How Important is Infinity Focus?

[robsteve]

Originally Posted by glenerrolrd

Rob How did you test those M lenses at infinity ?

Put the camera on a tripod, use the cable release and with the lens on the infinity stop take a picture of a distant scene. I live on a lake and just shoot the buildings down at the other end of the lake. For the wider angle lenses, I shoot an area of the lake that is narrower and the buildings are much closer. I then look for the sharpness of details in the pictures, such as detail in the roof shingles or window trim.

I will try to find one of my old test pictures and post it here as an example.

Robert

[marknorton]

I'd just add that you should take images starting with the lens set to less than infinity and change the focus each time by the smallest increment until you are at the infinity stop. You should find they focus improves throughout the series and does not peak and then decline - if it does, the lens is focussing beyond infinity.

Trouble is, while you can tell if the lens is focussing beyond infinity (because the focus peaks and then declines), you cannot easily tell if, when the lens is on the infinity stop, there is "more to go" because of the depth of field. Even at f1, the Noctilux will have everything in focus from 82m to infinity.

[usefeet]

My 50 Summilux ASPH doesn't focus to infinity (and has been confirmed on a collimator), as it goes past,
but I can't be bothered sending it to Solms and taking the chance that it won't be fixed properly either, so I just compensate when using short distance subjects,
particulary when using large aperture openings.

If I get to travel to Photokina this year, then I might bribe service to fix it while I wait, so don't you get that idea too!

[lars_bergquist]

What is infinity? It means that the rays from a point object do not diverge when hitting the lens, but are parallel. In practice, while that distance would theoretically be at the end of the universe, the distance is taken to be at 1000 x the focal length of the lens. So a 135mm lens should be tested at 135m (1m = 1000mm) while a 21mm lens is at practical infinity for objects at 21m.

Testing at much longer distances is not practical because of the intervening atmosphere, with its dispersion and mirage. Any image test should be done on a clear, calm day and from a sturdy tripod.

The old man from the Age of the Darkroom

[marknorton]

Originally Posted by lars_bergquist

Testing at much longer distances is not practical because of the intervening atmosphere, with its dispersion and mirage

A voice of reason! I agree... Trying to take a picture of some minor star in Ursa Minor is hardly likely to be a good test for the infinity setting of your camera and lenses. Astronomers have enough trouble when that's what they're trying to do. If the lens focuses well at 1000f, it's fine.

[usefeet]

>>>In practice, while that distance would theoretically be at the end of the universe, the distance is taken to be at 1000 x the focal length of the lens. So a 135mm lens should be tested at 135m (1m = 1000mm) while a 21mm lens is at practical infinity for objects at 21m.<<<

I'm in full agreement with this statement as the lens focussing did not change whether I focussed on the bright moon (that we have here in our clear conditions),
or the television mast in the distance. The mechanical disrepancy was about the same.

[Tony C.]

Originally Posted by lars_bergquist

Testing at much longer distances is not practical because of the intervening atmosphere, with its dispersion and mirage.

The old man from the Age of the Darkroom

That is an interesting argument, but I'm not sure that I agree with your conclusion. I say that not only because I – and apparently many others – have had quite good success using a star to calibrate our M8s to infinity, but because it strikes me that any distortion due to atmospheric conditions shouldn't matter at all.

What I mean by that is that the distortion to which you refer has no practical bearing on what we see and focus on through the viewfinder. It shouldn't matter whether a distant star is actually 10mm, 10 inches, or 10 miles to the left or right of where it appears to be through the viewfinder, as we are focusing on the object where it appears, not where it actually is.

By way of analogy, consider a situation in which you were photographing a subject which was on the other side of a window. Now, the glass will, to some extent, distort the subject, yet you wouldn't attempt to calculate how to adjust for the distortion – you'd simply focus on the subject and shoot. And the subject would be in focus, in spite of the fact that it would not have been in focus had the glass not been there.

Perhaps I'm wrong about all of this, so feel free to correct me!

[robsteve]

I think some posters are confusing the infinity setting of the rangefinder with what the original poster was asking, the optical infinity setting of the lens.

With digitals, the optical infinity rule quoted doesn't seem to apply. I can focus a fast 50mm at different objects well past the distance quoted and still not have ininity objects in focus. If weather permits, I will take some samples with my Noctilux or 90mm APO.

Anybody know the formula for optical infinity? I think it may be 1000 x the focal length ^2. For a 50mm, that would be 250meters. Does this formula assume a certain aperture? In other words, does a f1.4 lens have a different optical infinity than a f2 lens?

[t024484]

Originally Posted by robsteve

I think some posters are confusing the infinity setting of the rangefinder with what the original poster was asking, the optical infinity setting of the lens.

With digitals, the optical infinity rule quoted doesn't seem to apply. I can focus a fast 50mm at different objects well past the distance quoted and still not have ininity objects in focus. If weather permits, I will take some samples with my Noctilux or 90mm APO.

Anybody know the formula for optical infinity? I think it may be 1000 x the focal length ^2. For a 50mm, that would be 250meters. Does this formula assume a certain aperture? In other words, does a f1.4 lens have a different optical infinity than a f2 lens?

(1000 * focal length) exp 2 is not 250 but 2500 metres if I'm right, which seems a bit far to to test for infinity, and 1000 * (focal length exp2) is only 2.5 meter.
From experience 250 meter is too short to test infinity for a 50mm lens.
500 meters would be better.
The exp 2 seems to make sense, because twice the focus length, means 4 times as many pixels for the corresponding part of the picture.
I would suggest 200,000 * (focal length exp2), everything expressed in metres.
For a 50mm this gives 500 meter, for a 21mm the result is 80 meter and for a 75mm it is 1120 meter.

Hans

[marknorton]

We do seem to agree though that 400,000 km to the moon is a little over the top...

[robsteve]

Originally Posted by t024484

(1000 * focal length) exp 2 is not 250 but 2500 metres if I'm right, which seems a bit far to to test for infinity, and 1000 * (focal length exp2) is only 2.5 meter.
From experience 250 meter is too short to test infinity for a 50mm lens.
500 meters would be better.
The exp 2 seems to make sense, because twice the focus length, means 4 times as many pixels for the corresponding part of the picture.
I would suggest 200,000 * (focal length exp2), everything expressed in metres.
For a 50mm this gives 500 meter, for a 21mm the result is 80 meter and for a 75mm it is 1120 meter.

Hans

Hans:

It has been a long time since I took math. I thought the order of operations was to do the power of two before the multiplication.

The formula is in Erwin Puts' book, but that may relate to film and not digital.

I have been waiting for the right weather to be able to shoot a test using the lake in my back yard. If it hasn't been snowing or raining over the past few days, it was too sunny to shoot wide open. I have a mapping program that has my lake in it and I measured some areas that I can test. For example, the houses at the end of the lake are about 550m away, while a hotel part way down the lake is about 290m away. In my testing of lenses, I think the hotel has always been far enough away. I will confirm the next overcast day we have so I can shoot the Noctilux and Summicron at f1 and F2 respectively.

Robert

[robsteve]

I took the time to test for infinity on a version IV 50mm Summicron. Using a mapping program I estimated the distances to my target subjects. The nearest I tested was about 95mm and the furthest was 550m.

These were shot at F2 and the lens on the infinity stop. Incidentally, the rangefinder also aligned at these distances.

In this shot, the aprtment building is about 190m away and about 100m behind it is a hotel. At f2 everthing was sharp at this distance.



Full resolution crop into the above picture.



Here I am shooting the other way towards a closer shore. The wharf and objects are about 95m away. The netting in the hockey net is plenty sharp, so I would say we are still in the infinity range of the lens.



The crop:




My conclusion is you probably don't have to shoot the moon to see if your lens is sharp at infinity.

[glenerrolrd]

I wish I would have saved the shots. During that period when some M8 owners (not me of course) were "experimenting with the allen wrench adjustments, I found a building about 800 yards for my back deck. There was a large tree maybe 500 yards out ..I could be off a little . First There was a slight difference in focal point on the 500 verse 800 yard targets and you could see it in the image. Second I also noted that if the rangefinder didn t quite make it to infinity ..the 800 yard target suffered. This was far from a controlled effort but I was surprised that you could see the difference when I focused on the near subject(about 500yards) . I guess another way of saying this is that at 500 yards I hadn hit true infinity and it showed in the subject. When I have some light I will try this again.

[robsteve]

Originally Posted by glenerrolrd

. I guess another way of saying this is that at 500 yards I hadn hit true infinity and it showed in the subject. When I have some light I will try this again.

I also noticed the Noctilux at F1 needed the 500m plus to reach infinity, as opposed to the 50mm Summicron that was in focus at 95m and f2.

It may be the super speed lenses are just not meant to shoot infinity. In the case of the Noctilux, the edges were in focus at the 200m distance, but the center wasn't until I shot distances of 500m or more. This is probably the curvature of field they talk about coming into play.

[glenerrolrd]

My Noctilux is back and it feels good. This one is my baby ..I have had it for over 25 years maybe a little longer. So tomorrow I shoot again after 10 months..but if its really on ..I will forget the time it took. No question Noctiux users are a bit obsessive ..