Having Trouble with Autofocus

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jsphoto1 Junior Member • Posts: 35
Having Trouble with Autofocus

Hi! First post on DPReview! This is because while I've dabbled in photography basically whole life, am just getting really serious about it now, and am still trying to get to know my DSLR camera (the Nikon D750).

As the subject line clearly indicates, one thing I'm struggling with, somewhat surprisingly, I think, is autofocus. I say "somewhat surprisingly" because I thought that this would be something that would make my photography life easier, but so far that's not the case. Specifically, when both my camera and lens are set to AF, sometimes my camera essentially refuses to take the picture when I hold down the shutter button! It's like just nothing happens. Unfortunately, I've missed a few decent shots as a result. The solution is always to just switch both to manual mode, but then, obviously, I have to use the focus ring to focus, myself, which I rather not have to do. So, I would think that the problem has something to do with the camera being unable to find focus while on AF, but why would that be the case?

By the way, in case it makes a difference, I recently switched to Back Button Focus, upon the recommendation of several veteran photographers. But I'm pretty sure this happened even before then.

Thank you!

Denis Van der Staey
Denis Van der Staey Regular Member • Posts: 422
Re: Having Trouble with Autofocus
1

Supposing your AE-L/AF-L button is assigned to AF-ON.

When using back-button AF, autofocus is normally set to continuous (AF-C), you don't need AF-S because at any moment you can take your finger from the AF-ON button to keep the current focus position.

Your problem might then be related to custom function a1 (or a2 when using AF-S). Set them to Release.

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Chuck Yadmark
Chuck Yadmark Veteran Member • Posts: 5,041
Re: Having Trouble with Autofocus

You should get acquainted with the settings, reset custom settings and go back in and look

My guess is you are set to AF-Single and Focus Priority which would greatly decrease many hits

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PHXAZCRAIG
PHXAZCRAIG Forum Pro • Posts: 19,325
Re: Having Trouble with Autofocus

Was the subject black?   Something hard to get focus on to start with?

And - what focus mode and settings in the camera?

AF-ON Only?

Release or Focus priority?

Those two have to be set together.

Try this:

1. AF-ON only (back button focus)

2. Release priority (or release+focus)

3. Spot focus.   (Put focus point on subject and hit AF-ON - but YOU must decide that the subject is in focus before pressing shutter release.)

AF is complicated because cameras aren't really smart enough to figure everything out for you.  Some modes work perfectly in some situations, but no mode works perfectly in all of them.  You get choices.

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BernhardF
BernhardF Forum Member • Posts: 63
Re: Having Trouble with Autofocus

Hi, as a pleased user of the D750 for several years I agree to others here that you should try to use back button af only + release mode. That way the camera will fire even if the subject is slightly out of focus. But from what you describe I find it puzzling that nothing happens when you pull the trigger. My experience with this camera is that the af is, under most circumstances, fast and accurate so you should at least get one shot even when using focus mode. With af-c (continuously) and if the subject isn’t moving very fast the camera should be able to refocuse almost instantly and allowing you to get several shots in one burst even with focus mode, but not as long bursts as when using release mode. Of course this can be affected by which lens you use. If the lens doesn’t hold up af speed wise with the camera then the shutter will not fire when using focus mode.

Happy Shooting!

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OP jsphoto1 Junior Member • Posts: 35
Re: Having Trouble with Autofocus

Thank you to everyone who responded (as a quick aside, as DPReview newbie, I checked the box for notification of replies but haven't received any, so I didn't even know until now that there were any -- not sure what that's about). Anyway, I'll try to respond to as many points as possible here.

- Yes, using the AE-L/AF-L button for the back-button focus, which seems to work, as I do hear the "focusing noise" when I press on it and can then press the directional wheel to move the focus point around). On that note, yes, it's set to AF-S as almost all of the photography I'm interested in involve static subjects. I'm not sure what "custom function a1 or a2 refers to.

- I'm also not sure what "focus priority" refers to. What does "greatly decrease many hits" refer to -- just not taking the picture at all when I press down the shutter?

- Well, a variety of subjects, but none of them black. Yesterday, for example, I took it out by a river on a cloudy, foggy, day, but that's hardly black. Again, I'm not familiar with the terms "focus priority" or "release + focus," so it sounds like those are concepts I'm going to need to research and get acquainted with!

The bottom line, though, is that it sounds like this should NOT be happening, correct? This goes back to what BernhardF said -- "the af is, under most circumstances, fast and accurate so you should at least get one shot even when using focus mode." That's kind of what I was hoping for and expecting. So, for sure, it's frustrating when I press down the shutter and nothing happens!

