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Focus at infinite


epand56

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Maybe this is a silly question for many people here, but it is not for me. Not a long time i'm into manual focus.

 

All my lens have the infinite in focus not exactly at the end of the focus ring run but, say, half a millimeter before the end of the run. All of them. Is this correct?

 

The focus on pictures is ok, but i always wonder if the infinite focus should be at the end of the ring run for the camera to work properly.

Thanks for any advice.

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x

yes, that happens when focusing on very distant objects such as buildings, mountains and even clouds. The picture la okay but the infinite focus is slightly befor the end run or the ring. If i end the run objects get out or focus again.

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I understand that your lenses are correctly focused when scale-focused at infinity, but the rangefinder images do not coincide perfectly.

 

If all your lenses show the same phenomenon, then I suspect you should have your rangefinder checked.

 

Practical infinity is usually reckoned to be focal length x 1000, so for a 90mm lens it should be 90 meters. But the M rangefinder does focus farther than that, so 200m should be safe.

 

The old man from the age of Scale Focusing

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You need to check it on a star or planet.

Even IF it looks off when checking it on a star or a planet the main thing is does the camera focus correctly at shorter distances? If it does stop worrying about infinity alignment and take some pictures.

 

I have many M lenses and most all place the split/double image at different places when turning the lens all the way to infinity.

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Because of my not proper english maybe i've not been completely clear.

The rangefinder images do coincide perfectly at infinite, but this happens just an half millimeter before the ring is at its end run.

When a distant object (a mile or so) is in focus, my focus ring could still be turned to the left of an half millimeter or even less.

I wonder if the normal behaviour of the rangefinder is to focus on infinite at the total end run of the focus ring.

In this case mine have to be checked.

 

And yes, Shootist, lenses generally focus properly at short distance.

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Infinity is not 1 mile away or 10 miles away or 100 miles away.

That is why I suggest to use a star or a planet.

 

Like I said in my previous post, "If the camera and lenses are focusing correctly at your normal shooting distances then don't worry to much about how the images line up at the lens infinity stop".

 

Yes at any distance other then looking/focusing on a star or planet I can always turn the ring a little more.

Normal.

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This is nothing new, nothing uncommon and absolutely nothing to do with the M8. Many lenses are made with some tolerance - the ability to focus "beyond infinity" - to allow for focus variation due to, for example, variations in ambient temperature. This is a feature not a fault.

 

Regards.

 

Bill

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This stuff about stars and planets is baloney.

 

To a first approximation (to keep the pedants at bay), camera lenses follow the simple equation:

 

1/u + 1/v = 1/f

 

where u and v are the subject and object distance and f is the focal length. For an object at infinity, 1/u = 0 and the object distance, v, is the same as the focal length, f, and the image comes to a focus behind the lens. For all possible values of u, v >= f.

 

The question is, if instead of using a celestial object several light years away, you revert to terra firma and instead use that Dunkin Donuts/<substitute your favourite junk food brand> sign over there, how big is the error in where the image comes to a focus? Taking Lars number of a junk food sign 200 metres (656 feet in old money), the error is 1/200,000 mm = 5 um which is the same tolerance within which Leica mounts the sensor in the camera. 200 metres is good enough.

 

If the infinity stop instead allows the lens to come closer to the focal plane that it should (v < f), it follows that u is negative so that nothing in front of the lens is in focus. The imperative on a Leica lens therefore is that the infinity stop should not be set beyond infinity but a fraction less than true infinity and rely on the depth of field to cover the difference.

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Infinity is not 1 mile away or 10 miles away or 100 miles away.

That is why I suggest to use a star or a planet.

 

Like I said in my previous post, "If the camera and lenses are focusing correctly at your normal shooting distances then don't worry to much about how the images line up at the lens infinity stop".

 

Yes at any distance other then looking/focusing on a star or planet I can always turn the ring a little more.

Normal.

 

 

Infinity is strictly defined by Leica. It is an object at a distance of 1000x the focal length.

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Infinity is strictly defined by Leica. It is an object at a distance of 1000x the focal length.

 

Jaap, following Leica statement, in the case of a 50mm it is 1000 x 50 = 50,000, but 50,000 what, millimeters? Centimeters? Meters?

Never been an hot shot in Math.

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Enrico, if I'm not wrong it's 50000 mm, i.e. 50 meters.

Btw I'm encountering the same "infinity imprecision" with my lenses, but it never bothered me so much (unless you can't reach an infinity focus at all!).

 

Saluti da Torino!:D

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Infinity is strictly defined by Leica. It is an object at a distance of 1000x the focal length.

 

I hope that's not what they use when setting up the range finder on the M8. AFAIK the rear cam on a lens (which actuates the range finder arm wheel) has to be set so that it's infinity position is the same for all lenses. When I've taken the M8 into the countryside to check infinity focus the range finder can clearly distinguish between objects 1Km and 5Km away.

 

According to the 1000x rule:

 

21mm lens -> 21 metres

50mm lens -> 50 metres

135mm lens -> 135 metres

 

I suspect the 1000x focal length figure is somehow related to circle of confusion and degree of enlargement etc and relates back to early film days.

 

Bob.

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You need to check it on a star or planet.

Even IF it looks off when checking it on a star or a planet the main thing is does the camera focus correctly at shorter distances? If it does stop worrying about infinity alignment and take some pictures

The M rangefinder won't be able to discern between an object at 200 meters and one at 200 lightyears, neither does it have to. Mathematical infinity and optical infinity are not the same thing. Besides, not even Sirius is at mathematical infinity! The distance is measurable, i.e. finite.

 

Enrico, your rangefinder indicates infinity while your lenses are at less than infinity, so you have two infinity indications. You say that the infinity stops of your lenses are optically correct, i.e. they give you sharp images at infinity. So these are the correct ones, and your rangefinder is wrong. Get it fixed in Solms.

 

The old man from the Age of Scale Focusing

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I agree that Infinity should be seen in the right perspective.

Taking a 50mm lens, and an obiect 200 meters away, gives me:

 

1/x + 1/200.000 = 1/50.

 

Solving x gives; x = 50.01 mm, so the lens is 0.01 mm shifted from the infinity position, which is still well within the mechanical scope of the distance setting.

Reducing this to 5 micron, a figure that Mark has mentioned, brings the distance y to:

 

1/50.005 + 1/y = 1/50

 

Now the distance y is 500 meters.

 

So I would say just from a mechanical point of view: Infinity in mm is 10.000 * focal length.in mm.

 

Hans

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