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Definition of Infinity?


ravinj

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n divided by zero, where n is any number?

 

(Just kidding. ;) )

 

Infinity should be no different for the M9 than for any other camera. The question is, when you say "need to focus slightly less than infinity", do you mean judging by the infinity mark on the lens, or judging by where the RF images coincide?

 

Basic test - the moon (if available). Focus using the RF. The RF images of the Moon should coincide exactly at the point the lens focus ring hits the infinity stop and can't be turned any further. Picture of the moon taken at this point should be sharp.

 

If the moon is not available, things at a couple of miles/kms (distant buildings, mountains, etc.) can substitute. 200 meters is borderline (barely far enough).

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In my experience, I can see a "just noticeable difference" on my M9 rangefinder between a tower about 500 meters distant and a mail box about 75 meters distant. I see no difference between that tower and the moon, so IMO the tower is effectively at infinity for me.

 

Regards, Jim

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The theoretical definition is 1000x the focal length. So, for a 50mm lens this would be 50 meters. Practically, I have found the same as you, I am quite surprised not to hit the infinity stop on my lens when the distance is quite far, and some quick tests showed that the rangefinder was correct. AF SLR lenses have a very short throw from close focus to infinity and depth of field scales are disappearing in order to accommodate faster AF speeds. This makes me appreciate Leica more than ever.

Edited by darylgo
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Theoretically, the definition of optical infinity is that rays from a subject point are parallel when they enter the entrance pupil, but f x 1000 is a useful approximation.

 

A mathematical pedant (if not that is a tautology) would say that any numerable distance is finite; not even the most distant galaxy on the edge of the visible universe is infinitely far away, strictly speaking. "Infinity" is a mathematical and not a physical concept.

 

The old man who lives at Hilbert's Hotel

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The theoretical definition is 1000x the focal length. So, for a 50mm lens this would be 50 meters.

 

Let's lay this to rest. It's not a "theoretical definition", it's a practical rule of thumb, IIRC from Zeiss not Leitz, from about 80 years ago, and has no relevance to fast lenses or high-resolution sensors.

 

With a "50mm" Leica lens, 1000 times the focal length is about 52m.

 

Pre-war Zeiss 35mm cameras assumed a 0.05mm circle of confusion. On this basis, any of the pre-war 50mm lenses focused at 52m would have depth of field stretching to infinity (even the f/1.5 Sonnar wide open).

 

Leica assumed a 0.03mm CoC. On this basis, the rule of thumb is fine for the standard pre-war lens but not for fast lenses:

 

  • f/3.5 (Elmar) - infinity
  • f/2.5 (Hektor) - infinity
  • f/2 - just under 400m
  • f/1.4 - about 135m
  • f/1 - about 90m.

To make the most of the M9 (or of modern films and lenses) a smaller CoC is required. At 0.02mm:

 

  • f/3.5 - infinity
  • f/2.5 (Summarit) - about 190m
  • f/2 - about 125m
  • f/1.4 - less than 90m
  • f/1 - less than 75m.

QED.

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Let's lay this to rest. It's not a "theoretical definition", it's a practical rule of thumb, IIRC from Zeiss not Leitz, from about 80 years ago, and has no relevance to fast lenses or high-resolution sensors.

 

With a "50mm" Leica lens, 1000 times the focal length is about 52m.

 

Pre-war Zeiss 35mm cameras assumed a 0.05mm circle of confusion. On this basis, any of the pre-war 50mm lenses focused at 52m would have depth of field stretching to infinity (even the f/1.5 Sonnar wide open).

 

Leica assumed a 0.03mm CoC. On this basis, the rule of thumb is fine for the standard pre-war lens but not for fast lenses:

 

  • f/3.5 (Elmar) - infinity
  • f/2.5 (Hektor) - infinity
  • f/2 - just under 400m
  • f/1.4 - about 135m
  • f/1 - about 90m.

To make the most of the M9 (or of modern films and lenses) a smaller CoC is required. At 0.02mm:

 

  • f/3.5 - infinity
  • f/2.5 (Summarit) - about 190m
  • f/2 - about 125m
  • f/1.4 - less than 90m
  • f/1 - less than 75m.

QED.

 

Wow, the knowledge on this board continues to amaze me and the more I learn about optics the more I appreciate Leica. Do you have a good source for this information?

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Wow, the knowledge on this board continues to amaze me and the more I learn about optics the more I appreciate Leica. Do you have a good source for this information?

 

You can confirm the figures with any good depth-of-field calculator. I usually use Barnack.

 

There are references to the Zeiss 0.02mm CoC in this forum and elsewhere on the internet, or you can look at the depth of field scale on an old Contax and check it against a DoF calculator (or a Leica lens). Virtually all Leitz and Leica lenses for 35mm cameras have DoF scales based on 0.03 or 0.033 mm, and that has also been the standard for just about every other 35mm camera since including all the Japanese SLRs.

