Question about Lens Focus Adjustment

[k-hawinkler]

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Let's assume a lens has either front focus or back focus.

How does a technician actually adjusts the lens?

What are his/her options?

What severity can be adjusted? What can't?

I am particularly interested in a 35 lux ASPH, made in 2000, with quite a bit of back focus.

Thanks.

[Bo_Lorentzen]

Im assuming that your lens is on target wide open. and focus shift when you stop down.? (as that is typical for that lens). I have heard of some users adjusting the lens to front focus a very little at wide open to compensate for the back focus when stopped down, not sure how well this really works.

 

however if your lens is backfocusing a bit when wide open, that CAN be adjusted by a tech. if you want to play, you can cut a little piece of 3M tape and stick it on top of the cam and see what happens, that will bring the focus a little forward. I have adjusted a couple of lenses with a bit of thinned down black fingernail polish. (which is not normally my color but) Im sure that is not the factory recommended process.. but it worked.

 

 

.

[colorflow]

Front or back focusing can usually be adjusted by taking away or adding shims between the lens element collar and the lens barrel. Front focusing is a result of the lens elements being too far from the image plane. Therefore they need to be moved closer to the image plane by either taking away existing shims (if there are any) or machining away a small amount of the lens element collar. Backfocusing is a result of the lens elements being too close to the image plane. Therefore they need to be moved farther from the image plane by adding shims. These are pretty straight forward adjustments for competent technicians.

 

Focus shift is an entirely different matter. It is inherent to the lens design. The focus plane shifts farther (backfocus) as the aperture is stopped down. Luckily the increasing DOF usually masks the shift. In newer designs the shift is minimized by the so called 'floating elements".

[k-hawinkler]

Im assuming that your lens is on target wide open. and focus shift when you stop down.? (as that is typical for that lens). I have heard of some users adjusting the lens to front focus a very little at wide open to compensate for the back focus when stopped down, not sure how well this really works.

 

however if your lens is backfocusing a bit when wide open, that CAN be adjusted by a tech. if you want to play, you can cut a little piece of 3M tape and stick it on top of the cam and see what happens, that will bring the focus a little forward. I have adjusted a couple of lenses with a bit of thinned down black fingernail polish. (which is not normally my color but) Im sure that is not the factory recommended process.. but it worked.

 

 

.

 

Bo,

 

Many thanks for your help. I tried to measure as a function of f-number how far off focus is, i.e. the middle of DOF.

I first list the f-number and then in cm the back focus. Provided I focused correctly it looks something like this:

 

1.4 5

2.0 6

2.8 7

4.0 7

5.6 7

8.0 7

11. 7

16. 7

 

For the larger f-numbers it's hard to tell as focus is pretty good most everywhere.

So, it looks to me if the lens were set to be spot on at f=2 that would be pretty good.

I presume Leica NJ or Solms or any of the non-Leica repair folks should be able to fix this?

 

Again, thanks for your help. With best regards, k-hawinkler

 

PS: I like your dry humor. Real funny.

[k-hawinkler]

Front or back focusing can usually be adjusted by taking away or adding shims between the lens element collar and the lens barrel. Front focusing is a result of the lens elements being too far from the image plane. Therefore they need to be moved closer to the image plane by either taking away existing shims (if there are any) or machining away a small amount of the lens element collar. Backfocusing is a result of the lens elements being too close to the image plane. Therefore they need to be moved farther from the image plane by adding shims. These are pretty straight forward adjustments for competent technicians.

 

Focus shift is an entirely different matter. It is inherent to the lens design. The focus plane shifts farther (backfocus) as the aperture is stopped down. Luckily the increasing DOF usually masks the shift. In newer designs the shift is minimized by the so called 'floating elements".

 

Alan,

 

Many thanks for your lucid explanations.

That sounds pretty encouraging.

From reading these forum posts and some lens reviews, it looks like some but not all 35 Lux ASPH lenses are a prime candidate for the focus shift. It looks like I got one of those. No problem. I am sure I will enjoy the lens once it's properly adjusted.

 

Have fun in the Livermore valley. A long time ago I used to live in Walnut Creek.

 

Thanks again for your feedback. With best regards, k-hawinkler.

[D&A]

Bo Wrote: >>> "I have heard of some users adjusting the lens to front focus a very little at wide open to compensate for the back focus when stopped down, not sure how well this really works.">>>

 

Bo is exactly correct! The best focusing Leica 35mm 1.4 ASPH lenses I've shot with, are adjusted this way where wide open they front focus just a bit (still usable but at 100% one can see the front focusing)...and from there on out (stopping down)...the rearward focus shift is covered by the growing depth of field by stopping down. If one wants to look at it another way, these lenses appear to be optomized for f2.8 (not f2.0)....since at f2.0, one can still detect a very slight bit of front focusing, but only very slight...and focusing completely "dead on" at f2.8. The other kind of 35mm f1.4 ASPH lenses I've tried seem to be optomised at f1.4 but once you stop down, the subject quickly goes completely out of the depth of feild till something like f4.5-f5.6. Anything between f2.0 and f4.5-f5.6 essntially puts the subject out of the dfepth of field. Users shooting with film often don't see the degree of focus shift as digital users do, since examination of focus is done at 100%.

 

Dave (D&A)

[k-hawinkler]

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Bo Wrote: >>> "I have heard of some users adjusting the lens to front focus a very little at wide open to compensate for the back focus when stopped down, not sure how well this really works.">>>

 

Bo is exactly correct! The best focusing Leica 35mm 1.4 ASPH lenses I've shot with, are adjusted this way where wide open they front focus just a bit (still usable but at 100% one can see the front focusing)...and from there on out (stopping down)...the rearward focus shift is covered by the growing depth of field by stopping down. If one wants to look at it another way, these lenses appear to be optomized for f2.8 (not f2.0)....since at f2.0, one can still detect a very slight bit of front focusing, but only very slight...and focusing completely "dead on" at f2.8. The other kind of 35mm f1.4 ASPH lenses I've tried seem to be optomised at f1.4 but once you stop down, the subject quickly goes completely out of the depth of feild till something like f4.5-f5.6. Anything between f2.0 and f4.5-f5.6 essntially puts the subject out of the dfepth of field. Users shooting with film often don't see the degree of focus shift as digital users do, since examination of focus is done at 100%.

 

Dave (D&A)

 

Dave,

 

Many thanks for your help. I also have a 35 Cron ASPH that seems to be pretty much on focus at f=2. I haven't checked that one for focus shift yet. In light of that, may be the Lux correctly adjusted for f=1.4 is the better option? What did you find most useful?

 

Thanks again. With best regards, k-hawinkler.

[colorflow]

My Lux 35 asph is adjusted to focus right on at 1.4 and lucky for me the focus shift is mostly covered by the DOF when stopping down. I understand other samples of this lens have more severe focus shifts.

 

As I mentioned in other threads, Don Goldberg at DAG is the best bet for this type of work. I have sent 4 or 5 lenses to him for adjustments even though a couple of them were still under Leica warranty.

[k-hawinkler]

My Lux 35 asph is adjusted to focus right on at 1.4 and lucky for me the focus shift is mostly covered by the DOF when stopping down. I understand other samples of this lens have more severe focus shifts.

 

As I mentioned in other threads, Don Goldberg at DAG is the best bet for this type of work. I have sent 4 or 5 lenses to him for adjustments even though a couple of them were still under Leica warranty.

 

Great. He is at DAG Camera Parts, right? I'll be in touch with him tomorrow.

Thanks again.

[colorflow]

Great. He is at DAG Camera Parts, right? I'll be in touch with him tomorrow.

Thanks again.

 

That's him.

