Toning is a method of changing the colour of black-and-white photographs. In analog photography, it is a chemical process carried out on silver-based photographic prints. Most toners work by replacing the metallic silver in the emulsion with a silver compound, such as silver sulfide in the case of classic sepia toning, which gives a black-and-white photographic print a brownish, "warmer" tone and enhances its archival qualities. Beginning in the 1880s, sepia tone was originally produced by adding a pigment made from the Sepia cuttlefish to the positive print of a photograph taken with any number of negative processes. Nowadays, toning can be simulated digitally, either in-camera or in post-processing. The in-camera effect often uses just a simple tint. More sophisticated software like Adobe's Photoshop tends to implement sepia tones using its duotone feature.
Sepia photography has been popular among portrait photographers for years. In terms of design, a sepia tone is a classic/vintage photo effect and can give an olden, aged or worn look to most portrait photographs. Sepia triggers a nostalgic feeling and makes a modern portrait look like an instant classic (especially in combination with a subtle, almost subliminal black retro vignette)...
"Each of us chooses the tone for telling his or her own story. I would like to choose the durable clarity of a platinum print, but nothing in my destiny possesses the luminosity. I live among diffuse shadings, veiled mysteries, uncertainties; the tone of telling my life is closer to that of a portrait in sepia..."
"Choosing sepia is all to do with trying to make the image look romantic and idealistic.
It's sort of a soft version of propaganda."
(acc. to Martin Parr)
Blog entries about practical editing techniques for street portrait photographs: