Before the rush

Before the rush
by evan-pak

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The title pretty much says what I want to ask.

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up vote 3 down vote accepted

Auto-focus always focuses at the aperture wide open to ensure maximum light reaches the sensor. While aperture will impact the depth of field (and thus how far outside the focal plain is acceptably in focus) it will not impact the actual focus plane on most lenses.

Therefore, it should not be necessary to refocus when changing aperture unless you are trying to backfocus or front focus something intentionally. (ie, you want to control what is just barely in focus rather than the central point that is in focus). This is not generally a normal practice though.

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Theoretically, camera always autofocuses at the largest possible aperture, and then right before shutter being released, it closes it down. Therefore, no need to do it, since DOF can only increase.

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I really liked your answer but accepted @aj-henderson 's for pointing out that "While aperture will impact the depth of field ... it will not impact the actual focus plane". Thank you very much for your answer though! – yannisf Nov 9 '13 at 6:49
@yannisf - Please see the links in Michael Nielsen's answer. There are lenses that suffer from focus-shift when aperture changes. – Esa Paulasto Nov 10 '13 at 6:08

Focus shift happened with some of the well known vintage manual lenses (read about them on pentax forums). This was always noted in reviews because then you could avoid focusing wide open and then stopping down if it was a known issue. Nowadays with auto focus, you dont get that option. But you can check the focus using the aperture test button. Modern lenses are designed for the "wide open focus, then stop down" mechanism, so the engineers minimize the drift. Howver, it seems sometimes it goes wrong for them:

Even on pro versions

and renowned Zeiss

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Excellent references! – yannisf Nov 10 '13 at 9:45

It really depends...

With higher F-stop value you will get a deeper depth of field. So, if you foucsed with say F/16, and then you change aperture to a lower value, let's say F/2, the diaphragm opens and your depth of field gets shallower, so you might need to refocus to be sure your subject ist still focused.

However, if you change from a low F-Stop to a higher value, your depth of field gets deeper and you shouldn't need refocusing.

You could check into this as a reference:

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Unless you are focusing while pushing the DOF preview or have a lens with a manual aperture, you are always focusing at max aperture. – AJ Henderson Nov 8 '13 at 18:01

There is something called "focus shift" where what's in focus at one aperture might not be 100% in focus at another aperture.
But it's most likely to occur when you're dealing with fast apertures where very little is in focus to begin with.

This article explains it in detail:

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