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What is the focal plane for an off centre AF point? Is it the same focal plane as centre AF point?

Can off-centre AF point be as sharp focus as the centre AF point? If not, will the sensitivity of an off-centre AF point vary according to distance from the centre AF point?

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    "As sharp" and "sensitivity" are two totally different things.
    – Michael C
    Apr 5, 2016 at 7:26

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There are two answers to that question, on dSLRs the focal plane and focus sensor can be a bit off (misaligned), hence why you may need micro-adjustments - usually however it will not be misaligned by the z axis so it shouldn’t be based on AF points, but in theory it could be.

In mirrorless cameras, the focus is done on the images sensor so you will not have any misalignments.

However, not all AF points are created equal and off centre focus points are usually of lower quality (meaning often non cross type and almost always non f/2.8). High end dSLRs do have many cross-type off centre, lower end have fewer - for example Canon 7D mk. II has 65 autofocus points, but they are all of the cross-type, meanwhile the Canon 1200D offers 9 AF points, with just the centre point being cross-type). The centre focus is always a cross type and most often also f/2.8 optimised.

So off centre AF point will typically be less accurate than the centre AF point. And with phase AF, the closer to the edge of the sensor/frame it is, the more difficult it is to get proper focus – this is why you don’t have the full coverage of AF points, they are usually clustered around the centre of the frame.

However, by using hybrid focus* (both phase and contrast detection) Sony have in their mirrorless E-mount made it possible to have full coverage by making the edge AF points contrast detection only (like A6300 and A7R II) and the centre a combination.

Mentioning field curvature has been suggested, however field curvature is very lens dependant, I have two macro lenses, both tack sharp corner to corner - and a few primes with at least fair corner to corner sharpness. So it's certainly not all lenses that suffer visibly from this.

However, many lenses (especially zooms) have visible corner softness at certain apertures, these lenses will naturally affect off centre AF when the sharpness and contrast is failing to provide the autofocus with the informations needed.

Both contrast and phase AF will be affected by field curvature when the lenses performance drops. Meaning AF performance is not lens independent.

  • Not to be confused with Sony's A-mount hybrid AF system in A99M2, this is phase AF only, however combined on sensor phase AF and a dedicated tradition phase detection AF sensor. Sony uses the same terms incl. 4D auto focus, but the two systems works differently.
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  • Upvoting and suggesting mentioning field curvature. Apr 5, 2016 at 8:59
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    It's worth noting that many modern DSLR autofocus systems have cross-type focus points off-center too.
    – mattdm
    Apr 5, 2016 at 10:20
  • And also worth noting that not all modern DSLR AF systems are sensitive to f/2.8, even with a center cross type point. Many are limited to f/5.6. cough Nikon cough. Many Canon cameras have dual cross type points at the center. The vertical and horizontal lines are usually sensitive at f/5.6 or even f/8, while the two diagonal lines are the ones sensitive to f/2.8.
    – Michael C
    Apr 6, 2016 at 10:07
  • Cross-type AF points are simply two conventional AF points overlaid on top of one another. Typically one is vertically sensitive and the other is horizontally sensitive. Dual cross-type AF points are four conventional AF points: one vertical, one horizontal, one diagonal from lower left to upper right, and the other diagonal from upper left to lower right. In the typical Canon implementation, the vertical one is sensitive to apertures as narrow as f/8, the horizontal one is sensitive to f/5.6, and the diagonal are only sensitive to f/2.8 and wider apertures.
    – Michael C
    Nov 19, 2016 at 5:18

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