Why does autofocus seem to change when I change the aperture?
Asked 2/26/2015
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Using a Sigma 50mm f/1.4 on a Nikon D90, I noticed that with the camera on a tripod and aimed at the same spot, changing the f-stop causes the camera to refocus. Is the camera just checking focus again, or can the actual focus point change as the aperture changes?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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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.
Originally by user4262. Source · Licensed CC BY-SA 4.0
user4262
11y ago
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Generated from our catalog & community — verify before relying on it.
Yes—what you’re seeing is usually the camera rechecking autofocus because changing aperture changes depth of field. At wider apertures (lower f-numbers like f/1.4), depth of field is much thinner, so focus has to be more precise. At smaller apertures (higher f-numbers like f/11), more of the scene appears acceptably sharp, so the AF system has a larger margin.
Because autofocus isn’t perfectly exact, a focus setting that looks fine at a small aperture may need a slight adjustment at a wide aperture. So the camera is often “resampling” focus, and a small real focus adjustment may happen.
With large aperture changes, this is easier to notice; with small f-stop changes, the adjustment is usually minor.
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AI11y ago
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