Am I imagining things? (AF accuracy when half pressing vs full pressing shutter)

[Kit Lens Jockey]

I tend to do something out of habit and somewhat based on the results I've seen in the past, but I wanted to get everyone else's take on it.

When shooting a moving subject, camera set to one shot AF, it feels to me like the AF tends to nail the focus better if I half-press the shutter button, let the AF do its thing, wait for the camera to give me the red confirmation blink, and then immediately press the shutter button fully. This, as opposed to just holding the shutter button down fully right from the start, and trusting the camera to not release the shutter until its determined that it's achieved focus.

It's a subtle difference, but does make somewhat of a difference time-wise when you're trying to fire off a quick shot. Obviously it's going to be faster to just hold the shutter button fully down and let the camera automatically release the shutter as soon as it gets focus, but it's always kind of seemed to me like the camera kind of "rushes" it when I do this, and takes the photo before the focus is quite locked on, resulting in a blurry subject more often.

Has anyone else noticed something like this, or am I just imagining it? My 5D MkIV seems to do this, and as far as I remember, the MkIII felt similarly.

Now, before you reply, let me clarify a few things...

1) Yes, I know you can set the camera to prioritize taking the photo faster, or achieving better focus while sacrificing some shooting speed. I always have it set to prioritize focus. I've still noticed this phenomenon.

2) Please don't chime in with "You should be using AI servo for moving subjects." I've tried it, one shot just works better in the conditions and at the apertures I'm usually taking photos at.

3) This isn't meant to devolve into a discussion of the merits of back button focus. I've tried this too. While I see its advantages, it has never seemed to be quite as fast as having autofocus driven off of the shutter button. If it is for you, that's fine, but it hasn't seemed to be for me.

 

[neuroanatomist]

Are you using a lens with image stabilization? If so, the IS system takes up to about half a second to fully stabilize the image. If you just mash down the shutter without letting the IS stabilize, you will often get an image with more blur than not having IS at all (i.e. the IS system causes what appears as camera shake).

 

[Mt Spokane Photography]

Are you using a lens with image stabilization? If so, the IS system takes up to about half a second to fully stabilize the image. If you just mash down the shutter without letting the IS stabilize, you will often get an image with more blur than not having IS at all (i.e. the IS system causes what appears as camera shake).

Yes, that is a error I make too often, and one reason I only want IS for lenses that absolutely give a big benefit such as telephotos. With a lens like my 24-70, I would not want to wait for IS to settle before taking the photo, its frustrating enough with a lens like the 70-200L.

 

[Mikehit]

If you are waiting for the image confirmation bleep I wold have thought the IS would be fully engaged in that time.

I would have thought that once the focus has locked on, in single shot it would not keep tracking the subject and in your reaction time, the subject wold have moved - but this is variable with the direction of travel (movement parallel to the camera will be less of an issue than movement towards you, a walking person less than a running horse). In that respect your experience seems counter-intuitive.
Having said that, even when shooting AI Servo I will press the shutter for the first shot only when I believe that focus has been hit so maybe what you are seeing is that the system is not predicting anything so if you hit it you hit it well.

 

[Mt Spokane Photography]

If you are waiting for the image confirmation bleep I wold have thought the IS would be fully engaged in that time.

I would have thought that once the focus has locked on, in single shot it would not keep tracking the subject and in your reaction time, the subject wold have moved - but this is variable with the direction of travel (movement parallel to the camera will be less of an issue than movement towards you, a walking person less than a running horse). In that respect your experience seems counter-intuitive.
Having said that, even when shooting AI Servo I will press the shutter for the first shot only when I believe that focus has been hit so maybe what you are seeing is that the system is not predicting anything so if you hit it you hit it well.

My lenses focus almost instantly, but IS takes much longer to settle and lock on. There is no beep or indication that IS is locked on, I have to watch thru the viewfinder and see it suddenly stabilize. Thats why many just turn IS off unless they have a need for it and are willing to wait. With FF bodies, a bump in shutter speed to compensate for no IS is not a big deal, the ISO can rise with little impact. When long telephoto lenses come into play, IS is often worth the delay.

 

[LonelyBoy]

If you are waiting for the image confirmation bleep I wold have thought the IS would be fully engaged in that time.

I don't think the camera knows if IS has settled.

Isn't this the cause of the behavior I've seen written about, many times, where often the first frame in a burst is blurry compared to the rest?

 

[Kit Lens Jockey]

Nope, this is primarily with non-stabilized, wide aperture primes (being shot at wide apertures.)

I agree that the results are totally counter-intuitive. It should yield better results to just point the selected focus point on what I want to focus on, mash down the shutter button completely, and this would result in the shutter firing right as soon as focus locks, and theoretically giving the subject the least time to move out of focus. But. it just simply doesn't seem to yield as good of a result as me half-pressing, waiting for the viewfinder to blink, and then pressing fully as fast as possible. Like I said, it seems like maybe the camera is over-eager to release the shutter, even with the camera set to prioritize focusing over speed of shooting.

And in fact, now that I'm thinking about it, the very existence of an adjustment to prioritize focusing or releasing the shutter kind of proves that the camera has some leeway in what it considers good enough focus to release the shutter. So, perhaps, even with the priority set to focus, there is still a little bit of error that the camera allows for, which, at wide apertures, might be enough for the focus to end up being far enough off to ruin the photo.

Would be really interesting to know specifically what the decision process is within the camera for when it decides to release the shutter with the shutter button pressed while the camera is in the middle of focusing.

 

[Mikehit]

If you are waiting for the image confirmation bleep I wold have thought the IS would be fully engaged in that time.

I don't think the camera knows if IS has settled.

Isn't this the cause of the behavior I've seen written about, many times, where often the first frame in a burst is blurry compared to the rest?

I did not mean the camera will know if IS has settled, just that in the time you half-pressed the shutter and waited for the focus confirmation then pressed the shutter, in that time the IS would have settled.

If you are in AI Focus then the temptation is to fully press the button is greater and in that time the IS may not have settled.

 

[neuroanatomist]

Could also be the physical action of pressing the button all the way down causing a bit of camera shake, compared to a slow and easy half- then full-press.

 

[Mikehit]

I'd say that's the most likely explanation.