----- Original Message ----- From: <[EMAIL PROTECTED]> > A "slide" is only a mechanism for viewing. > It is more pricisely termed a "reversal". > On the film plane it's flipped horizontally and vertically, > completely reverse of what your eye sees. > It is a optical-chemical production. > (snip) ---------------------------------------------------------------------------- --------------- RESPONSE:
Slides are called reversal not because of their viewing orientation but because the chemical process involves a reversal step. All silver halide based photography (Ilfochrome is a diazo process) involves the 'burning' of the sensitised material to a darkened state, i.e. more light = darker image. Prints are positive because they are images of a negative. To get to a positive image without a printing stage, reversal films first get 'ordinary' development that doesn't activate the colour couplers. This is to isolate the unwanted negative image from subsequent colour development of the positive image. The positive image is left both unexposed and undeveloped. At this point some old processes called for the film to be re-exposed to light to prepare the positive image for colour development. Beginning with E-4, Ektachrome style films have used a chemical fogging agent to get the same result without the need for elaborate light baffling in continuous processors. This action is the 'reversal' step after which 'reversal films' are named. The newly fogged film is now colour developed, which also locks into position the globules of yellow, magenta and cyan dye that accompany the silver halides of the blue, green and red sensitive layers respectively. Remember, though, that the negative image had ordinary development so its colour dyes were not locked-in. Finally, the unwanted silver image is bleached away, the emulsion fixed, and the dyes that remain to form the desired image are stabilised so they don't 'wander' (i.e. diffuse) away from their original locations. That was a grossly oversimplified description of reversal development. Some comany's processes split each action into a different bath, while others combine one or more actions to get a faster, simpler process. What is constant is that the film is 'reversed' in the process so that what would otherwise be a negative becomes a positive. regards, Anthony Farr
