Measuring Autofocus Speed

sk66 said:

A real image only exists at the plane of focus (not sensor necessarily); otherwise it is just scattered/mixed light. A virtual image is a portion of that scattered/mixed light that will become the real image when focused.

E.g. at every point on the objective element there is some of the light radiated from every point within the scene... there is a complete image in terms of light/information, but it is not a real image at that point; it is a virtual image.
I.e. an objective element can be any diameter w/o restricting the focal length. And a faster lens is faster because it is stacking more images (light from additional objective element area). But at any other point beside the plane of focus they are virtual images.

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That's how a split prism works as well... the prism has 1/2 that looks left and 1/2 that looks right as the image is being formed (or up/down).

Edit: I probably should have drawn the objective element much larger.

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Ahh I see now thanks. I was thinking of virtual more in terms of 'made up'/artificial but I understand it now thanks.

I am getting a bit lost in the technical details.

Coming back to the question, "AF speed" on its own doesn’t tell you very much. Acquisition, tracking reliability, subject recognition, reacquisition and consistency probably matter more.

Thinking back to my Canon 1DXM2, its on-sensor dual pixel AF tracking in live view was already better than the dedicated DSLR AF system. And the R1 is streets ahead.

I still can't think of a situation where the 1DXM2 AF would be better, except when you need the speedlight AF assist pattern in complete darkness (which for me is never).

TimHughes said:

I am getting a bit lost in the technical details.

Coming back to the question, "AF speed" on its own doesn’t tell you very much. Acquisition, tracking reliability, subject recognition, reacquisition and consistency probably matter more.

Thinking back to my Canon 1DXM2, its on-sensor dual pixel AF tracking in live view was already better than the dedicated DSLR AF system. And the R1 is streets ahead.

I still can't think of a situation where the 1DXM2 AF would be better, except when you need the speedlight AF assist pattern in complete darkness (which for me is never).

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I posted the other day that it's probably low-light af where there could be further improvements, but I never really thought of the opposite. What about high contrast situations, such as very bright car headlights disrupting the AF system, is this an area were AF currently struggles and could be improved on?

gman said:

I posted the other day that it's probably low-light af where there could be further improvements, but I never really thought of the opposite. What about high contrast situations, such as very bright car headlights disrupting the AF system, is this an area were AF currently struggles and could be improved on?

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High contrast is what you want, but I get your point re headlights etc. AF also struggles with reflective surfaces sometimes.

Another area where AF could be improved would be to have an option to ignore fencing, when shooting motorsports through catch fencing it will still sometimes focus on the fencing even with the focus range set from 3m to infinity, circuits are putting a secondary fence 2-3m back from the catch fencing meaning that it’s still within the focus zone.

I wish Sony would let you set the focus zone in camera so in situations like this I could set it from 5m to infinity or 10m to infinity so it would completely ignore the catch fencing. I’m sure I had a camera once that I could do this with, but I can’t for the life of me remember which

Edit: maybe it was the Sony A77 II.

Edit 2: it was indeed the A77 II. I wonder why we don’t have this option now

gman said:

I posted the other day that it's probably low-light af where there could be further improvements, but I never really thought of the opposite. What about high contrast situations, such as very bright car headlights disrupting the AF system, is this an area were AF currently struggles and could be improved on?

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These are some of the situations the Z9 warns about:

Caution: 3D-Tracking and Subject Tracking
The camera may be unable to track subjects that:
are similar in color, brightness, or pattern to the background,
change visibly in size, color, or brightness,
are too large or too small,
are too bright or too dark,
move quickly, or
are obscured by other objects or leave the frame.

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TimHughes said:

I still can't think of a situation where the 1DXM2 AF would be better

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Back before the D850/D5 Nikon's "blocked shot" was exactly that and the tracking would stay with the subject all day unless it was blocked/lost (left the frame). With the D5/D850 it became more of a "time out" setting and really has nothing to do with if the subject is blocked/lost even though they left the nomenclature the same... if I select too small of an AF area and the subject leaves that area (but still not "lost") the system will quickly time out and revert to the original restricted area/initial point... the Z9 behaves the same; I much prefer the earlier behavior.

When the subject is smaller/faster I find the Z9 locks onto the BG far more often; and it refuses to let it go even with subject detection/limiting active; so I have to use focus recall to pull it back and start over. I also find that the AF is more likely to blow right past the subject if it is not rather large in the frame when starting from OOF; so pre-focus and zone focus type tricks become more important (delaying AF activation until you can make out the subject reasonably well). I also find things like DX mode/TC/zoom for a tighter composition helps the AF (larger subject in AF/viewfinder feed), but equally hurts my ability to acquire/track (subject is effectively faster/more erratic). And the BG doesn't have to be "busy" in the sense that it would record very OOF if the camera focuses on the subject... just something other than blue sky (blank BG).

With mirrorless I still find that when the subject/situation is fairly difficult it is necessary to give the AF less free range/control/points... that's not any worse than the DSLRs, but it's not any better either.

Now, when the situation is relatively easy I can put the AF into full auto and expect it to do quite well... and I guess that is a notable improvement over the DSLR; it just doesn't help me much. And with an easy enough subject and subject recognition I can expect it to find the subject and then step down the focus appropriately for me (find/focus on the eye); but I see that as more of a convenience.