Infinity focus

[delander †]

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Hi All,

 

I have just spent a short time looking at the infinity focus of my lenses.

 

When I focus on an upright bare (dead) branch against the sky about 75 yards away, my 50mm ASPH cron and my 35mm ASPH cron both find best focus with the lens at infinity. If I focus on a high altitude jet plane they both show a double image of the plane. Not far apart but it is there; the plane cannot be brought into really coincident images.

 

With my WATE, 28 mm ASPH elmarit and 90 mm elmarit, the branch is brought into focus just short of infinity and a high altitude jet plane is in focus at infinity.

 

I have tested this on two M8 bodies and the lenses behave identically on both.

 

How can I explain the magnitude of this? Imagine the bare branch is about 2 inches (50mm) in diameter. On the three 'correctly' focusing lenses, at infinity the two images have passed through and are out of focus so that they are just separate.

 

At long distance these small focus differences may not be noticeable? But what is the effect at middle and shorter distances? Is it large enough to have the two summicrons corrected back at the factory. Should I really be seeing no discernable differences?

 

I have done the angled ruler test at close focus and the two crons are spot on wide open and then show back focus as you stop down.

 

Any advice appreciated, I really dont want to send the lenses in.

 

Jeff

[andybarton]

Do you notice this when you take real pictures?

 

Does this out of focus-ness appear in prints?

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[marknorton]

Take a look at the side of the lens bayonet where the cut-out is. With the lens set to infinity, the lens cam should be a fraction of a mm ( below the top surface of the bayonet flanges. As you reduce the focussing distance, the focussing cam moves down to a distance of about 4.5mm below the bayonet flanges.

 

If you set all your lenses to infinity, can you see a difference between the lenses which work and those which do not?

[jaapv]

75m is definitely not infinity; the jet is acceptable,a star is better.

[Tony C.]

I am hardly an expert, but the essence of my experience with focussing issues on the M8 is this:

 

First and foremost, adjust the body to infinity using this technique (be SURE to use a distant object such as a star, rather than a relative close object):

 

http://www.l-camera-forum.com/leica-forum/leica-m8-forum/13933-new-backfocus-thread-solution.html#post146014

 

Then, test all of your lenses. If all but one or two focus correctly, then you probably have issues with those lenses, and should consider sending them in. If you have problems with most or all of them, then the body may need further adjustment (by Leica).

 

Regards,

 

Tony C.

[fursan]

the moon, the moon!!

i see a little more than 1 moon!! at the lhs. am i doing good?

 

is it that time of the year already? i should try it at the full moon or the eclipse just

to avoid user error!

 

what a sight..all leica m8 owners looking to the moon with their cameras to their eyes uttering strange gutteral

sounds!

[ho_co]

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Jeff--

Non-technical answer: My feeling is that if you're getting good images, you're okay. Perhaps the reason the two lenses don't bring the rangefinder to infinity is that they've been optically corrected rather than mechanically. That's inconsistent, but if they're in focus wide open it sounds as if they're okay. In other words, if you don't see a problem in the images, the discrepancy could likely be ignored.

 

More technical: The rangefinder should reach infinity with a star, and yours doesn't. There's definitely something peculiar going on; and it bothers you. Since you're aware of it, you're not going to be happy without getting it fixed. I would send the lenses to Leica with an explanation of the problem.

 

Before doing that, I recommend you check them on another camera or two; if the rangefinder doesn't show coincident images at infinity there either, the problem is the lenses. But if the rangefinder lines up properly on those cameras but not on yours, I would contact Leica about the problem. They'll likely want the body as well as the two misbehaving lenses, and they may want to see the other lenses as well.

 

--HC

[delander †]

Take a look at the side of the lens bayonet where the cut-out is. With the lens set to infinity, the lens cam should be a fraction of a mm ( below the top surface of the bayonet flanges. As you reduce the focussing distance, the focussing cam moves down to a distance of about 4.5mm below the bayonet flanges.

 

If you set all your lenses to infinity, can you see a difference between the lenses which work and those which do not?

 

Mark it appears I can. The cams on the two summicrons appear to be lower but it is marginal. I tried to measure it with a mitutoyo vernier gauge which indicated about 0.5 mm difference then gave up in case I scratched a lens element. I would have thought Leica would easily have detected 0.5mm.

 

Howard, I have two M8 bodies, the lenses perform the same on both.

 

The lenses seem OK with regard to general performance.

 

I am not really a fan of 'testing' unless something is really wrong, so I'll work with the lenses some more and then see.

 

Thanks for all your advice,

 

 

Jeff

[delander †]

Jeff--

Perhaps the reason the two lenses don't bring the rangefinder to infinity is that they've been optically corrected rather than mechanically.

 

--HC

 

Howard,

 

Does that mean that there are two methods of calibrating a lens?

 

Jeff

[ho_co]

Does that mean that there are two methods of calibrating a lens?

 

Jeff, no, it's my overly terse and clumsy description of something that can occur in the assembly process.

 

With modern Leica rangefinder lenses, you've got the rangefinder coupling (which is either always or almost always part of the focusing helicoid) and you've got the lens head (which is mounted within the focusing helicoid). (E Puts says the Cosina Voigtländer lenses are somewhat different, but I don't understand his description.)

 

So with the Leica rangefinder lens, it's possible to set the helicoid wrong (so it doesn't reach infinity), but then locate the lens within it so that the lens is in focus whenever the rangefinder says it should be.

 

In other words, the rangefinder tracks accurately as far as it can, but the helicoid is blocked before it can reach infinity.

 

Coming at it from the other end (I don't know in what order the parts of the lens are assembled, so the following is hypothetical): The lens head is first mounted in the helicoid so that its location matches that of the helicoid in its relation to the rangefinder follower in the camera. Then the helicoid is mounted in the focusing mount (the engraved part that we rotate to focus) so that when the lens head in the helicoid is at infinity (in other words, lens optical unit and rangefinder are both at infinity since the rear of the helicoid drives the tracking roller in the camera), the scale on the focusing mount also shows infinity. At that point, the lens head, the focusing helicoid and the focus mount are locked down in relation to each other. (All this is far more complicated on a rangefinder lens than on an SLR lens.)

 

So my surmise is that on your two improperly-functioning lenses, the lens head is properly mated to the focusing helicoid (because you said the lenses focused properly at closest distance), but that combination is improperly mounted in the focusing mount. The infinity stop then is at the right place for the lens, keeping it from 'going past infinity,' but because the helicoid is not properly set in the focusing mount, the mount can't be rotated far enough for the rangefinder to reach infinity; if it did reach infinity in this situation, the lens would be getting garbage because it is 'focusing beyond infinity.'

 

Just perhaps a half-millimeter rotation could be enough to mess up the relations. There is no excuse for that to happen, in my opinion. I don't have an explanation how it could happen. Maybe there was a shift change or a lunch break between doing one part and doing the other. It's wrong and shouldn't occur, and Leica can and will fix them quickly.

 

My description is doubtless erroneous in detail, but I think it holds in general. I hope I haven't been too wordy trying to explain what I meant; but you can see now why I just said, "Perhaps the reason the two lenses don't bring the rangefinder to infinity is that they've been optically corrected rather than mechanically."

 

--HC

[marknorton]

The cams on the two summicrons appear to be lower but it is marginal.

 

That tiny distance is all it takes, we are talking fractions of a mm here - the focussing roller moves about 4mm as you focus from 0.7m to infinity and two thirds of that 4mm is used up by the time you get to 2m, meaning that the roller only moves about 1.5mm as you focus from 2m all the way to infinity.

 

I agree with HC's diagnosis; a lens like the 35/2 has a double helicoid; as you rotate the focussing ring, the focussing cam moves at one rate (X mm/degree rotation) and the lens head moves at another (Y mm/degree rotation), X and Y being determined by the machined pitch of the helicoids. For a wide-angle lens, Y < X, for a longer lens, Y > X, typically. X and Y are chosen to give as accurate focussing as possible which is then made exact by profiling the lens cam.

 

The infinity adjustment is done by rotating the helicoids with respect to the lens mount and each other; with the focussing ring at infinity, the helicoids are rotated in the lens mount (and secured in position by set screws) so that the focussing cam and lens head are both in the correct position, using the bayonet flange as a reference.

 

I think these two lenses need to go back to Leica for calibration.

[ho_co]

My gosh, Mark, thanks!

 

Wow, what I put into a page you distilled clearly into a paragraph.

 

Jeff--skip my 'article' and just read Mark's post!

 

--HC

[delander †]

Thanks Mark and Howard for taking the time to make these detailed replies. I will check this infinity focus again just to make sure that I am not seeing things before I send the lenses in for adjustment.

 

Jeff

[ioriw]

I have the problem on a 50 lux where I cannot get the mast atop a tall skyscraper to fully coincide in the rangefinder.. I hit infinity and run out of focussing room with just a little more left to go. On my 90/4, I can of course get the images to coincide a little before hitting infinity on the lens. Is the 50/1.4 out of whack a little? I've had quite a bit of trouble nailing focus with it sometimes, I put it down to bad eyesight or focus shift...

[lars_bergquist]

75m is definitely not infinity; the jet is acceptable,a star is better.

 

Infinity with a LENS is in normal practice reckoned to be at about 1000x the focal length: 35m for a 35mm lens, 50m for 50mm etc. No need to go into celestial photography. There is no such standard for rangefinders! I seem to find however that an M rangefinder is incapable of producing a double image at distances longer than about 200m. So if yours double-images a jet plane, or anything else at a longer range, adjustment is called for.

 

Remember that if you are going to judge correct focus in the image produced by a lens, then the small rate of reproduction of a wide angle lens makes distant objects very small indeed. Simple atmospheric facts (haze, mirage) plays havoc with sharpness. This is mostly the cause of the often heard complaints that wide angle lenses cannot make sharp pictures at long distances.

 

When you test, remember to make clear to yourself whether you are testing the lens or the rangefinder. Don't try to test both at the same time!

 

The old man from the Age of the Standard Lens

[filmboy]

No need to go into celestial photography

 

Just as well, for me. I live in London - difficult to find a star to focus on, even if I did want to stand outside my flat at night in order to adjust the focus on my M8! (I don't...)

 

Thinking of standing by the side of the river, and focusing down at Canary Wharf in the distance. That should be acceptable.

[ho_co]

Iori--

What you describe is annoying, but probably of no practical import.

 

You virtually never focus at infinity, right? Doing so means throwing away all your far depth of field.

 

For the same reason, in normal usage--shooting a mountain range, for example--the lens' depth of field would cover any practical focus difference.

 

If you do focus to infinity--for astronomical images, for example--the lens may or may not be at infinity optically. That is, the lens' infinity stop may be at the right position, but the mount may be slightly off, preventing the rangefinder's reaching infinity. (See Mark N's post just above.)

 

But despite all of that, it's still annoying, and you might want to have Leica adjust either the lens or the camera or both.

 

--HC

[Guest guy_mancuso]

I have the problem on a 50 lux where I cannot get the mast atop a tall skyscraper to fully coincide in the rangefinder.. I hit infinity and run out of focussing room with just a little more left to go. On my 90/4, I can of course get the images to coincide a little before hitting infinity on the lens. Is the 50/1.4 out of whack a little? I've had quite a bit of trouble nailing focus with it sometimes, I put it down to bad eyesight or focus shift...

 

Sounds like your 50mm lux may need a calibration shim, mine did also. May want to send that to Solms since you are in Germany.

[ioriw]

No, I hardly ever focus at infinity but I was thinking it is just an indicator of a lens that is not quite bang on, and whether focussing on the moon or a close up portrait, if the rangefinder view is impossible to coincide at 300m or however far away that mast was from me, then im not getting an accurate rangefinder view at 3m right? I'm never happy with the focus on this 50 lux, it is rare that I nail it how I wanted.. I'm just not eager to post off the camera again.

 

Guy: did you need to get the lux shimmed for use on a particular body?

 

I'm hoping this is just a lens-thing with the 50lux and that my rangefinder is in perfect alignment

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[ho_co]

No, I hardly ever focus at infinity but I was thinking it is just an indicator of a lens that is not quite bang on, and whether focussing on the moon or a close up portrait, if the rangefinder view is impossible to coincide at 300m or however far away that mast was from me, then im not getting an accurate rangefinder view at 3m right? I'm never happy with the focus on this 50 lux, it is rare that I nail it how I wanted.. I'm just not eager to post off the camera again. ...

Iori--

That's why I mentioned the two possibilities. In all likelihood, the lens is simply not focusing correctly, as you surmise. It's quite likely that the lens is out at 3m as well as infinity. But what Mark and I said above is that with rangefinder lenses, that isn't necessarily so.

 

Ship or deliver the lens to Solms. There is definitely something wrong with it. If you go to a shop that carries Leica, or if you go to Solms, you can try another lens on your body, or try your lens on another body.

 

Judging from the fact that other lenses align the rangefinder at infinity, but not this one:

 

1) It's extremely unlikely that the body will need adjustment.

 

2) It's possible but also unlikely that the lens is okay except for an improperly-set infinity stop.

 

3) At a minimum, the infinity stop is wrong on the lens; and very likely the lens needs a focus recalibration.

 

Good luck!

 

--HC