PS. I did purchase the camera used (and the lens as well), so it's possible that there could be some weird settings the previous owner programmed in that are now giving me problems -- I don't know. I guess that also theoretically means that something is just broken, but it sounds like I have several options to exhaust before I start having to assume that.

bjn70 Veteran Member • Posts: 4,762
Re: Having Trouble with Autofocus
1

If you're set to focus priority then it won't release until it confirms focus.  In some conditions this can be difficult.  I get in the habit of putting the focus point on something that I know it can focus on, let it focus, then recompose and shoot.  There can be issues with this method but it generally works well.  If you are trying to shoot quickly then it might not work reliably.  If you set to release priority then it will take the photo even if it is slightly out of focus, or a lot out of focus.  This is highly dependent on your lens and how well/quickly it responds.  Either method can work for you or can be problematic depending on circumstances and how they are used.  Regarding back button focus, it has no affect on your problem.  There are situations when back button focus is helpful and situations where it doesn't make any difference.  It is the release priority mode that makes the difference in how the camera responds.

briantilley
briantilley Veteran Member • Posts: 7,296
Re: Having Trouble with Autofocus

jsphoto1 wrote:

The bottom line, though, is that it sounds like this should NOT be happening, correct? This goes back to what BernhardF said -- "the af is, under most circumstances, fast and accurate so you should at least get one shot even when using focus mode." That's kind of what I was hoping for and expecting. So, for sure, it's frustrating when I press down the shutter and nothing happens!

From what you've told us, it's likely that things are working as designed.  I'd say the real bottom line is that you need to spend some time learning about your camera and how to configure its many options to suit the way you want to shoot.

One of the many guide books to the D750 would be a great place to start - I can recommend this one, for example:

https://www.amazon.co.uk/Mastering-Nikon-D750-Darrell-Young/dp/1937538656/

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BernhardF
BernhardF Forum Member • Posts: 63
Re: Having Trouble with Autofocus

Hi again, I believe I was a little unclear in my last post - sorry for that

In the menus you can find a choice between focus priority and release priority, and it works just as has been described in this thread. You find it the custom settings menu - the pencil icon. Under submenu a you click right on the directional pad and then your first choice is if you want focus or release priority for a1 Af-c (continues shooting) and a2 for af-s (single shot). Click right and make your choice, confirm with OK. 
You choose between af-c and af-s by pressing the little button on the side of the lens mount, where you also can switch between manual and auto focus, and at the same time turn the back wheel (beside the ae-l af-l button). If you turn the front wheel (on top of the grip) while pressing the same button you can choose between focus areas like single point, group area and so forth.

I have found the videos from Steve Perry very helpful for my learning curve, try a YouTube search on for instance “Steve Perry back button focus” and you find a few helpful videos.

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JohnnyLuddite Senior Member • Posts: 1,843
Re: Having Trouble with Autofocus

One thing that might help while you're learning the camera and settings is simply to put the beep sound on the D1 setting - in single servo focusing (AF-S), it'll beep when focus is gained.

As others have recommended, BBAF + AF-C is a powerful way of working.

Don't feel shy about doing a load of test shots in varying conditions and subjects, and if you use NX Studio, that can display the focus point(s) in some modes.

Steve Perry's Secrets to the Nikon AF system is valuable as you want to learn more, since effective AF is subject dependent and according to what you personally find effective.

Not wanted to presume, but I know I've been guilty sometimes of expecting to nail ultra-shallow depth of field in a dynamic situation and failing - often, stopping down, getting further away and planning a bit more works wonders.

The other thing I found tricky learning- coming from film - was controlling highlights more carefully - the D750 does support Highlight Weighted Metering which can be valuable in some high-contrast situations; and also the preview can be set to show histograms.

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Robert Cohen
Robert Cohen Veteran Member • Posts: 4,582
Re: Having Trouble with Autofocus
4

>> PS. I did purchase the camera used (and the lens as well), so it's possible that there could be some weird settings the previous owner programmed in that are now giving me problems -- <<

If there is a possibility that there are some unusual settings that are tripping you up, you could try resetting the camera to factory settings, then start setting it up from a blank slate.

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OP jsphoto1 Junior Member • Posts: 35
Re: Having Trouble with Autofocus

Well, one positive is that with all of the talk about focus priority, I did locate this topic on p. 327 of the camera's official instruction manual, including the statement that with release priority "photos can be taken whenever the shutter-release button is displayed," which sounds like it might very well be the problem (I don't have my camera with me right now so I'll have to check for sure and then test it out). It also looks like that's in the Custom Settings menu, which I haven't really messed with yet so probably why I'm not familiar with it.

Still, even if that is the solution, to me it kind of begs the question of WHY the camera is so often not in focus even though Autofocus is turned on. And then the next question, does that mean that the image truly is out of focus, or that the camera just thinks it is for some reason?

BernhardF
BernhardF Forum Member • Posts: 63
Re: Having Trouble with Autofocus

Still, even if that is the solution, to me it kind of begs the question of WHY the camera is so often not in focus even though Autofocus is turned on. And then the next question, does that mean that the image truly is out of focus, or that the camera just thinks it is for some reason?

