(Another) AF Issue thread... X-T20

Started 7 months ago | Discussions thread
OP Guyps Forum Member • Posts: 92
Re: Potpourri

Wow. Great post! much appreciated

Doug Pardee wrote:

I won't pretend to know the answer(s) for your situation. Here are some general thoughts.

1. Mirrorless AF is still behind DSLR AF, no matter what some folks maintain. It's good enough for many purposes -- even my X-T10's AF is certainly good enough for my purposes -- but when the going gets rough, DSLR AF will generally beat out mirrorless AF. Mirrorless wins on avoiding front/back focus due to AF sensor misalignment and on coverage of a wider part of the scene; DSLR wins on locating and tracking a subject.

2. The only current mirrorless camera with cross-type on-sensor AF points is the Olympus EM-1 mk II. All others are sensitive to vertical lines only (up to maybe 45 degrees each way for Fuji). In at least one picture you seemed to be focusing on the mouth, which is a horizontal line that mirrorless AF points will be blind to. A simple test to see if this is part of your problem: take some pictures in portrait mode, so that the AF sensors are sensitive to horizontal lines instead of vertical.

I didn't realise that.

3. Because mirrorless cameras use the main sensor for AF, they're limited to reading out the AF sensors once per scan. For most mirrorless, that's about 20 times per second (the Sony A9 is the big exception). I don't think this is relevant to your situation, though.

4. Because mirrorless cameras use the main sensor for AF, they're pretty much blind to infrared. Near-IR focus-assist beams aren't very effective. This doesn't seem relevant to your situation.

5. Fujifilm puts its on-sensor AF detectors only at green pixels. It essentially sees green and black. A pink face with black shadows is going to be a more challenging target than most. I don't know how other mirrorless brands do their sensors.

6. Fujifilm's single-point AF system will fall back to contrast-detect if the phase-detect fails to find anything to focus on. This will be slower, but you won't get the !AF (AF failed) indication unless you're pointing at a blank wall or something. Zone and Wide/Tracking behave differently.

7. Fujifilm's AF-C operates while stopped down to the shooting aperture. If you're shooting at f/9 or above, the AF sensors are blind so the camera goes to strictly contrast-detect AF. Since your examples are at f/2, this isn't relevant to those particular cases.

Maybe not relevant in this case but that is good to know that if I increase the F number the reliability of the AF will suffer. This is quite a big difference as my DSLR centre point that I used 95% of the time was always f2.8 (I didn't have any slower lenses). Just had a play about and noticed how much it hunts at F8 compared to F2. Interesting!

8. Fujifilm's Zone mode doesn't let you select the specific AF point to start tracking from. Wide/Tracking mode does, but it uses contrast-detect AF unless you're in CH drive mode (and at f/8 or wider and in good enough light for phase-detect to work).

9. Contrary to Canon terminology, Fujifilm's idea of "release priority" means "wait for focus, and if it fails, go ahead and snap." Fuji cameras should never take a picture while focusing is still in progress, which Canon's AF will do (except for the second and subsequent shot in a burst). This means that the actual shutter release can occur some time after you've pushed the button.

Yes, Unfortunately this is what seems to happen.

10. After you press the shutter release, a mirrorless camera has to close the first curtain and reset the sensor before it's ready to take the picture. A DSLR has to raise the mirrors before it can take the picture. Which one is faster is a question mark. Both also need to stop the lens down, although for a Fuji in AF-C, the lens is already stopped down.

11. For single snaps, you might find that using AF-S along with the "mash the shutter release" technique works better on a Fuji. The shutter will be released at the instant that focus is obtained.

Had a go at that lastnight, does feel a bit awkward after years of half depressing until the focus is obtained.

Added: 12: If you're using back-button focus and have it set for AF-C, be sure you keep it held down. On a Canon you can just tap the back-button to get AF-S type behavior, but a Fuji will simply abandon the AF operation if you let up. You can't reliably get AF-S behavior out of back-button AF-C on the Fuji. Similarly, if you're using back-button AF-S on Fuji you need to keep the button held down until focus is achieved. This (obviously?) isn't an issue if you're using the shutter release to run the AF.

I did try back button focus but due to the small size of the camera and the layout of the buttons it was a bit awkward. I just stick to the shutter release.

13: There has been at least one report that the X-T20 sometimes gives the "green box" even though it didn't even try to focus at all, when the "instant preview" mode was active that plays back each image right after you take it, unless the beep was turned on. This report was for an earlier firmware version, and may have been corrected.

I have the image preview off and beep off normally, I also read about the green box showing/shutter releasing without acquiring focus on some cameras that could be detected by a lack of the beep. I have temporarily turned the annoying beep back on but obviously this only works in AF-S mode which I don't really have much issue with.

[sigh. There was one other point I was going to add, and I forgot what it was.]

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Co-founder of the open-source LightZone Project: http://lightzoneproject.org/

Didn't get much time last night but did pop off a few shots.

AF-S, Single point on her right eye, 1/180, Auto ISO max 6400 (camera selected 2000), No Flash

AF-S, Single point on her right eye, 1/180, ISO200, Godox 350 Flash bounced off ceiling

Now I know she isn't really moving away/towards me so there is no real need to use AF-C but I flicked it over to AF-C mode. Personally from previous experience I would have expected the AF-C to focus as well as in AF-S. It just wouldn't need to adjust much. Is this a wrong presumption?

Whilst the photo when viewed on here, or on Instagram via a phone isn't bad, if you have this as full computer screen or zoom in it is clearly not as sharp as the previous. Maybe I am a bit too picky and should stop looking at full screen photos! Tried a half depress and also the button mash technique. Similar results.

AF-C, Single point on her right eye, ISO200, Godox 350 flash bounced off ceiling, AF-C custom setting I think was on 1.

I'm at the Man City vs Bournemouth game today so will try to experiment more tomorrow with moving subjects if I get time. Also the 3 year old has a mini ATV from Santa so that will give me opportunity to test the AF-C success rate outdoors again over the festive period and perhaps play with the Zone focussing rather than always using single point.

One interesting thing on the Fuji AF site is the use of AF-S + zone mode for slowly moving subjects, I may have to try this with the button mashing technique.

Thanks again for the advice! I really want to get to grips with this system

 Guyps's gear list:Guyps's gear list
Fujifilm X-T20 Canon EF 85mm f/1.8 USM Canon EF-S 17-55mm f/2.8 IS USM Canon EF 70-300mm f/4-5.6 IS USM Fujifilm XF 18-55mm F2.8-4 R LM OIS +1 more
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