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Fujifilm's new camera uses contrast-detect autofocus, and the current Internet hand-wringing is over whether this is a horrible critical flaw or not. As often happens in such discussion, a bit of street wisdom has arisen: it's better to just push the shutter the whole way. For example, this blog comment:

AF speed on the XPro isn’t that slow, the problem is that most people use it as they would an SLR/DSLR for focusing, i.e half press the shutter and wait for AF confirmation then press the shutter all the way, in most cases with the XPro and the X100 you don’t need to do this.

And there's at least one thread on DPReview, but as characteristic for such discussions, there's a lot more smoke than fire.

The X-Pro 1 manual says nothing about this, and in fact says:

S (single AF): Focus locks while the shutter button is pressed halfway. Choose for stationary subjects.

In my experience, if the subject is stationary, the half-press method works as it always does, and if it's moving the full-press method isn't as good as using AF-C (continuous autofocus). I've used plenty of point-and-shoot cameras as well, which of course use contrast-based AF, and the half-press focus lock has always served me well.

But then, a lot of people are saying this is a big deal. Is there any reasonable basis, or is it just wishful thinking?

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I'm not sure about whether it's reasonable, but I can at least guess at what point they're after: the CDAF uses the main sensor, so focusing heats up the sensor (at least a little). The less time it's active, the less noise you should see. Personally, I doubt it makes a visible difference, but that seems to me the main thing you'd hope for. –  Jerry Coffin Apr 26 '12 at 16:14
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This makes no sense at least on the X100. Did not every see an X-Pro1 yet. –  Zak Apr 26 '12 at 17:17
    
Your experience will tell. I was used to AF-C being the most accurate method to focus for both my X100 and X-Pro1, but it sucks battery. I've been practicing the "press all the way" method in AF-S since I heard about it 2 weeks ago, and indeed, it... works, though I would only use it for those situations when I don't have the time to do the usual half-press. –  sebastien.b Apr 26 '12 at 18:00
    
@sebastien.b — yeah, the claim seems to be not just that going straight to full press often works (which wouldn't be too surprising with a modern system), but that half press is somehow doing it wrong — that it makes things worse. –  mattdm Apr 26 '12 at 18:29
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Is the expectation that C-AF will perform as well as the PD-AF of a DSLR? If so, I think the expectation is what is flawed. You can make contrast based AF work pretty well, but no matter what if focus is off, a certain amount of hunting is required, even with very advanced software driving it. The advantage of phase-shift detection AF is that usually (there are exceptions) the AF detection strips intrinsically know how far off AF is automatically, and correcting the discrepancy is usually a one-step process. –  jrista Apr 26 '12 at 18:34

2 Answers 2

up vote 2 down vote accepted

No, it is not. There is nothing magical about not waiting at the halfway point.

What you read is silly, as if waiting would make the focus take longer or something. It does not work that way. You can press all the way as fast as you want and you'll get a shot in focus or not. The longer you wait at the half-press (in AF-S) mode or before fully pressing (in AF-C mode), the higher the probability the shot will be focused. Waiting too long might make you miss action but it won't make the camera miss focus.

Note, having an X-Pro1 one in hand and, for all I tried, I cannot see how not waiting at the half-press can improve focus.

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Based on the manual, here is what I understand about the X Pro-1 AF:

  1. The AF system is not particularly capable with moving subjects, and is not very capable with fast moving subjects period.

    • This is not surprising, since it is contrast-detection AF

    • See Pg. 44

  2. The AF system does perform AF when the shutter button is half-pressed, locks focus and exposure once it is achieved, and keeps it locked so long as the shutter remains half-pressed:

    • See Pg. 43

    • 1 Focus: Position the subject in the focus frame and press the shutter button half-way to lock focus and exposure. Focus and exposure will remain locked while the shutter button is pressed half way (AF/AE lock).

  3. For moving subjects, I believe the only really viable option is to use AF Mode MULTI and AF-C

    • If you use a single focus point, and the subject moves away from it, contrast detection will either focus on something else, or not focus at all
    • Use of both MULTI point and continuous focus would be necessary to focus a moving subject
    • Fast moving subjects (and fast seems potentially subjective here...I would figure anything faster than a crawling baby is probably what this means) are unlikely to focus regardless.

Personally, I believe one of the key battlegrounds where supremacy between DSLR and Mirrorless will be decided over the next few years is the AF performance. Contrast-detection based autofocus has some key intrinsic limitations, despite some of its benefits. It will take some pretty astounding software to drive a CD-AF system well in general, and even with amazing software there are going to be areas where it suffers. Namely, objects in motion/subject tracking and low-light AF. DSLR's still reign supreme when it comes to AF, and may always reign supreme, despite some of the inherent weaknesses of phase-shift detection AF (i.e. AF strips are usually designed to operate at specific apertures, below which they no longer have enough light to function...often limiting AF to apertures at least as wide as f/5.6.)

If someone figures out how to embed a PSD-AF system in a mirrorless camera, then mirrorless cameras will gain in an area where they are inherently very weak (by design, since the light through the lens passes on to the sensor without obstruction). I've never had a particularly high opinion of contrast detection AF systems, even the ones that are very good, because the limitations tend to outnumber and outweigh the benefits, where as PSD-AF has few limitations and so many benefits. I wouldn't get hopes up for DSLR-level AF performance in mirrorless cameras until someone announces something incredibly innovative.

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They already did. Nikon 1 series mirrorless cameras use Phase-Detect AF embedded in the sensor. This was first use in the compact Fuji F300 EXR and Z800 EXR which use CCDs. Somehow, Fuji lost that ability when they went to CMOS sensors while Nikon 1 cameras actually use CMOS sensors. I can attest those systems are all quite fast too! –  Itai May 26 '12 at 20:30
    
Looks like Funji's focal-plane phase detect (FPPD) AF is done by masking off part of each pixel in a line or diagonal, and making a microlens that only directs light from one side of the lens or the other. Interesting that it works that way. If thats all it takes, and if that kind of FPPD-AF is as fast and accurate with a larger sensor as a dedicated AF unit, then the primary weakness I see with Mirrorless might be a thing of the past... –  jrista May 27 '12 at 3:39

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