problem with front focusing

[dante]

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I don't want to break up the math party, but the real issue is that the lens - like every fast lens - has some amount of undercorrected spherical aberration that causes a focus shift (backward) when stopped down. If this lens had perfectly centered DOF at 1m and f/1.4, it quite possibly would have bad back-focus at other apertures.

 

As DAG explained it to me once, it is pretty typical for a fast lens to be closer to front-focusing wide-open and to shift focus backward (relative to the intended point as determined by the RF) as the lens stops down. This is where increasing DOF masks a multitude of sins - and it's why you can't use a 75 Summilux as an aid to DIY RF adjustment.

 

The lens in this test looks fine. This magnitude of error is overcome by turning the lens toward distant just to the point where the RF image is still looks aligned but just about to break out of alignment.

 

Dante

[jaapv]

You are responding to posts made in 2011 by a person who is not a member any more.

[thighslapper]

I don't want to break up the math party, but the real issue is that the lens - like every fast lens - has some amount of undercorrected spherical aberration that causes a focus shift (backward) when stopped down. If this lens had perfectly centered DOF at 1m and f/1.4, it quite possibly would have bad back-focus at other apertures.

 

As DAG explained it to me once, it is pretty typical for a fast lens to be closer to front-focusing wide-open and to shift focus backward (relative to the intended point as determined by the RF) as the lens stops down. This is where increasing DOF masks a multitude of sins - and it's why you can't use a 75 Summilux as an aid to DIY RF adjustment.

 

The lens in this test looks fine. This magnitude of error is overcome by turning the lens toward distant just to the point where the RF image is still looks aligned but just about to break out of alignment.

 

Dante

 

Yup..... I use my Noctilux (used to use the 75/2) when calibrating my rangefinder and this effect is very obvious between f0.95 and f2...... a compromise position needs choosing... and I've found if it's ok for the Nocti then all my other lenses are usually ok as well.....

 

Going back to te original post ..... if NJ have been calibrating his rangefinder by adjusting what is effectively a coarse adjustment/end stop on the mechanism (which is mangled in the photos) then I'm not surprised it was never right