D4s AF stopped working - help needed.

[jeremyaustin]

The issue: I am using a Nikon D4s and all of the sudden, none of my lenses will autofocus in the viewfinder mode. I read Nikon d4s specs and don't understand what to do. It maxes out and then comes back to infinity. So every time, the background basically focuses but not the subject. However, in live view mode, it focuses flawlessly and sharp. I do not have this issue in my Nikon D7500. Any ideas why it won’t work unless in live view? I have checked all of my AF settings and it’s the way it needs to.

 

Just to clarify.

1: All of the standard checks for auto focus issues have been done.

2: All Nikon and Sigma lenses did work before, none will now (when in auto using viewfinder).

3:All of these lenses work perfectly on my D7500.

4: My original guess is maybe the Phase Detect Autofocus Sensor? But don’t know how to go about checking that.

5: still works in manual mode and auto when using the live view.

[Andrew Garrard]

I think you're right - something's up with your phase-detect AF sensor, which wouldn't be used in live view (that uses contrast-detect - effectively taking lots of partial images until it thinks it has something in focus). Racking back and forth through the whole focal range is what Nikon lenses do when they can't acquire focus (on the assumption that the reason they can't see is that whatever they're trying to focus on is just very out of focus).

 

It's possible something bad has happened to the electronics of the phase detect sensor, or that something has knocked it. It's also possible that something has happened to the secondary mirror (behind the main mirror) that reflects light down to the AF sensors. However, it's also possible that you've just got a big bit of fluff in the mirror box and it's blocking the phase detect sensor - it's in the bottom of the box (in landscape shooting mode), so it's not so unlikely that something which got in might fall there.

 

Before sending things to Nikon for repair, perhaps blow some air around the mirror box (ideally with a bulb blower, since compressed air can leave propellant residue) just in case it's something you can blow loose? I wouldn't bother opening the mirror and shutter to do this, since you really don't want to blow something from the AF sensor onto the image sensor.

 

Others here with way more experience than me with the D4 or with camera repairs might have more specific ideas. Good luck!

[ShunCheung]

Behind the mirror, there is a smaller, secondary mirror that bounces light downward to the AF module at the bottom of the camera body. Maybe gently lift the mirror to see whether the secondary mirror is ok. Be extremely gentle if you touch those delicate parts. Please do not touch any of the reflective surface area of the mirror as you can easily leave a finger print or scratch it. With the mirror up, maybe blow some air to the AF module area to clear any dust.