Phase Detect AF in the X-T10 - experimental results
DISCLAIMERS AND NOTES
1. The following is my personal interpretation of results from experimentation, and there probably will be some incorrect or incomplete statements. The experiments were performed on a Fujifilm X-T10 with firmware 1.30. The same probably applies to the X70 and X-E2S and to the X-T1 and X-E2 with recent firmware. Changes might occur with future firmware.
2. Any directional words like horizontal, vertical, height, width, up, down, left, right, top, bottom, and sides are referenced to the “normal” camera orientation. When the camera is in portrait orientation, you’ll need to adjust those directions.
The phase detect autofocus (PDAF) in the Fujifilm X-T10 consists of three horizontal rows of sensors. The firmware divides each of these rows into three separate PDAF points in Point AF and into five separate PDAF points in Zone and Wide/Tracking AF.
Each PDAF point is sensitive to edges that are approximately vertical. Angles up to 45 degrees from vertical, or a bit more, are also detected. None of the PDAF points are sensitive to horizontal edges, which implies that none are “cross type.”
An unusual limitation: the current firmware seems to be relatively primitive in that if multiple edges are detected at a single PDAF point, it refuses to choose which edge to use and instead will ignore that point.
The horizontal extents of each PDAF point are exactly as shown in the viewfinder. The vertical extent, however, is a single pixel row across the middle of the point shown in the viewfinder. The gaps between PDAF points that appear in the viewfinder are genuine gaps, ignored by the firmware.
Based on reliable reports (not my own testing), it seems that the phase detect system prefers to lock onto subjects at approximately the current focus distance. This may be especially noticeable when the desired subject is seriously out of focus, with long lenses and/or wide apertures. The contrast detect system, on the other hand, prefers to lock onto nearby subjects.
As far as I can tell, the X-T10 doesn't currently offer true "hybrid AF" where the focus is initially set with phase detect, then contrast detect is used for fine-tuning. It seems that at any given time, it's using either phase detect or contrast detect.
Takeaway: a PDAF point is looking for a single vertical edge. It doesn't like horizontal edges, it doesn't like multiple edges, and it doesn't like edges that are badly out of focus.
INTELLIGENT FACE DETECTION
If Intelligent Face Detection is active, regardless of AF mode, the entire image is continuously searched for faces, even when AF isn’t active. If a face is found, the current AF configuration is ignored and contrast detect AF is forced. The current metering and exposure settings are also ignored.
Takeaway: if you're not using it, you're probably better off leaving Intelligent Face Detection turned off.
In Point AF, the firmware provides three PDAF points on each of the three rows. These are marked by large crosses. If you select an AF point marked with a small cross, no PDAF can be performed and contrast detect AF is used.
Point AF allows you to select five different sizes for the AF point. This is the bounding box for contrast detect AF, and it also expands the PDAF zone to the width of the box.
Fortunately, an enlarged box isn’t nearly as bothered by multiple vertical edges passing through it as I’d expected. It appears to me that perhaps the box is invisibly subdivided (horizontally, of course) into some number of PDAF points with no gap between them. It’s possible that these PDAF points overlap. An alternative deduction would be that the firmware “slides” a PDAF point-sized window across the line, looking for a spot with a single vertical edge. There’s no indication of what part of the PDAF line is being used, so the only way to tell is by judging what’s in focus.
In the event that PDAF is unable to locate a reference edge, the camera falls back on contrast detect AF. As already noted, the contrast detect zone covers the entire box.
The takeaway: PDAF is active on the nine large crosses and not active on the forty smaller crosses. If PDAF fails, contrast detect will be used. A larger box size increases the chance of PDAF locking onto something, but what it locks onto might not be what you wanted, and you might not be able to tell. Perhaps a medium-sized box is the best compromise.
In Zone AF, the firmware provides five PDAF points on each of the three rows. These are marked by large boxes. If you select a zone that includes only large boxes, PDAF is always used and, unlike Point AF, if PDAF fails to find a reference edge, you get !AF (autofocus failure warning); the camera will not switch to contrast detect AF. If you select a zone that includes any small boxes, contrast detect AF is always used, even at PDAF points.