[gwelland]

The other good thing about Don is that he can set up your lens to suit your preferences. I had him sort out my first Noctilux that came from the factory back-focusing terribly. He asked me if I wanted it to be spot on at f/1 with slight focus shift as I stop down, or optimized for f/1.4 - f/4. No doubt he can set up the 35 'lux in a similar manner based on your needs.

[ho_co]

Don can adjust to your specification, as noted above, while Leica prefers to return items to their original specifications.

 

Remember, on most Leica rangefinder lenses, there is a focusing collar, a rangefinder cam (flat on almost all), and the actual helicoid that adjusts the lens forward and backward. If I understand correctly, the lens can be adjusted forward and backward in the helicoid; the helicoid can be adjusted within the rangefinder cam linkage; and of course, all that is held by and adjusted within the focusing collar.

 

If I were having a lens checked or adjusted, I would choose at the minimum a person like Don Goldberg or Sherry Kräuter, who were both trained by Leica; or I would send the lens direct to Leica.

[scott kirkpatrick]

...

I first list the f-number and then in cm the back focus. Provided I focused correctly it looks something like this:

 

1.4 5

2.0 6

2.8 7

4.0 7

5.6 7

8.0 7

11. 7

16. 7

 

.

 

Thanks for sharing your careful measurements, but at what distance did you do them? At 0.7 m, I can see that this is annoying, but at 2m it might not be noticed by some people. Anyway, it looks like the commonly observed focus shift of 2 cm is accompanied by an offset of 7 cm. As others point out, the offset is fixable to taste, either by making 1.4 or 2.0 spot-on. And some of the more adventurous forum members do that for themselves with the rangefinder linkage.

 

scott

[k-hawinkler]

Don can adjust to your specification, as noted above, while Leica prefers to return items to their original specifications.

 

Remember, on most Leica rangefinder lenses, there is a focusing collar, a rangefinder cam (flat on almost all), and the actual helicoid that adjusts the lens forward and backward. If I understand correctly, the lens can be adjusted forward and backward in the helicoid; the helicoid can be adjusted within the rangefinder cam linkage; and of course, all that is held by and adjusted within the focusing collar.

 

If I were having a lens checked or adjusted, I would choose at the minimum a person like Don Goldberg or Sherry Kräuter, who were both trained by Leica; or I would send the lens direct to Leica.

 

Howard,

 

Many thanks for your feedback. I have already heard back from Don Goldberg and I am delighted that he will make the adjustments.

 

With best regards, k-hawinkler.

[k-hawinkler]

Thanks for sharing your careful measurements, but at what distance did you do them? At 0.7 m, I can see that this is annoying, but at 2m it might not be noticed by some people. Anyway, it looks like the commonly observed focus shift of 2 cm is accompanied by an offset of 7 cm. As others point out, the offset is fixable to taste, either by making 1.4 or 2.0 spot-on. And some of the more adventurous forum members do that for themselves with the rangefinder linkage.

 

scott

 

scott,

 

Many thanks for your feedback. I measured exactly at close focus distance of 0.7m - or whatever the actual close focus distance is for my particular lens. The way I did the measurement is described in "M9 Delivery Register", Page 44, #874 (permalink) http://www.l-camera-forum.com/leica-forum/leica-m9-forum/100872-m9-delivery-register-44.html, namely:

 

"I set aperture to f=1.4 and close focus distance = 0.7m on the lens, positioned myself accordingly relative to the focus target point, gently rocked my head (and M9) back and forth, and seeing the target slowly weaving in and out of focus."

 

Thanks for pointing out that the lens can be fixed for either f=1.4 or f=2.0. Earlier in this thread, #6, D&A recommended either f=1.4 or f=2.8. As I also have a 35 Summicron-M ASPH that seems to have pretty accurate focus throughout from f=2 to f=16, I may ask Don to adjust the 35 lux for f=1.4 but I have not made up my mind completely yet.

 

Thanks again for your help. With best regards, k-hawinkler.