3

I feel like I quite consistently get:

  • Blurrier autofocus
  • Lower exposure

when shooting with my T5i's viewfinder than its LCD (i.e., in phase-based vs. contrast-based mode).

But whenever I search online, people seem to hail phase-based autofocus and say that contrast-based is just what cameras do when they can't do phase-based autofocus... and nobody even mentions anything about the exposure difference, making me wonder if I'm just... hallucinating?

Here's an example (phase-based vs. contrast-based). (Yes, they were taken at the same settings: ISO 1600, aperture f/5.6, shutter speed 1/64s) with stock 18-55mm lens.

Both images cropped from original JPG. Original, uncropped images here

If I'm being honest, I find it quite frustrating. :)

Am I misunderstanding which one is actually better?

If I actually care about quality, would I ever use phase-based autofocus?

Or is that just something I use when some other concern overrides quality (e.g. speed)?

Community
  • 1
user541686
  • 646
  • 1
  • 5
  • 16
  • 1
    Both shots seem to be done in very low light/high ISO (there is lots of color noise). I doubt that any effect you see in such situations can be transferred to normal light. – Aganju Jul 18 '16 at 10:33
  • Were the 2 shots really taken 3 seconds apart? So you took a photo via the viewfinder and then went to live-view and took another 3 seconds later? Can you post the shutter speeds for each? They both look underexposed. RAW/JPEG won't be an issue. – Steve Ives Jul 18 '16 at 10:44
  • @Aganju: They are ISO 1600 indeed. But really, this isn't "abnormal" light; Obviously if the lighting conditions are ideal then pretty much anything would give good results, but what would that even prove? I obviously need to know how to use my camera when the sun isn't bright and shining directly on everything I'm shooting. So that's really a non-argument... – user541686 Jul 18 '16 at 11:19
  • @SteveIves: 2.59 seconds, to be exact. No, I'm not trying to trick you by faking the clock or shooting on a different day or something... you can have a little faith in me here. The settings for both were ISO 1600, aperture 5.6, shutter speed 1/64. So if my common sense is correct, you can't really attribute the blurriness to the settings. – user541686 Jul 18 '16 at 11:25
  • This sounds like a back or front focusing issue. – jmn Jul 18 '16 at 11:25
  • @jmn: Okay but what is my takeaway from that? Does that mean contrast-based is actually better quality? Or is phase-based sometimes better quality? – user541686 Jul 18 '16 at 11:40
  • 1
    @Mehrdad: My point is that both Autofocus behaviors get very poor and unpredictable in low light. Any comparison in phase vs. contrast in low light will not give you a useful result for Autofocus method selection. – Aganju Jul 18 '16 at 11:47
  • @everyone: P.S. what actually shocks me about all of your questions is, are you asking just out of curiosity, or are you asking because you've never seen contrast-based perform better than phase-based when everything else is controlled for? I get this behavior at a huge variety of settings and lighting conditions, and I thought the T5i was a pretty typical camera, so I'm confused why so many alternative explanations are being sought. Is this behavior actually so rare? If so, I would really like to know. Otherwise, if there's a common explanation for it, then maybe it's just that? – user541686 Jul 18 '16 at 11:49
  • @Aganju: That's only true if you're saying phase-based might actually give better quality than contrast-based in other not-completely-artificial situations. Otherwise, the answer would still be that contrast-based doesn't do worse, and hence I should be using that all the time if I can afford the time required to focus. So is that what you're saying? If so, when would I expect phase-based to do better? If you tell me maybe I can just try to reproduce that to verify? Right now I'm completely skeptical it ever does better in any conditions I can find myself in. – user541686 Jul 18 '16 at 11:51
  • @Mehrdad. If you are trying to do a conntrolled test, then why rush the shots so much? Are these on a tripod with a remote release? Plus you are being paranoid re your RAW files and no, I'm not trying to trick you into putting one online – Steve Ives Jul 18 '16 at 12:24
  • 2
    Possible duplicate of Is it normal for phase-detect autofocus to be inaccurate with a Canon EOS 750D and EF-S 18-55mm lens? – Michael C Jul 18 '16 at 14:44
  • See also:http://photo.stackexchange.com/questions/65456/why-is-my-camera-focusing-fine-in-liveview-but-getting-it-wrong-with-the-viewfin – Michael C Jul 18 '16 at 14:45
  • @michael-clark: it looks like the issue is not focusing process at all but rather the difference between output images at fixed settings and focusing. – Euri Pinhollow Jul 18 '16 at 15:24
  • @MichaelClark: So I'm not sure how to tell if that's actually what I'm seeing. If you told me it was, I would definitely buy it, but if you told me it wasn't, I'd also definitely buy it, so I'm not sure how to tell if it's a duplicate or not. One thing that confuses me about your particular answer there is that you say "PDAF systems work based on the amount of contrast* that they can detect"*?! I'm confused, isn't that what contrast-based autofocus is supposed to be doing? – user541686 Jul 18 '16 at 19:18
  • 1
    @mehrdad: phase detection is not a miracle and requires sufficient details in the scene, which is nothing more for a camera than contrast. – Euri Pinhollow Jul 18 '16 at 21:34

2 Answers2

4

As a first-order approximation:

  • Contrast-detection autofocus is more accurate and more flexible
  • Phase-detection autofocus is much faster
  • Contrast-detection always requires "hunting" back and forth to find the best focus; in ideal conditions phase-detect moves certainly to the right point
  • Contrast-detection which uses the main sensor is likely to hurt battery life
  • And as a practical matter, it means using the LCD screen or an EVF instead of the optical viewfinder

If your phase-detect system (camera and lens) are carefully micro-adjusted for the aperture, focal length, and focus distance you are using, results will be perfect.* Otherwise, phase detect results are likely to not be perfect — but they're generally good enough for 99.9% of people in 99% of cases. And the big speed advantage is real. So, that's why people count it as desirable.

