50mm 1.8f slightly off focus??



  • funtagraphfuntagraph Posts: 265Member
    edited February 2015
    @heartyfisher I'm sorry but I don't think we are on the same planet of thinking about focus shift. I admit it's not easy for me to communicate subjecting such a complex matter, but I will check for different articles about that issue, maybe I'd understand those better.

    I interprete your eyplanation still as happening in the wrong corner. We're not talkig about the distance of a plane, measured form sensor to center of plane (if that's perpendicular to the sensor) and measured form sensor to the extreme corners of the FoV. As those are farer away, they would be out of focus.

    Anyway, like I said, I'll check for different explanation models and am suspecting I'm misunderstanding you.

    Okay, just found on Photozone.de:

    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.</i>

    A 50 mm is not exactly what I'd call wide or ultrawide.
    Post edited by funtagraph on
  • FlowtographyBerlinFlowtographyBerlin Posts: 477Member
    edited February 2015
    You'll need to expand on your theory there Flow. If METALBLADE focuses at three metres f1.8 he has a DoF of 38cm if he then moves to f8, it is 1.85m - how is focusing going to shift that far?
    No expansion of theory needed, let's focus on the practice :-) Focus shift isn't as bad as the name suggests, but it really depends on what you're shooting. I.e. if you calibrate for f/1.8, your focus plane is shifted at f/8. You notice this e.g, with the 85mm/1.8 when you shoot portraits. Not that the focused plane will be blurred, but your in-focus area is not centered around what you focused on, rather it will be on the edge of your in-focus area. This may matter more or less, depending on what you're shooting, as I said.

    For sure, if you calibrate at f/8 and then shoot a portrait wide-open, your focused point will not in focus, and hence not be "sharp", or as sharp as it gets wide open.
    Post edited by FlowtographyBerlin on
  • HammieHammie Posts: 258Member
    edited February 2015
    I use FoCal. It is software that does all of the thinking for you.

    I have calibrated all of my lenses (35mm, 50mm, and 24-70mm) except the 70-200 with my D750.

    My 35mm f/2 was the most off at -12 or 13. All the other lenses were right around +6-8.
    Post edited by Hammie on
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