On Apr 11, 2005, at 11:02 AM, Cory Papenfuss wrote:
And more: with non A lenses, P-TTL isn't available !
Hrm... that's a rather significant limitation, no? How does P-TTL differ from TTL? I'm assuming that in P-TTL the camera wants to know how far away the subject focus is so it doesn't like not knowing.
P-TTL works by make a preflash and ambient exposure, integrating them, then making the actual exposure. Full knowledge of aperture is required, and that's only available with the aperture ring on the A setting.
When the DS cannot do P-TTL, with pre-A version lenses, or A and later version lenses not locked to the "A" setting, it reverts to simple, full power flash output, not TTL flash metering. A guide number is provided in the manual for you to set the aperture by distance per a given ISO setting. The DS ONLY supports P-TTL flash metering.
External flash units that support both P-TTL and TTL flash metering work as designed on the DS. TTL flash metering measures flash illumination DURING the actual exposure, and thus will work with any lens.
Godfrey
