M Focus tuning with Field Curvature and focus shift?

[everyXnewXday]

I recently picked up my first Leica, an M2, plus 35mm Summicron V4, 50mm Summilux V2, and an M-Rokkor 40mm f/2 and am trying to understand how these lenses are SUPPOSED to behave regarding focus shift and field curvature relative to the rangefinder patch.

For instance, the summilux exhibits field curvature such that, at wider apertures, if the center is focused at infinity the field is slightly front focused and if the field is focused ideally then the center is slightly back-focused. Assuming everything was calibrated to Leica specifications, would the lens's infinity stop be set for the center or the field?

To put it simply, if I wanted to test the rangefinder patch accuracy should I be looking at the center of the image or at the field? What about at close range?

Same question for focus shift. My M-Rokkor exhibits pretty strong focus shift. At what aperture would Leica normally tune a lens for optimal focus?

Oh, also, where should the actual numbers on the lens focus scale line up with the focus mark? The center? The front edge of the number? I ask because my summilux is supposed to focus down to 1 meter, but the focus ring turns well past the "1".

Thanks!!

 

[mod2001]

For instance, the summilux exhibits field curvature such that, at wider apertures, if the center is focused at infinity the field is slightly front focused and if the field is focused ideally then the center is slightly back-focused.

Hi, is this based on your experience or just based on technical data from the lenses or worse experience with digital? I never experienced any focusshift on film, even with the Zeiss Sonnar 50/1.5 which is known for the shift depending on the initial setting either for 1.5 or 2.8 its hard to tell with film, ate least in my experience.

Yogi

 

[ferider]

There is no back-focus at infinity.

The lens' infinity stop is set for image distance = focal length, and most Leica lenses have the exact focal length specified on them.

Everything else should be adjusted to center accuracy and wide open. Now, with 20+ year old lenses from ebay, there is a large chance that a used lens / camera was adjusted to a shooter's preferences for close focus. Just check out how often people here talk about CLA this, CLA that. Over the last 10 years or so, the used lenses that I've seen easily have variied +- 2inches wide open, at 1m focus.

You can either adjust all your lenses to your preference, or make sure the lens behaves as it should at infinity (wide open center resolution), and "learn" how each lens behaves at close focus.

At close focus, I personally prefer - in particular for moderate wides - a slight front focus (1-2 inches, depending on focal length), to compensate for cosine errors at 1/3rd composition, and make the lens sharper when it shifts closing down 1 or 2 stops (all of your 3 lenses shift backwards noticeably). Other users will be different.

In a nutshell, at the accuracy levels that you are talking about, Leica lens compatibility is a myth. With used and even with new lenses, in particular if you add other brands (Minolta, CV, Canon, Nikkor, etc).

Roland.

 

[Rob-F]

There is no back-focus at infinity.

Good point! Nothing is farther away than infinity.

Well, maybe if you are Stephen Hawking . . . 🙂