Focus shift - True?

Discussion in 'Lens Lust' started by Fabrian, Jul 29, 2008.

  1. As I've been reading an exhaustive amount of review, photozone makes reference on a lot of Nikkor tests that focus shift a different apertures is common for Nikkor lenses. Is this true? I'd hate to think that it is for most lenses and it's something that people just accept as normal.
     
  2. mikechang

    mikechang

    27
    Jun 9, 2008
    San Diego
    Can you explain what do you mean by "Focus Shift a different apertures" ?
    Do you mean front focus / rear focus??

    If so, I heard more of these problems occurs on Canon DSLR/Lens.
    I seldom hear Nikon user complain about their body or lens has front/rear focus problem.
    The most common one I see is the Sigma 30/1.4 that has this problem.
     
  3. No, not front/back focus.

    For example, go here and about 1/3 of the way down under MTF (resolution) it's mentioned and I've seen this mentioned for several other Nikkor lenses.

    Taken from photozone's review of the 16-85:

    "MTF (resolution)
    The resolution characteristic of the Nikkor is impressive. Unsurprisingly the "weakest" performance is at 16mm - the center quality is about as good as it gets whereas the borders and extreme corners are on a very good level within the focus zone. However, the lens suffers from some field curvature in the extreme corners and there's also a bit of a focus shift when stopping down (residual spherical aberrations).
    "

    Taken from Photozone's FAQ:

    "Q: What are residual spherical aberrations ?
    This defect is actually a quite evil one. Residual spherical aberrations are focus shifts when stopping down e.g. if the lens has a max. aperture of f/2.8 and you stop down to f/8 the focus distance shifts from 0.80cm to 0.90cm. Remember that you focus at working aperture (f/2.8 in the example above). At long focus distances the defect will be compensated by the increased depth-of-field but expect focus errors at closer focus distances. Some ultra-wide and wide angle (<35mm) lenses suffer from this.
    "

    This has been well documented on the Canon 50 f/1.2. Say your camera and subject are fixed and you're at f/2.8. You focus dead on and take the shot - great. Stop down to f/4, for example, and now your DoF is in front, or behind the subject. That's what I'm talking about when referring to "focus shift".
     
  4. rvink

    rvink

    Mar 21, 2006
    New Zealand
    I suspect the increased DOF as you stop down more than covers any focus shift on most lenses. I wouldn't worry about it.
     
  5. This is actually a bit more complicated than just spherical aberration, it is really more spherochromatic considering we are concern near the center of the field of view and over the spectrum. This can get a bit technical, so let me just give you an example instead. Consider that we have a typical fixed-focus 100mm lens having a Double Gauss form with 6 elements in 4 groups. I quickly modeled such a lens and at F/3 the rms spot diameter is about 16 um (combining all of the colors) with a back focal length of 57.3097mm. If you now stop the lens down to F/6, the rms spot diameter decreases to < 5 um with the back focal length (BFL) at 57.4553mm. So, the act of stopping the lens down from F/3 to F/6 results in a change in focus of 0.1457mm, which is rather significant. For this lens, the BFL will remain almost constant at the value for F/6 as the lens is stopped down further. Between F/6 and F/3, the BFL will change.

    This behavior is very specific to each lens design. Lenses that are very fast (low F/#) often shown this behavior. There are other implications, but I think this answers your question.

    Regards,
     
  6. cotdt

    cotdt

    Jul 14, 2007
    Bay Area, USA
    But BarryD3, shouldn't the chip in the lens correct for any focus shift?
     
  7. gd1418

    gd1418

    195
    Feb 3, 2008
    Gurgaon, India
    I think this is typical of spherical and aspherical lenses. SIGMA came out with APO lenses that correct this problem.
     
  8. cotdt

    cotdt

    Jul 14, 2007
    Bay Area, USA
    APO lenses are basically like Nikon ED lenses. It doesn't completely get rid of abberations.
     
  9. Leif

    Leif

    Feb 12, 2006
    England
    I don't think so. APO tends to apply to telephoto lenses, and means that the lens is better corrected for spherical and chromatic aberrations (corrected for 3 colours for example).

    In fact it is typical of very fast lenses, which are not fully corrected for spherical aberration when wide open. The result is that the outer parts of the lens bring the light to a different focus to the inner parts. Wide open the outer parts dominate (more area), but the image is a bit soft. Stop the lens down, and you are only using the centre part. Hence the focus appears to shift.
     
  10. Well, that was explained better than mine - I think I'm in love :Love: LOL!

    Thanks for that very specific explaination. One of the reasons I'm moving to Nikon (amongst other things) is so I can have lens adjustment (D300 - less than half the price of a 1D MKIII) since I'm pretty critical of focus accuracy. I wonder if I gave myself a false sense of security since this behavior is seen only on 1 known modern Canon lens AFAIK.
     
  11. Hi,

    I don't understand your question. What do you mean by "chip" please?

    Thanks,
     
  12. mdg137

    mdg137

    309
    Aug 7, 2007
    Michigan, US
    I think thats largely true-- the only time Ive ever found focus shift to be a problem was with a Leica Noctilux.
     
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