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How do the following compare in terms of autofocus accuracy for night photography:

  1. Mirrorless cameras like the Sony NEX-5R, in hybrid autofocus mode.

  2. The NEX-5R, in contrast-only autofocus mode.

  3. A typical Nikon or Canon SLR in live view mode.

  4. The SLR when using the OVF.

Note that this question is only about accuracy (what percentage of photos are mis-focused?), not the autofocus speed (I can certainly wait for a second for the camera to focus). In other words, I want to avoid situations like Is it normal for autofocus to produce blurry photos under low light? .

Am I correct that the order is (4) > (1) > (2) > (3), where > means "is more accurate than"? Or, to put it differently, phase-detection > hybrid > contrast-only?

Am I further correct in assuming that anything other than a contrast-only system works well most of the time (say, produces correctly focused photos 99% of the time)?

As I said, this question is about night photography (I have had no issues with mis-focus during daytime, with any of my cameras). It's also about a static scene like a landscape, and not, say, sports (which is why I specifically excluded speed of focus from this question). The scene does have a decent amount of contrast (bright streetlights and dark fields), unlike, say, astrophotography.

When I said SLR, I mean typical digital SLRs exclude specific brands, and Sony SLTs from this discussion, so that we discuss the big picture (pun not intended) and not get side-tracked into discussing the pros and cons of each of these camera systems.

Similarly, when I said mirrorless camera, let's please discuss only the NEX, again to keep the discussion focused (pun not intended) and not get into Sony vs Olympus vs Panasonic minuteae.

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1 Answer 1

up vote 6 down vote accepted

In general, contrast-based autofocus systems as found in mirrorless cameras and DSLRs in live view are identical, and have the potential to be the most accurate.

Phase detection tells the system which direction to go, though, so it can be very fast — contrast-based systems have to seek around to find out. The main downside comes from using a separate sensor, which means that there's problems with alignment, and even when alignment is adjusted perfectly, problems with different light paths. Contrast-detection reads from the main sensor, and therefore avoids these problems.

The accuracy of contrast-detection AF comes down to a) whether you're actually focusing on exactly what you meant to and b) when the system decides it's spent enough time and gives up.

In low light, you'll likely run into both of these, as good contrast might be hard to come by in the scene. It's important to recognize that this means good contrast in at your desired focus point — the pools of light and shadow from streetlights you mention may be completely irrelevant if you didn't want to focus on one of those edges.

As for hybrid AF, think of it as contrast-based plus hints. Ideally, it should seek to the perfectly accurate focus in the end, but again may give up on the interest of getting you to "good enough" in a reasonable time.

So, going back to your symbols, it's roughly

(1 = 2 = 3) > 4

assuming enough contrast to work with and that the system doesn't give up.

That's the theory, by the way, behind this method of micro-adjusting phase-detect autofocus systems.

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Thanks, Matt. I guess I run into issues with the contrast-based autofocus system hunting for a while, then deciding it has spent enough time and giving up. So, maybe phase-detect autofocus will work better for me because it's faster. BTW, I do have good contrast at my desired focal point (the entire scene is farther than the hyperfocal distance, so focusing on any part of it will do). –  Kartick Vaddadi Feb 16 at 16:09
    
Just because all of your scene is beyond the hyperfocal distance doesn't mean any focus distance beyond the hyperfocal distance will yield equally pleasing results. There is still only a single distance of sharpest focus and everything closer or further is out of focus to one degree or the other. The edge of the DoF is not a hard line beyond which everything is totally out of focus and everything inside it is in sharpest focus. –  Michael Clark Feb 17 at 21:47
    
I'm talking about the scene being grossly mis-focused, resulting in a photo that's unusable: i.stack.imgur.com/JxRmK.jpg . I'm not talking about the minute differences that might occur when focusing 200 feet away vs 300 feet away. –  Kartick Vaddadi Jul 13 at 1:19

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