I have a Canon EOS Rebel T5 camera and I've been running into an issue lately that I do not know how to solve.
The autofocus while using viewfinder does not work at all; I could not even take a photo. The lens will zoom in, and out whenever I half-press the shutter. However, the camera will autofocus when I use live view instead.
Is this issue something I can fix myself? Or should I bring it in to have it checked out?
You can try at least following and see if any of it helps :
If any of the 2-4 helps, it would sound a technical problem with lens.
Best bet to make sure is try another lens on. If the problem persists, it would be with camera.
After this, the best bet would probably be repair shop.
There is a difference between autofocus when using the viewfinder and when using LiveView.
In LiveView, the image sensor is acquiring the correct focus whereas there is a separate focusing sensor that is used when using the Viewfinder.
The LiveView is usually most accurate, as it has exactly the same distance to the area that the image is being projected to as the sensor itself (obviously, as it IS the sensor doing the focusing). But when using the separate focusing sensor, there could be a minor difference between the distance of the projected image on the sensor and on the focusing sensor. That is why many photographers are Autofocus Micro Adjusting their lenses.
I don't think that is possible on a Canon EOS Rebel T5 camera though.
But reading your question, I wonder if the problem really is that the focusing sensor does not receive enough light to acquire focus? again, LiveView uses a different technology and it is possible that it struggles less in the same light. You say that is focuses in and out, but will not let you take the focus. That sounds like the lens is focus hunting and then gives up. This will usually occur in low light or on subjects that have low contrast (e.g. being mostly a uniform black surface). Shooting in better light, or with a lens that lets in more light can remedy this.
Live view on the T5 would, I think, be using contrast detect for focus and non-live view focus method is phase shift.
These have different pros and cons and sometimes you can get focus with one and not the other.
Both focus methods will be affected by the amount of available light. No focus system likes low light but, again, one may be better than the other depending on the scene and circumstances.
The phase shift focus system is entirely separate from the main sensor and it may be a fault in that that is causing the problem, but also make sure it isn't just e.g. too little light for phase shift to work. The contrast detection system is using the main sensor itself.
