lens has been through 5 (FIVE!!) versions and each and every one has had the same bad press on their inability to sharply focus!  If you ask Leica about them you are told to set your aperture to f/11, focus and shoot hoping that the DOF will overcome the shortcomings of the focus cam system inside of the lens!

Now, I badly desired a longer reach lens with a M mount but was convinced that I could not overcome the focus issues with these lenses.  So one of my students from the Arcanum (a online photograph school) had one of the 135s and sent it to me to experiment with.  It was one of the older models, looked kind of funky but had a tripod mount on the lens!  Reviews of ALL the 135s said that this model was the best one for the money!

So with the lens on camera (M 240) I set out to test the rangefinder focus vs. Electronic focus with peaking…

Suffice it to say that the lens BACK FOCUSES so badly that unless you are shooting at f/11 or f/16 you have very little chance of a sharp image!  I focused the lens using the range finder then again with the electronic peaking system and discovered the following failure:

  • 7 meter subject.  The lens needed to be adjusted to 6 meters to be in focus!
  • 50 meter subject.  The lens needed to adjusted to 20 meters to be in focus!
  • Infinity subject.  The lens needed to be adjusted between Infiniti and 50 meters!

The adjustments were the difference between the rangefinder focus and the peaking focus!

Well….

If you were to turn over and look into the mounting flange on it you will notice that there is a focus cam that moves up and down as you focus the lens.  This is NOT adjustable!  But… I had an idea!

Enter High Tac Copper Tape…

High tack copper tape

High tack copper tape

Tape Application Leica 135mm

Tape Application Leica 135mm

What if…

What if I were to tape one or two layers of this really sticky copper tape onto the end of the focus cam?  Could I at least get the infiniti focus point dialed in? 

YES!  It can be done!

So, I ordered a very thin roll (1/2 inch) from Amazon, cleaned the cam and cut 1 thin sliver of the tape and applied it to the end of the cam and redid my focus tests!

After cleaning the cam with Alcohol  I applied the tape, smoothing it down with a wooden dowel and allow it to sit for an hour giving the adhesive a chance to cure.

Mounting the lens again onto my M 240 I re-ran my range finder and electronic peaking focus tests on the same subjects and distances! I carefully (on a piece of tape attached to the lens marked the two focus marks for each test on the lens so that I could visualize both the size of the focus error as well as where it was on the overall focus scale.

Here are my results with only ONE layer of copper tape.  I knew that my target was to get my infinity focus dead on and that the closer distance focus points would still be off but hopefully by smaller amounts.

Test data is marked on the lens showing both RF focus/Electronic Peaking focus points.  The peaking points are dead on and hopefully we could bring the two closer together!

If you look at these points you will see the non cam taped focus dat on the bottom and the corrected cam data on the top.  The V stands for Electronic Peaking and the M for Range Finder focus.

7 Meter Focus Tests…

7 Meter Focus Test Leica 135mm

7 Meter Focus Test Leica 135mm

So for the non corrected cam we see a subject at 7 meters as focused with the rangefinder, but with the Peaking focus it is moved down to 6 meters to be sharp!  This is a 1 meter difference and unless you are shooting with a small aperture you will get a very soft focus!  When we do the test after the cam change the difference between the range finder and peaking is actually a bit wider.  This was expected…

Now, The 50 meter tests…

100 foot focus test on Leica 135mm

100 foot focus test on Leica 135mm

As you can see on the uncorrected cam side focusing at 50 meters on the rangefinder we needed to adjust down to 20 meters… That is WAY OUT OF RANGE!  Yet at f/11 we can get it into focus!  

With the corrected cam the difference between the range finder focus and the peaking focus gets closer together at about 30 meters which is a 10 meter reduction of focus error!  Better and better!

Infinity Focus Test

Infiniti Focus tests on the Leica 135mm

Infiniti Focus tests on the Leica 135mm

Ahh, the pre cam measurement using the rangefinder sets the lens to the infinity as expected but the peaking focus sets it down about 1/3 of the way towards 50 meters.  Way off to be sure but small than before.    

BUT…

The corrected cam BOTH THE RANGEFINDER AND THE PEAKING focus match at infinity!!  We are now dead on on the far end of the lens!

What this means is that knowing the differences between the rangefinder and peaking that we can adjust the 135mm lend down about 1/3 of the way between the major distance measurements to get much closer to being dead on.  As long as we shoot at a reasonable aperture (f/8) we can be reasonably assured of a sharp focus!

I have found that these measurements are the same with the current Leica 135mm as well!

 

So there you have it.  The online posts of issues with these lenses have been endless over the years.  They are known as very difficult lenses to get a sharp focus with and now we know why and more importantly how to overcome it!

 

I hope that you enjoyed this discussion on what could be a very nice and powerful lens to have in your bag!

 

 

 

 

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