Back-focus or back-rangefinder ?

[rpo]

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I want to make a question to all M8 users.

Please check your M8 to infinite, also with a 50 mm lens but at least to 500 meters or more, and check the rangefinder coincidence. This is good? All two images overlap perfectly? If your M8 suffers by back-focus on short distances with a Summilux or a Noctilux full open, really often by back-rangefinder and is sufficient to calibrate the rangefinder coincidence to the infinite: simply no more.

The digital sensor focal plane is not the same of the film because the film is not flat but a little curved and little behind and a factory rangefinder calibration like all the film camera probably could be not corrected. I hope nobody offense...

Thanks for your answer

Roberto Piero Ottavi

Venice- Italy

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[lars_bergquist]

Roberto,

 

if the plane of best focus moves outwards when you stop the lens down,

 

--------------- and don't change the lens focusing ------------------

 

then you have backfocusing of the lens, and not a rangefinder error. Use a tripod and be careful to touch the aperture ring only. – The important difference between film and a solid-state sensor is that the film emulsion has a depth that the sensor lacks. Lack of film flatness is a fault that can work both ways, and Leica cameras (and 35 mm cameras in general) have excellent film flatness.

 

The old man from the Age of Wooden Tripods

[rpo]

May be, but check your M8 at the infinite with a 24 or 28 mm lens...

Thanks however for your answer

Roberto

[ho_co]

Roberto,

You are correct that the rangefinder must be working correctly before judging the performance of a lens.

 

But keep in mind that whether the rangefinder is properly adjusted has nothing to do with focal length: The rangefinder mechanism is part of the camera, no matter which lens is attached. Therefore, specifically checking the rangefinder with a 28mm or a 135mm is meaningless.

 

If the infinity alignment of the rangefinder is out with a specific lens, that is a lens problem.

 

Your point is good that there are lens problems and rangefinder problems; and you are right that one must check the rangefinder before embarking on lens critiques. But you may be 'preaching to the choir' in that regard. Most of the threads concerning lens backfocus, focus shift and spherical aberrations contain discussions of exactly this point.

 

--HC

[Guest Olof]

a few days ago i wasnt able to get a sharp picture with my 1.4 50mm at 16, then i remember the bachfocus discussion and wit another aperture everything was fine

[mjh]

a few days ago i wasnt able to get a sharp picture with my 1.4 50mm at 16, then i remember the bachfocus discussion and wit another aperture everything was fine

At f16, I wouldn't expect any problems with focus-shift induced backfocus. A more likely culprit would be diffraction, but it's hard to decide without having seen the image.

[lars_bergquist]

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May be, but check your M8 at the infinite with a 24 or 28 mm lens...

Thanks however for your answer

Roberto

Apples and oranges ... you cannot expect a 24 mm lens to resolve, say, the Eiffel Tower or Chrysler Building at inf. as well as a 50 mm lens, because the magnification is just half that of the 50 mm. It would have to resolve 40 lp/mm just as well as the 50 mm resolves 20, which is of course impossible.

 

The old man from the Age of Resolution Targets

[lars_bergquist]

At f16, I wouldn't expect any problems with focus-shift induced backfocus. A more likely culprit would be diffraction, but it's hard to decide without having seen the image.

I have tested my Summilux ASPH 50 mm pretty rigorously for backfocusing and have found none. My 35 'lux and 28 'cron are in Solms for coding, so I can't say anything about these lenses. I do however suspect that this may be a pseudo-problem.

 

The old man from the Age of Tape-Measure Focusing

[rpo]

Apples and oranges ... you cannot expect a 24 mm lens to resolve, say, the Eiffel Tower or Chrysler Building at inf. as well as a 50 mm lens, because the magnification is just half that of the 50 mm. It would have to resolve 40 lp/mm just as well as the 50 mm resolves 20, which is of course impossible.

 

The old man from the Age of Resolution Targets

 

I repeat: check your M8 at infinite...

 

Roberto Piero Ottavi