I'm presuming you mean the 60D (there actually was a D60 but it's more than 10 years old.)
Focusing systems need (a) enough light and (b) an object with some contrast.
For example: It would have a hard time focusing on a plain colored wall in dim lighting. But if you give it something with contrast it'll have an easier time focusing.
What sort of lighting do you have and what sort of subject are you trying to photograph?
Have you attempted to test the system under ideal circumstances (nice bright lighting and contrasty subjects?)
Incidentally... using the camera through the viewfinder will always be faster than using liveview. The camera actually uses different focusing systems depending on how you use it.
When you use the viewfinder, the camera uses "phase detect" auto-focus sensors. These sensors are not located on the imaging sensor. The reflex mirror bounces some light into these sensors. The sensors look for contrasting edges. Through a prism-like efect, part of the image is shifted in one direction and part in the other (imagine using a pair of scissors to cut a photograph in two halves and then slightly mis-aligning the image along the cut). The amount of the shift varies based on the focus distance to the subject. The camera is able to analyze the pixels along the edge to determine how the two halves should fit together and with this, it can determine how the focus needs to be adjusted to bring the image together.
There is one additional benefit to the phase detect system. The system is able to analyze all AF points at the same time. The direction of the shift tells the camera whether it needs to focus closer or farther. That means in an instant, the camaera knows _which _ AF point is on the nearest subject and it knows which way and how far the focus needs to adjust before the image comes to focus. That means phase-detect AF systems generally don't need to "hunt" for focus... they know exactly which way to turn focus and how far to go to achieve a lock.
The phase-detect system is very fast.
When the camera uses "live view" mode, the reflex mirror has to swing clear so that light can hit the imaging sensor. This means the tiny mirrors that bounce light into the phase-detect focus sensors are not in play so the camera cannot use that system.
Instead, it analyzes the image on the sensor trying to achieve maximum contrast in target areas.
The best example I can think of is to imagine a "UPC code" (barcode) and imagine what this looks like when it's "in" focus vs. "out" of focus. If it's in focus, you have a white background with sharp, crisp, black lines. But if you think of the colors involved... every pixel is either "white" (background) or "black" (the stripes). There are no "gray" pixels. Now imagine that same barcode horribly out of focus.... you have a white background, but as you approach a black line, you transition through lots of "gray" pixels getting progressively darker until you get to the black parts of the stripe. The camera knows it has focused when it has achieved the highest attainable contrast between adjacent pixels.
Unlike "phase detect" focus, the "contrast" system doesn't know which direction it needs to adjust in order to achieve focus. It has to take samples... (adjust and see if it's getting better or worse... if worse, reverse the focus direction and try again). Not only does the computer have to do more analysis to detect if the image is focused... it has to do more iterations because of the focus hunt problem.
Point & shoot cameras and "live view" mode on DSLR cameras typically use this "contrast detection" focus system. The system is extremely accurate... but the downside is that the computer has to analyze the image for contrast multiple times and it's not nearly as fast as the phase-detect sensors (which are usually extremely fast).
Cameras struggle with focus when there is nothing with high contrast upon which to base it's analysis. They also struggle in low light because there's not a lot of difference between the dark areas and light areas.
Canon has a new system they call the "dual pixel" AF. Essentially they've combined the "phase detect" technology right onto the sensor by creating micro-prisms on the surface of the imaging sensor itself. The camera is effectively able to do "phase detect" style auto-focus directly on the sensor surface (never before possible and Canon are the only company who have this.)
The new 70D is the first (and currently only) DSLR to get the system.
If you shoot video, you have no choice... the mirror has to be up in order to record video so the camera can only use contrast-detect focus when shooting video* -- except for the 70D because it can effectively do "phase detect" focusing directly on the sensor.
Tim Campbell
5D III, 5D IV, 60Da
