I just use a tape measure and point/angle my lens at approx 45 degrees......not sure if thats a decent test or not :shrug:
Potentially it is not. The AF sensors are a lot larger than the little squares in the viewfinder suggest. If you point at a tape measure at an angle, where you have areas of high contrast all the way along it, the AF sensor could decide to focus anywhere it finds a sharp edge. That might be at the bottom of the sensor, a few mm towards you, or at the top of the sensor, a few mm away from you, or slap bang in the middle.
That is why you need a focus target that is either square on to the lens, or very thin, such as my thin strip of white paper. You cannot use a focus target accurately that has any depth to it. That is why one example of a "standard" focus target is like this....
You need clear, contrast free space around your AF target so that you can be certain exactly what the camera is focusing on. In this example you focus on the black bar in the middle of the paper to give the AF a clear target to focus on. There needs to be clear space around the target point. The scales on the sides of the paper allow you to judge whereabout your focus actually has ended up, but the target itself - the thing you point the sensor at - must be simple and unambiguous.
My test target is a modification of this test sheet, using the patterning on the back of some hardboard to show precisely where my focus is. The strip of paper provides the unambiguous focus target. That said, I think I may have to review my adjustments once again, as I know the camera can focus on the hardboard dimples too, so the strip of paper is not a concrete guarantee that the camera it the right point. What I really need is a square of plain paper, with a single strong black line as the target, stuck in the middle of the hardboard. Then I will have the best of all worlds (I hope).
There is another design of paper test target that has been doing the rounds but just now I cannot find an example. Basically it requires the paper to be at 45 degrees to the camera, and has a graduated scale at the edges. However, you cut out a flap in the centre of the paper and actually lift that up so that it stands proud and upright and square to the camera. With a nice big X on it or something similar you have a foolproof point at which to aim your AF sensor. There should be nothing infront of or behind the target on that flap that could trick the sensor, so long as you are close enough to the paper to completely cover the sensor with the target on the flap with your chosen focal length.
Here's nother example, once again with similar principles and a clear explanation....
http://regex.info/blog/photo-tech/focus-chart/
