At first glance, this method looks OK. But, there is a BIG weakness. Suppose you are checking the focus of your 400mm f/5.6 lens. At f/5.6, there is a fair amount of depth of field at 33 feet distance from the target (the recommended distance for a 400mm lens). You could look at a flat target and see that your lens is in sharp focus. But what you cannot tell from a flat target is how much is in focus in front of the target and how much in back of the target. In other words, within the depth of field zone, where is your lens focusing?
I use the LensAlign system which incorporates a flat focusing target and an angled ruler. The camera focuses on the flat target and the ruler shows you very clearly where your depth of field focus "zone" lies. Having just done this with fellow Canon shooter, we saw his lens was in complete focus on the target. But most of the depth of field "zone" was in front of the target. Remember the rule of thumb, one third of the zone in front and two thirds behind. Using the LensAlign, we were able to calibrate precisely his 400mm lens on his 7D with one third in focus in front of the target and two thirds behind the target. Almost like focusing on an imaginary flying bird.
You would encounter this same issue on all big lenses - 300, 400, 500, 600, and 800. Don't you want to control the exact position of your depth of field? A flat focusing target will not show you this.
I do not work for LensAlign. I paid for my LensAlign just like everyone else. I like it because it works and I can repeat and reproduce the results accurately every time. It is not cheap. But neither are the wonderful big lenses you own!
