Understanding Focus Shift

Started Oct 10, 2012 | Discussions thread
Marianne Oelund
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In reply to Teak, Oct 11, 2012

Teak wrote:

I am trying to understand the whole issue of focus shift especially as it pertains to 28 mm 1.8g.

I was over at the diglloyd site and I quote below

Maybe my technique is wrong however, if I frame at f 2, I shoot at f2. Why would you frame at one aperture and shoot at another?

Inquiring minds want to know. Thanks all.

This is an extensive and complex subject.  That fact too often results in misunderstandings, misstatements and outright errors.

The first concept you need to understand is spherical aberration, or SA.  Although this fundamental type of lens aberration can be well controlled, at least in moderate-speed lenses, designers often under-correct it, in order to produce pleasing background bokeh.  In the case of fast lenses, especially f/1.8 to f/1.2, it's quite difficult to eliminate and typically is significant.  Spherical aberration is the result of the light rays which pass through the perimeter portions of the lens, becoming focused in front of the point where the central rays are focused.

Fundamental point:  Fast lenses used at wide apertures do not have a single plane of sharp focus.  That is, "precise focus" isn't achievable and different users may even tend to select different focus settings when manually focusing fast lenses.  Generally, we prefer focus settings where most of the rays come into acceptable focus, which will end up being different than where the central rays are focused, because the peripheral arrays account for most of the light passed by the lens.

With such a focus setting, if we then stop the lens down a couple of stops without re-adjusting focus, we are left with just those mis-focused central rays, resulting in an out-of-focus photo (or one where a more distant portion of subject is in focus, than we wanted).  This is especially a problem when the increase in DOF from stopping down, isn't sufficient to cover the focus shift.

Manual Focusing and Modern Focus Screens

Viewfinders on modern cameras do not show us the image that will be obtained, if the lens is much faster than f/2.8 and we plan to shoot at f/2 or faster.  If we intend to take the photo at f/2.8, there will be no problem:  What you saw in the viewfinder is what you end up with in your image.  To manually focus for exposures at f/2 or faster, only Live View will show the true focus.

Autofocus

If you thought that the f/2.8 limitation of viewscreens was serious, get ready for this:  Nikon AF only uses the f/5.6 portion of the lens exit pupil.  The fact that the lens is held wide open while AF is finding focus, doesn't help at all, because the AF optics mask out all of the peripheral light.

So, how can AF focus a lens that will take an exposure at wide apertures like f/2.8 to f/1.4?  It cheats.  There are compensation tables built into the camera and/or lens CPU, which allow AF to estimate where best focus will be at wide apertures, even though it isn't seeing the wide-aperture image while focusing the lens.  The success of this approach is down to how well the lens is characterized, and also how well the individual production lenses follow the expected SA behavior.  As you can imagine, there is quite a bit of room for errors to creep in.

Caution for Live View Manual Focusing

To avoid unpleasant surprises when shooting at narrow apertures, be sure to stop down to your intended shooting aperture to check focus.  Although Live View is the only way to manually focus accurately for wide-aperture photos, it may mislead you for narrow-aperture photos if you don't do the stop-down check.

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- Marianne
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