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Thread: Focus shift

  1. #1
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    Focus shift

    All,

    I am hoping to achieve a conclusive understanding of focus shift. I have never experienced the problem firsthand, but see that the topic comes up fairly common in posts (e.g., regarding the Lux 35mm). I always thought it mean that, when focusing a lens at a certain f-stop (e.g., 4), the focus would be off, being either front or back focused. But, when recently reading an explanation on digilloyd, I think I was wrong. It seems that focus shift "is*a displacement of the sharp plane of focus when the lens is focused wide open, but the image is made with the lens stopped down." Such a problem is significantly better than what I had thought, as you can compensate for it simply by focusing/refocusing at the aperature at which you plan to shoot. Is that correct? Thus, if I had a Lux 35mm*with focus shift, I could focus and shoot at**f/1.4, focus and shoot at f/2, focus at f/2.8 to shoot at f/2.8, etc.

    Thanks,

    R

  2. #2
    Senior Member thrice's Avatar
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    Re: Focus shift

    Not with a rangefinder lens on a camera without live view, since you aren't looking through the lens are you.

    Focus shift is when the plane of focus shifts (usually further away) as the lens is stopped down. That is why you see mention of "optimised for 1.5" when referring to the Zeiss ZM C-Sonnar 50/1.5, meaning it focuses perfectly at f/1.5 and the plane of focus will shift further away as the lens is stopped down. Whereas the earlier "optimised for 2.8" version would focus perfectly and f/2.8, the plane of focus at larger apertures would be closer and once again, further at smaller apertures.

    All this is moot if you're using a live view (Nex/4:3) camera, just make sure you focus stopped down (which can be a problem at times).

  3. #3
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    Re: Focus shift

    Thrice:

    Great thanks for the clarification. I guess I had it right before and then became confused by the digilloyd explanation (for DSLRs etc.).

    Thanks for setting things straight ...

    R

  4. #4
    Senior Member thrice's Avatar
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    Re: Focus shift

    No problem

    An easy way to see it and why it is a problem with rangefinders:
    Focus shift is a function of the optics with relation to aperture.
    Focus on a rangefinder is mechanical and independent of aperture.

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