As algorithms improve, and faster hardware comes to newer cameras, the speed advantage will come down, but it's hard to get around the hunting. The state of the art right now is hybrid modes, which use on-sensor phase-detect areas to get into the ballpark very quickly and then contrast-detect to fine-tune. I expect that eventually, pure phase-detect will become rare. But then, ten years ago, I thought we'd all be using Lytro-based cameras by now. It's hard to make predictions, especially about the future. :)

There is no inherent reason for exposure to be different. However, it is likely that as a side effect, your camera is also metering differently, using the main sensor instead of separate metering sensors. That probably accounts for the difference you are seeing in exposure, but I'd count that as a quirk of your particular camera model rather than something inherent. (And in any case, which exposure is chosen is a matter of preference, not something necessarily better or worse.)

* In fact, using contrast-detection mode is my favorite way to do micro-adjustment — but unfortunately, Canon doesn't provide that as a user-accessible feature for your camera.

Community
  • 1
mattdm
  • 143,140
  • 52
  • 417
  • 741
  • +1 for the bullets. When you say "micro-adjust", do you mean adjust the camera for the particular lens? Because I haven't done that (I'm not sure how to either), I just put in the lens and started shooting. However, the issue isn't that the phase autofocus is always bad. If I'm out shooting landscapes in the sun, it usually turns out fine. It's when the conditions become less than perfect that this is no longer the case -- and I get this behavior quite consistently. So is the problem really an adjustment issue? I can buy that the metering might be different though, I'd have to look at that. – user541686 Jul 18 '16 at 19:01
  • Yeah, micro-adjustment is aligning your camera to work with a particular lens. Unfortunately, Canon reserves user-accessible micro-adjust as a feature for differentiating their higher-end models. You'll have to send your lens and camera in for service for them to do it for you. (Personally, I wouldn't buy a camera with phase-detect AF without this feature — but, again, obviously many, many people do without.) – mattdm Jul 18 '16 at 21:13
  • @mattdm Pse see Mehrdad and my comments on my answrer. I'm happy for you to comment on any areas where our answers may vary. I think they overlap quite closely but I'd expect your detail to be better than mine. – Russell McMahon Jul 19 '16 at 09:52
  • The difference in exposure can probably be chalked up to a flickering light source illuminating the scene. Dim lights tend to flicker the most with the alternating current powering them. Or maybe the light coming through a window was affected by rapidly moving clouds? – Michael C Mar 10 '23 at 10:16
3

1600, aperture 5.6, shutter speed 1/64 is 'low light'
Not dark by any means, but roughly 300 lux if I didn't do something wrong (no guarantee). (1 x 1600/100 x (5.6/1)^2 x 64/100 ~= 320)

The 1/64th shutter speed may be marginal shake-wise depending on what your actual (unstated) focal length is and whether you are using an (unstated) tripod.

Phase focusing is generally far FASTER than contrast focusing and usually no worse than contrast focusing provided mecahnical effects which may affect the results have been adjusted out.
One effect that can occur with one and not the other is that in an SLR the focus point on the main sensor is inferred by the focus sensors which use a different in-camera optical path. Lens seating can adversely affect the phase focusing as the optical path length can vary with mounting whereas contrast focusing detects the actual focus condition at the main sensor.
Have you "micro-adjusted the AF setting for this camera/lens combination?

You seem surprisingly resistant to people's attempt to help.
People are trying to establish your complete environment as it is extremely common for much time to be spent on questions before the full relevant details are provided. Your rep on others SE / SO sites indicates you will be well experienced in how questions are often misleading.

Russell McMahon
  • 19,630
  • 1
  • 44
  • 79
  • 1
    "Have you micro-adjusted"... OP's stated the body is a T5i--the dRebels don't have AFMA capability, they have to be sent to Canon service for adjustment. – inkista Jul 18 '16 at 16:46
  • "Usually no worse than contrast focusing" -- but mattdm below says contrast-detection is "more accurate and more flexible". Do you disagree with him? If not, how should I reconcile the two? As for the last comment -- I'm not resistant to help obviously, rather it's that some questions like whether I "really" took these photos 3 seconds apart (is the assumption that I'm trying to trick people?) are kind of silly and frustrating when it's not even obvious to me if people really think the results are so unbelievable at all. – user541686 Jul 18 '16 at 18:57
  • 1
    @Mehrdad I think that what Matt & I said are close to the same. If in doubt, choose his version :-) (really). He said that contrast detection is more accurate [in absolute terms], and also noted that in ideal circumstances phase detect moves 'certainly to the right point' and also noted generally good enough for 99.9% of people in 99% of cases.-> All that is ~= summarised in my "usually no worse than". ie most people most time do not ind any difference (statisticaly) significant. | I & he discussed microadjustment to make phase focusing work, and why it is required. – Russell McMahon Jul 19 '16 at 09:51