Originally posted by kkoether Not 100% sure but weren't the *istDS's ttl on the flash not pttl?
The *istD, *istDS, and *istDS2 offer both TTL and pTTL. Its contacts are somehow signalling to the flash it can handle both standards.
If an external flash signals it wants pTTL, the camera will automatically work with pTTL and start the communication.
If not, it will work in the old ("legacy") mode, but additionally send the flash-end signal when it has received enough light. If the flash is TTL-capable, it will then shut off the light immediatelly. But as most TTL-flashes also offer Flash auto mode, one can as well switch the flash to "A"; in this case, the flash will just ignore the flash-end signal, and use its own photo cell instead.
EDIT:
TTL is specially interesting when using legacy lenses in connection with anything altering the effective F-stop against the nominal setting, like extension tubes, TCs, and so on.
For ordinary ("real life") flash shots with only one flash the flash auto mode will get you in most cases more consistant results than TTL or pTTL.
And for outside night shots of people any of these methods will give you lots of poor results, as the background will be farer away so the foreground will become strongly over exposed. That's where manual setting and calculations with guide numbers can help. Old hands with experience from the pre-TTL time (seventies) certainly have an advantage.