The focus sensor is in the bottom of the mirror box, and it wouldn't take much dust, even a few specs, considering the size of the sensor (not the image sensor, which is totally separate).

I did forget to mention that I shoot AF-C and have tried release+focus and focus only.

It does fire as it's capturing focus somewhere.

I did flip the mirror to check for visible debris but could not see anything. There were some fibers (fuzz) on the small felt pad above the sensors. I used tape to remove the fibers. I can't imagine those little tiny (thinner than hair) fibers would interfere with focus.

@ R8R +1 Exactly... I would select "focus" for the "AF-C Priority Selection" in the "Menu button > Custom Settings menu". This will prevent an out of focus shot, although it can be a bit frustrating if the camera does not focus for some reason as the shutter will not release when we want it to. The other option is "Release + focus" in C mode as the camera will grab focus on the second or third shot in most cases.... I shoot at about 8 FPS for something like a fighter where I want the exact moment...sweat flying, etc.

I don't think you mentioned what you set as AF-C and AF-S priority selection (release, release+focus or just focus). This can have drastic effects on exactly *when* the release happens in relation to the AF, especially with fast moving subjects.

I would try the above test msmoto suggested but also with different AF priority selections. Pick a simple object in a dim room and focus on it while moving though the room. Vase full of flowers, table lamp turned off, etc. Focus on the same area from different points in the room, while standing and moving.

Review the pics in ViewNX to see where the exact focus points were in each shot. This may help you to narrow down whether it's user error or camera/lens error.

If you haven't had the camera inspected/cleaned in awhile, it might be worth it to send both the body and lens in to be serviced and calibrated, separately and in combo. Nikon has equipment to easily check if everything is within tolerances or if something has gotten knocked out of whack.

First, lens off, body with lens opening facing down, use manual blower to clean all dust out of interior. While still holding the body with opening facing down, use electrical contact cleaner on a Q-tip to clean the contacts of the body and lens. Remount lens.

Set to Continuous servo, 9 points and place the focus center in the middle of the viewfinder. Then, in a dimly lit room, move around and shoot sample images, noting where you had the focus point. If the focus is off, make certain the little dot is on when it is supposed to be in focus. (As opposed to the >< marks). If the camera shows it is in focus and the image is out by a few inches to feet, there is no doubt a malfunction in the camera. In your menu make certain Shutter/AF-ON is selected. It could be that somehow the AF-ON only is selected on the menu and thus the lens will not focus unless you have pressed and held this. I would also set the AF-ON button on the camera back so it initiates focus only while doing this check.

Check a couple other lenses to see if they have problems as well.

If none of this works, Nikon repair service?

If you weren't have problems before, but are now it might not be settings, but hardware. Have you ever taken the camera or lens in for a cleaning and fine tuning? Dust can build up on the AF sensor and throw focus off. Also with the lens, over time, focus can drift off slightly, due to being bumped around and used. If it is the lens, AF fine tune might fix the problem.

I have had my Nikon D3 for several years now and primarily shoot MMA and boxing. My primary lens is the 24-70mm f/2.8. The typical setup is manual, 640 to 800ths of a second @ f/2.8, ISO 1000 to 2000.

For live fights, I go for center-mass of the fighter to make most of the target.

Recently, I’ve been having a lot of problems with capturing images of fighters (while fighting) in focus. The usual problem is focus elsewhere behind or next to the fighter instead of the fighter themselves. I’ve run though the different focus options such as dynamic area 9/21/51 AF points and AF point selection of 11 and 51. The body is setup for AF-ON on the AF-ON button as well as the shutter. I also mix between dynamic and single AF and spot, matrix, and center-weighted metering. I have also tried focus tracking lock from off through 5 (longest).

This does not appear to be a back focus issue as the lens lines-up with testing on well-lit subjects.

The body has been reset to factory defaults several times; the issue persists.

Despite numerous combinations, I seem to catch focus on the fighters surroundings instead of the fighter. When on the fighter however, the image is tack-sharp.

I am looking for advice on what may be the issue. At this point, it could either be me or the camera settings. It seems like the camera will focus where it thinks it should focus vs. where I am telling it to.

Any help?