>Think of it like this. Your flash has only one brightness level, not like a >three way lamp in your house. It always puts out light at the same rate. >TTL simply controls how long the lamp stays on. If the object is close by, >the lamp is shut off quickly thru the marvels of electronics. The returning >light is measured thru the lens and the camera's sensor sends the quench >signal to the flash. > >If the object is far away, the lamp is kept burning until it gets enough >light back thru the lens, or the flash runs out of juice (electricity). So >the process is similar. The in camera sensor counts the light returning >until it sees enough illumination, then sends the quench signal to the flash. > >With TTL, I've found that you can be too close, especially in Macro. The >flash is so close to the subject, that it needs to be shut down >immediately. >The electronics isn't fast enough to quench it this quickly and things get >overexposed.
This is a nice way of explaining it, Bob. Another brand of camera who shall remain nameless puts 2 flashes together, one after the other in rapid succession. Here's what happens: the shutter button is pressed, the mirror swings up, flash one fires, a TTL reading is taken of the scene and the flash is quenched, the image is analysed and the exposure assessed depending on whether spot/center-weighted/matrix mode is chosen, any necessary adjustment is made to flash duration, shutter blades open, second flash fires and exposure is made onto chip/film, shutter closes, end of sequence. This happens so fast, I did not realise it was happening until I read it. The 2 flashes are almost impossible to seperate, and the exposures are usually flawless. Bounce flash, white ceiling at 7'6" height, camera about 4 feet off the floor, flash directed at ceiling slightly in front of camera, cam to subject distance about 8 to ten feet or so. And my oh my, a square crop! http://www.macads.co.uk/snaps/portraits/images/pic18.html (NB image prepared on a Mac, but brightness level determined as a compromise between PC and Mac gamma. If viewing on a PC, it may look slightly dark, on a Mac, slightly bright. Take my word, it's all there ;-) However, it is not flawless and sometimes spuriously underexposes, having to dial in a couple of stops of compensation. The only thing I would add to Bob's explanation is that some flash units will allow a flash setting (usually manual) that provides for a flash burst with reduced intensity, say about a quarter-power if you like. The AF280T is a good example with the H and L settings on manual. .02, Cotty ____________________________________ Oh, swipe me! He paints with light! http://www.macads.co.uk/snaps/ ____________________________________ Free UK Macintosh Classified Ads at http://www.macads.co.uk/ ____________________________________
