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I have a prime Sigma f1.4 50mm lens on my Nikon D90 in P mode and focus mode on a set point of the field and notice that when I am using a tripod and change the f-stop that the camera changes the autofocus. Is it just resampling and presumably getting the same focus or does the focus change from one f-stop to the next?

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    \$\begingroup\$ Focus shift is a common phenomenon in older lenses — not just greater DoF, but actually different focus. I don't know, however, if that's the case with this lens, and/or if the D90 is compensating for it with this, or if something else is going on — so I'm not answering, but I hope answers will address this possibility. \$\endgroup\$
    – mattdm
    Feb 26, 2015 at 21:27
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    \$\begingroup\$ the problem i have with current answers is that SLRs focus with the aperture at its widest. for this reason, a change in aperture can't have any influence on autofocus. \$\endgroup\$
    – ths
    Feb 26, 2015 at 23:39
  • \$\begingroup\$ I have the same thoughtbas @ths. If it focuses using wide open and then applies a per-apeture correction, it can recompute with the same reading. As for mattdm's point, how come we never worried about it in the pre-auto days, where we manually focused with the apeture wide open? \$\endgroup\$
    – JDługosz
    Feb 27, 2015 at 3:56
  • \$\begingroup\$ Also, what mode are you using? P-mode? Program mode? \$\endgroup\$
    – Charles Lee Roberts
    Feb 27, 2015 at 8:49
  • \$\begingroup\$ What is the focus mode set to? If it's set to automatically choose appropriate focal points, then maybe its choice is being affected by depth of field. But even then, as @ths says, your lens is wide open when focusing. \$\endgroup\$
    – Myridium
    Feb 27, 2015 at 9:51

2 Answers 2

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does the focus change from one f-stop to the next

Depth of field changes when you change the aperture, so the camera is probably adjusting to ensure that the object at the selected AF point remains in focus. This is particularly important when you increase the aperture (i.e. move to a lower f-number), giving you a narrower depth of field. AF systems aren't perfect, and what the system judged to be acceptably in focus at small aperture may not be acceptable a larger aperture. So yes, the camera is probably "resampling," but it's probably doing so because a small adjustment may be necessary.

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If you think about a bigger change in depth of field, like f1.4 to f11, it is a bit more obvious: the autofocus has more 'probably sharp' points to pin down, therefore he needs to recalculate. If you have a smaller change in f-stop, the same happens, but on a much smaller basis and is barely noticeable.

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    \$\begingroup\$ But since modern cameras focus wide-open, this seems unlikely. \$\endgroup\$
    – mattdm
    Jun 27, 2015 at 16:28
  • \$\begingroup\$ @mattdm Are you sure about that? Cant recall any of my cameras changing aperture while focussing. \$\endgroup\$
    – appleseedexm
    Jun 27, 2015 at 16:30
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    \$\begingroup\$ They don't change while focusing — they stay wide open until right before the shutter is released. \$\endgroup\$
    – mattdm
    Jun 27, 2015 at 16:31
  • \$\begingroup\$ That is definitely not the case, especially with the camera and lens mentioned. The aperture iris does not change in manual or AP mode. \$\endgroup\$
    – appleseedexm
    Jun 27, 2015 at 16:45
  • \$\begingroup\$ ... until you press the shutter, right? \$\endgroup\$
    – mattdm
    Jun 27, 2015 at 16:57

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