Originally posted by codiac2600 A mode is the way everyone should learn to shoot if not full manual. A mode makes the flash use the thyristor which is more accurate than in body TTL or on flash I/P/E-TTL. You note the ISO, aperture and shutter and balance for the distance you are shooting. I/P/E-TTL is fooled too easily by subjects that get in front of the sensor like a bride thats far away but some guy is in front of her and to the side. This is why I shoot almost fully manual because I understand the relationship between distance and how far light will carry. all the new fancy TTL's are way too dumb to understand that relationship and good luck making a perfect form of it.
Interesting that earlier in this thread you said, "It's quite accurate" and then this post says "It's not accurate, A is better" - and I agree with you on THAT.
Of course, since A is better, there's not much point of buying the AF-540FGZ for most situations when there are far cheaper units that do A mode (Vivitar 285HV and Sunpak 383 both have A mode, comparable output power, and are 1/3 the price).
What I find really annoying is that sometimes the AF-540FGZ insists on dropping out of A or M mode and back into P-TTL mode the moment the camera meters.
Edit: I can't speak personally for the 383's A mode, but the 285HV always exposes as well as or better than the 540 when the 540 is in A mode. Either unit in A mode blows away the 540 in P-TTL.