 

The need for a smaller CoC to suit modern lenses, sensors and especially print sizes has been discussed many times on this forum. The usual advice is to treat the DoF scale as if the f/8 marking was actually labelled f/4, the f/16 marking f/8 and so on, i.e. a two-stop adjustment. This corresponds to a CoC of 0.015 mm, half the traditional size.

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[*]f/2 - just under 400m

[*]f/1.4 - about 135m

[*]f/1 - about 90m.

To make the most of the M9 (or of modern films and lenses) a smaller CoC is required. At 0.02mm:

 

  • f/3.5 - infinity
  • f/2.5 (Summarit) - about 190m
  • f/2 - about 125m
  • f/1.4 - less than 90m
  • f/1 - less than 75m.

QED.

 

Thanks for the inputs. Based on what I have observed with my M9 and 4 lenses: 35mm Summicron ASPH, Zeiss 50mm F2 planar, 90mm Elmarit and 90mm tele elmarit, the infinity distance at F2 (F2.8 for 90mm) corresponds more to the pre-war list.

 

At approx 300m and F2 with both 35mm Summicron and Zeiss 50mm focused at infinity mark on the lens leads to a blurred image.

 

Probably the "moon" test seems to be best and I will have to wait for it to show up to try it out. Or, the rangefinder needs adjustment.

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Virtually all Leitz and Leica lenses for 35mm cameras have DoF scales based on 0.03 or 0.033 mm, and that has also been the standard for just about every other 35mm camera since including all the Japanese SLRs.

 

The need for a smaller CoC to suit modern lenses, sensors and especially print sizes has been discussed many times on this forum. The usual advice is to treat the DoF scale as if the f/8 marking was actually labelled f/4, the f/16 marking f/8 and so on, i.e. a two-stop adjustment. This corresponds to a CoC of 0.015 mm, half the traditional size.

 

Thanks for information. I assume that also the modern new Leica lenses use the "old" DoF scale?

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I think my lens (50-cron R MK1) may be the goofball of the bunch. Its infinity is the moon, whereas If I stand in williamsburg, focus at the empire state building (visible) and set to infinity, its blurred. If I set it back to JUST before infinity, razorsharp (I can see the little windows in the pic). However, what shots it does get, are quite nice and accurate / in focus. Clearly, if the Williamsburg bridge is over 2,000 feet long, and the rule is that infinity occurs at 1000x focal length (thus in my case 50m)... either I have a really good lens or a really goofy lens (I'd have to find another Leicaflex SL to ascertain the claim).

 

I'm gonna have to test out this next time the moon is out (and visible)

 

Jason

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As a practical matter, years ago when I began to acquire some rangefinder cameras, I noticed infinity focus wasn't always accurately indicated by the focus ring and sometimes the RF images didn't align, etc.. but the shots came out just fine. I learned to adjust the focus rings to reality, but the RF image adjustment always annoyed me, as it was inconvenient to use the moon or stars as my source for this adjustment. A friend pointed me to a convenient method which has worked splendedly check on the RF adjustment. You merely measure the interpupillary distance between the centers of the RF windows....transfer that distance to a heavy duty card (I laminate mine) and mark one with a large X and the other with a cross. To check your images (horiz & vert) for proper infinity adjustment....place the card about 8-15 feet from the camera, focus for infinity, and if everything is fine you get the image of an 8 pointed asterisk centered between these two marks.

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Thanks for information. I assume that also the modern new Leica lenses use the "old" DoF scale?

 

Let's call it "traditional", but yes, as far as I know all current and recent, and virtually all past M-mount and R-mount lenses have DoF scales based on 0.03mm or 1/30mm. A while back there was a thread here in which we found that some old 21mm M-mount lenses had something a little different, but I can't remember the details.

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With smaller CoC values, many good lenses could not be focused accurately any more. See http://www.l-camera-forum.com/leica-forum/customer-forum/219656-rangefinder-focussing-accuracy-rule-thumb.html

 

Be careful with the conclusions of that thread. Bert used a false assumption when calculating the resolving power of the human visual system. He used a standard visual acuity resolving power of the human visual system when, he should have used the hyper acuity value that the human visual system is capable of under viewing conditions such as what is observed in the optical rangefinder.

 

This is a big error. For example, human visual acuity is capable of detecting 0.6-1.0 arc minutes under normal viewing. But, it can improve to as much as 0.13 arc minutes under hyper acuity viewing, such as that utilized in lining up the range finder patch . I believe his final conclusion is a little off and lenses can be focused with a better precision than what he concluded.

 

The human visual system including the visual cortex is an amazing thing.

Edited by RickLeica
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2000 times the focal length or about 62 miles for a 50mm lens.

 

Y'all should see the fanatic calculations aerial/mapping/recon people go through for photography. First, all lenses are certified at widest aperture, and no conventional aircraft fly high enough for their long lenses. The Black Bird flies at a publicized ~17 miles altitude, although we know it goes higher, quite possibly to 90,000 feet plus. It literally skips along the atmosphere varying 1000 feed at a time, like a stone skips across water. Talk about focusing issues.

 

--

Pico - once stationed with the RAF @ Upper Heyford - where we knew this stuff. The U2 was easier to deal with.

Edited by pico
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