>From: "Tim Munro" <[EMAIL PROTECTED]> >with MLU set the flash fired once when the mirror locked up then again when >I hit the remote switch to open the shutter. Does this have any adverse >effect on the exposure? I would assume not.
No adverse effect. In fact, it occurs because you're using E-TTL, and that's the only way it _can_ calculate the flash exposure--with a preflash before the mirror goes up. >secondly, I was using mainly fill flash in Av mode to minimise any colour >casts and when I took my omni-bounce off, which I don't normally do, the >thought occurred to me - how does the system know how much fill to give >either with or without the Stofen unit fitted? I understand that it doesn't >matter when the flash is being calculated as part of the exposure OTF, but >as a minor fill ?? The percentage of light that gets from the flash tube to the relevant part of the subject depends on many factors, including distance (of course), but also if you have it set for bounce and whether you have a diffusor or any other kind of lossy or dispersive device on the unit. The idea of any automated flash is that it automatically compensates for all these losses by seeing what actually comes in the lens, and making the flash give off enough light to get a good exposure regardless. Now there is a considerable complication when the system is trying to augment (read: fill) ambient illumination, which is continuous, with flash illumination, which is almost instantaneous. With TTL OTF (Off The Film), the system has only the data coming off the film plane which represents a mixture of ambient and (at least at the beginning of the exposure) flash illumination. That's kind of a paucity of data with which to decide how to control aperture, shutter duration, and flash duration. (Oh, and the Nikon system has the valuable datum of focus distance too, but as soon as you tilt the flash head or put something over it, that datum becomes essentially useless.) With E-TTL, the system gets _separate_ readings of ambient only, and then ambient+flash, all _before_ the exposure begins. Even without rgard to the important addition of numerous metering zones, this gives enough data to solve the equations for the controllable variables. The exact way this is done, taking into account all the zones, is a Family Secret, and, as you may have noticed, the subject of some controversy. My educated guess is that they have a Fuzzy Logic engine cranking out the control variables. The amount of cranking they can make that poor little micro do, is limited by the fact this all has to happen _after_ you press that shutter release all the way down. dgw * **** ******* *********************************************************** * For list instructions, including unsubscribe, see: * http://www.a1.nl/phomepag/markerink/eos_list.htm ***********************************************************
