I'm starting a new thread for this because I thought it would make a good discussion. The original thread is
here and the post I'll be referencing is
this one.
The relevant quote is:
Originally posted by Ben_Edict And lastly I want to add, that these focus charts are more or less meaningless. As you can see, it expects the camera to focus onto that black line in the centre of the chart. As nobody actually knows, where the central focus point is pointing to (the small illuminated rectangle is only a very rough approximation) and the AF point is bigger anyway, these charts are misleading: the AF point often simply doesw not lock onto the focus line, but a bit obave that (mostly) or bewlow. In these cases it seems, the lens is misfocusing.
The only valid test targets ar completely plano-parallel to the camera sensor, i.e. the famous brick wall.
This is something I've read from other forum members before, and it got me thinking; does it really matter where the camera's focus point really is? When framing a picture and focusing on a subject, isn't it more important that the camera's auto-focus be calibrated relative to the center of the viewfinder rather than where its focus points reside?
I realize that the auto-focus system might be targeting slightly above or below the area shown by the focus indicator. But the way I see it, a proper focus chart sitting at a 45 degree angle allows me to calibrate my camera so that the auto-focus system is in line with what I actually see. The physical location of the focus sensor seems irrelevant.
Meanwhile, a brick wall would only calibrate the auto-focus system for distance from the lens, not for the center of the viewfinder.
By "proper" focus chart, I mean a real chart where the camera only has one specific line to focus on. A ruler is not a proper focus chart because the camera could use any of the ruler's lines near the focus point to make a focusing decision.
This is a proper focus chart:
This is not: