Canon EOS 800D - When using viewfinder won't autofocus but live view focuses well

Question

I have a Canon EOS 800D (Rebel T7i) camera.

The autofocus while using viewfinder does not work at all, tried all possible AF settings and it never works; I could not even take a photo, or if it eventually gives the good focus sign it will take the picture but as shown in the viewfinder unfocused so the picture as well. The lens will zoom in, and out whenever I half-press the shutter. However, the camera will autofocus perfectly when I use live view instead.

I've came across some similar post but not really a solution was found. Is this something that can be solved without Canon support ?

Thanks !

Answer 1

In "live view" mode, focusing is done using the main sensor, but when you use the view finder focusing is done using a sensor at the bottom of the camera:

(EOS 450D above, yours could looks a bit different).

You can see it/access it more easily if you lift the mirror up (which is done using the procedure for "manual sensor cleanup"in the camera menus). Check for dirt/obstruction, use a rocket blower if necessary, but make sure you aren't bowing the dirt to the sensor.

Otherwise, Canon service...

Answer 2

Just to close the loop here for future reference anyone reaching this post with a similar issue.

I've sent the camera to Canon Support, the following actions where taken from the report provided by support to fix the camera.

Product checked against any reported fault
 Function check
 Adjustment: Image Sensor
 Replacement: Image Sensor
 Adjustment: Mirror Box Assembly
 Replacement: Mirror Box Assembly

Answer 3

The focusing system used by the camera when you are using the viewfinder is an independent system from the focusing system used when you are using Live View.

Each system is designed and operates completely separately from the other. One works when the mirror is flipped down and the imaging sensor is "blind". The other works when the mirror is flipped up and the dedicated PDAF (Phase Detection Auto Focus) sensor is "blind" along with the viewfinder.

You haven't given us much to go on with regard to what might be causing the PDAF system to not work when using the viewfinder to shoot. You might check the following:

• Make sure you're actually telling the camera to initiate autofocus. It's possible via the settings menu to decouple AF start from a shutter button half-press. There's an option to only initiate metering with a shutter button half-press and assign AF start to another button, typically one on the back of the camera that can be reached by your right thumb without moving your shutter finger off the shutter button.
• Inspect the primary and secondary mirror to be sure light from the lens is being properly sent to the PDAF array in the floor of the mirrorbox.

Should you attempt to fix any obvious problems with the primary or secondary mirrors yourself?

As much as I hate to use the old cliche, this is pretty much a case of, "If you have to ask how, you probably shouldn't be doing it." Mirror adjustments are delicate and must be done precisely. It doesn't take much misalignment at all of the mirror (and the attached secondary mirror) to render a camera's AF system completely useless. The optical path from the lens to the AF sensor via the secondary mirror must be exactly the same length as the optical path from the lens to the camera's sensor. The same is true of manual focus: The optical distance from lens via the mirror to the view/focusing screen at the top of the light box just below the prism must match the optical distance from the lens to sensor.