Originally posted by bdery Thanks for quoting me! Glad to know I'm being read :P
Another way to see this is that, in auto mode, the flash will use its light sensor to decide when it has illuminated the scene enough. The flash communicates its recommended exposure parameters to the camera, which will adapt itself to expose according to the flash settings (with the AF280T there are two power outputs, with associated apertures). But it is the flawh which decides when exposure should be good.
are you sure about this? I don't think the flash does any exposure calculation at all. In simplest form, there is only one communication between camera and flash in auto mode and that is the trigger. You set the lens F stop and ISO on the flash manually, and with this information, and the light it receives back through it's sensor, the flash terminates when it has decided there is enough light, that is all. depending on metering mode of the camera, the camera still decides what the exposure will be, and will attempt to make that exposure without any assistance from the flash.
Quote: In P-TTL, the camera measures the exposure and informs the flash of the recommended settings. The main differences are that you can control the way the camera meters exposure (spot, etc) and you could be using filters or other stuff, which could change the required exposure.
close, the camera does not send recommended setting but in P-TTL mode it sets the flash duration, in advance, based upon meter reading. the main difference is that P-TTL is predictive, as opposed to auto which is reactive
Quote: The flash has no way of knowing if you put a polarizer in front of your lens, for instance.
no, and it does not care because the camera is doing the measuring through the lens
Quote:
I'd love, all things being equal, to have a more powerful flash, and I'd like it to send AF assist bursts. But since I use my flash indoors in low light, I refrain from using filters and I use lenses that are fast enough to focus in low light. That way the AF280T is an inexpensive and very reliable alternative.
No argument there, auto capable flashes are a dime a dozen, compared to P-TTL and there are some quite good flashes around. In low lit conditions, except for the case where you have a subject close and a background further away, Auto does quite well for overall scene illumination. Auto requires much more attention in fill mode, where you need to deliberately under expose with flash ald allow the natural light to bring up the remainder of the exposure.