Hi, I’ll try to answer your questions. First it could be something wrong with your camera or maybe a dirty af module but I wouldn’t think so. Instead it probably has to do with the circumstances under which you shoot. Like to dark, not enough contrast where the focus point is on the subject or a lens that focuses very slowly so you have to wait a little bit longer before triggering the release button. 
Try with different focus modes (single point, group mode, dynamic mode) and see if you get better success with one or the other. I wouldn’t recommend the af-a mode which often is slow to find what it thinks is the best way to auto focus for any given situation. If I remember correctly there is a description in the manual about af-a where it says something along those lines.
Your second question is partly answered above but I can add that at no time you can get the camera to do more than “think” weather that is in focus or out of focus. In really tricky situations one has to rely on focusing manually. Luckily for most of us casual shooters those occasions are rare.

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JohnnyLuddite Senior Member • Posts: 1,843
Re: Having Trouble with Autofocus

Please, reset your camera. You'll need to get familiar with your settings the way you want them, so you may as well start from a known base. Thom Hogan's manual provides a reasonable discussion of recommended settings, and in any case, getting a spreadsheet of the settings and what you might want is worth it in the long run, and you can use it to develop U1 and U2 profiles.

Once in a known state, it'll become more apparent whether there's an equipment issue - my experience is that the D750 is astonishing at getting focus even in poor lighting. You may not always be IN focus on what you want with AF-C in some conditions.

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bjn70 Veteran Member • Posts: 4,762
Re: Having Trouble with Autofocus
1

for a1 Af-c (continues shooting) and a2 for af-s (single shot).

Af-c means continuous focus not necessarily only for continuous shooting.

PHXAZCRAIG
PHXAZCRAIG Forum Pro • Posts: 19,325
Re: Having Trouble with Autofocus

Well, for typical back button focus, you have the wrong settings.

You want it to focus and take a picture when you a) press your back button to focus and then b) press shutter release.

That sounds obvious, but it leaves out the part where the camera decides when it is in focus.  It simply takes a shot when you press that shutter release.  The part where you press the back button to focus also needs to include your own judgement that focus has been achieved - even if you focus-and-recompose.

Your settings - AF-s, is a problem for the above.  It should be AF-c, for continuous focus.   That gives it maximum flexibility - you want to focus on a moving object?  Just keep back button pressed.  Want to focus and recompose?  Focus, then lift thumb.

In order for the camera to take a shot whether it thinks things are in focus or not you must set the camera to release priority.  (Release+focus ok too, but I think it's only supposed to help when shooting at a high frame rate.)  There are two distinct uses for this setting.  One is to allow you to focus, then recompose and shoot even though the focus point is no longer on the subject.  The other is to grab a shot either quicker than the camera determined focus was achieved or simply take shot when it isn't possible for the camera to even tell about - this includes shooting black subjects and sometimes foggy ones - low contrast conditions.

In addition, the camera will be more sensitive to vertical lines than horizontal ones in most situations.

With your camera set to AF-S and focus priority, it will not take a shot if it doesn't think focus has been achieved.  This includes focusing and then recomposing (camera will try to refocus).  Problematic subjects cause issues.  I ran into it years ago with a D300 struggling to maintain focus on a squirming black puppy.

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OP jsphoto1 Junior Member • Posts: 35
Re: Having Trouble with Autofocus

Thanks again to all of you! I'm not sure if any of you saw it in one of my posts above, but just to be clear, right now (and for the foreseeable future) I'm only shooting in AF S mode and with STATIC subjects (no wildlife, sports etc.). And it's strange because I kind of thought that that was the "simplest" mode. But I had still been led to believe that back button focus was still advantageous. Does this fact (AF S with static subjects) change your responses at all?

PHXAZCRAIG
PHXAZCRAIG Forum Pro • Posts: 19,325
Re: Having Trouble with Autofocus

jsphoto1 wrote:

Thanks again to all of you! I'm not sure if any of you saw it in one of my posts above, but just to be clear, right now (and for the foreseeable future) I'm only shooting in AF S mode and with STATIC subjects (no wildlife, sports etc.). And it's strange because I kind of thought that that was the "simplest" mode. But I had still been led to believe that back button focus was still advantageous. Does this fact (AF S with static subjects) change your responses at all?

It means you don't need back button focus.   Are you using single point AF?

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BernhardF
BernhardF Forum Member • Posts: 63
Re: Having Trouble with Autofocus

bjn70 wrote:

for a1 Af-c (continues shooting) and a2 for af-s (single shot).

Af-c means continuous focus not necessarily only for continuous shooting.

You’re quite right. I stand corrected

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OP jsphoto1 Junior Member • Posts: 35
Re: Having Trouble with Autofocus

Probably a stupid question, but you mean with the one, single box in the viewfinder, right? If so, yes.

It's just kind of interesting because I had been advised that back button focus is some kind of panacea (lines like "you'll never go back!"), but I've never really understood what the big deal is. It sounds like it would be a different story for moving subjects. But for static subjects, it has just seemed to me like doing with two fingers what I could otherwise do with only one by using "regular" focus. All of that said, I think I was having this issue even before I switched to back-button, but that's something I'll need to try again to make sure.

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