The takeaway: if you want to use only phase detect AF, Zone AF with one of the four zone configurations that don’t include any small boxes is the way.
In Wide/Tracking AF, phase detect AF is ignored and contrast detect is always used. Since contrast detect "hunts", it might be wise to set Release/Focus Priority to Focus in order to assure that pictures aren't taken during the hunting cycles.
In AF-C continuous autofocus, you can select which AF point to start with -- that is, which one will be over your subject when you half-press the shutter button. That AF point is shown in the viewfinder. The lens will "hunt" continuously while the shutter button is depressed.
The takeaway: if you don’t want to use phase detect AF, Wide/Tracking AF is one way. Seriously consider setting Focus Priority.
HIGH-SPEED BURST WITH THREE-DIMENSIONAL TRACKING (SPECIAL CASE)
If you’re using CH high-speed continuous drive along with AF-C continuous autofocus, the rules change a bit. Note: the shutter type needs to be set to MS (mechanical shutter) -- if it’s set to ES or MS+ES, the camera acts as if it were in AF-S and won’t track.
This special case uses phase detect AF exclusively. Accordingly, you’ll have to keep the subject within the bounds of the PDAF zone. The zone is marked in the viewfinder, but that doesn’t show the vertical positions of the three PDAF lines.
Face Detection is disabled because it can force contrast detect.
In Point AF, you can only choose one of the nine PDAF points. You can set the size of the point, but that only controls the width of the PDAF zone. There is no fallback to contrast detect.
Zone AF forces the zone to be fully within the fifteen PDAF points, so the zone size is restricted to either 3x5 or 3x3.
Wide/Tracking AF behaves like Zone AF but with the zone forced to all fifteen PDAF points, and you must pick which of those PDAF points to start with -- that is, which one will be over your subject when you half-press the shutter button.
The takeaway: Wide/Tracking AF might be the best choice in most cases, because you can specify which AF point to start with. Zone AF lets you choose one of the three 3x3 zones as an alternative to the all-fifteen zone, but that’s probably not generally useful. Point AF probably is a poor choice because of the challenge of keeping the subject on the PDAF segment, although there might be unusual situations where it could work.
AUTO MODE (X-T10, X70, AND XE-2S)
If the Auto switch is turned on, the camera overrides the current AF settings. Different scene types result in different AF setting configurations being used. Some use phase detect, some use contrast detect, and some probably use a combination. I haven’t investigated this in detail, and I don’t see much value in investigating it, so I don’t plan to.
In Point AF mode:
- PDAF point (large cross) selected: phase detect or contrast detect, depending on whether or not PDAF can find a suitable vertical edge to focus on. This is what Fujifilm calls "Intelligent Hybrid AF": phase detect if it works, falling back on contrast detect if PDAF fails.
- non-PDAF point (small cross) selected: contrast detect AF.
In Zone AF mode:
- 3x5 or 3x3 zone containing only PDAF points (large boxes): phase detect AF only.
- 5x5 or other zone containing at least one non-PDAF point (small box): contrast detect AF, even at the PDAF points.
In Wide/Tracking AF mode: contrast detect AF.
Override logic 1: if Intelligent Face Detection is on, and it detects a face, the camera will ignore your AF settings and will use contrast detect AF on that face.
Override logic 2: if CH high-speed continuous mode is set along with AF-C continuous focus (requires MS mechanical-only shutter), the camera will force phase detect AF. Wide/Tracking AF, which normally is only contrast detect, becomes very similar to Zone AF. The camera won't allow the AF point or zone to go outside of the phase detect area. Intelligent Face Detection is disabled to prevent that from switching to contrast detect AF.
Override logic 3: if the Auto switch on an X-T10, X-E2S, or X70 is turned on, all of the above is ignored and the camera temporarily sets AF parameters to suit the type of subject it thinks it's dealing with.
The open-source LightZone Project: http://lightzoneproject.org/
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