On an speedlite 600EX-RT, what is the practical difference between E-TTL II and Ext. A? I know the technical one, but how does it change flash exposure if you measure through the flash sensor rather than through the lens?
There is some situation where the flash sensor is just off:
With a non coupled lens stopped down (unless you can input the f-number into the flash...)
When dealing with a small subject detached from the background: spot TTL metering will do the job, flash sensor will overexpose the subject trying to get enough light to the background.
All the situation where the flash is not on the body grip (as example: I do macro with a cobra flash hold in one hand in this situation the flash sensor is not directed to the subject and is not at the same distance than the camera)
In theory, ETTL or iTTL for Nikon, should be more accurate in more situations, especially for closeups where the flash sensor is not pointed directly at the subject. However, it has some drawbacks. It is a much more complicated process which takes some time and requires a preflash test which is measured. The preflash and the calculation can take just long enough that some people begin the blink which is captured. In some situations, it will slow down fast multi shot sequences. Flash auto sensor is always extremely quick, is consistent in how it behaves, and does not cause blinking. So, neither is perfect, but the technology of ETTL, used with lenses that provide distance data, gets the nod for most situations.
Hope that helps.