A soft developer is a type of photographic paper developer formulated to produce a softer, lower contrast rendering of the tonal scale than a standard print developer would achieve with the same paper and negative combination, providing the darkroom printer with an additional means of contrast control beyond the selection of paper grade or variable contrast filtration alone. By using a soft developer either as the sole developer in a standard single bath development sequence or in combination with a harder, more contrasty developer in a two bath development approach, the printer can exercise a finer and more nuanced degree of control over the tonal gradation, shadow detail rendering, and highlight separation of the finished print than paper grade selection alone affords.
The softer contrast characteristic of a soft developer is typically achieved through a combination of formulation choices that moderate the overall activity and contrast building tendency of the developing solution. Lower concentrations of the primary developing agents, the use of developing agents with inherently softer working characteristics - such as metol or phenidone used alone or at higher proportions relative to hydroquinone than in standard developers - milder alkaline accelerators, and higher levels of restraining agents such as potassium bromide all contribute to a developer that develops the photographic paper more gently and selectively, building shadow density more slowly and producing a longer, more gradual tonal scale with greater separation in the highlight and mid-tone regions of the print.
The two bath development technique exploits the contrast controlling capabilities of soft and hard developers most fully, allowing the printer to achieve intermediate contrast results and more refined tonal control than either developer could provide individually. In two bath development, the paper is given a partial development in the first developer bath - either a soft or hard developer depending on the desired result - for a carefully timed period that initiates development to a controlled degree, then transferred to a second developer bath of different character to complete development. When a soft developer is used as the first bath followed by a harder, more active developer as the second bath, the shadow areas of the print - which develop most rapidly and are most advanced in the first bath - are relatively restrained in the second bath because the developing agents absorbed into the emulsion during the first bath partially exhaust the available silver halide in the shadow regions, while the highlight areas - which are less developed after the first bath - respond more fully to the second developer and build additional density and contrast. The result is a print with restrained, detailed shadows, rich mid-tones, and well separated, luminous highlights - a tonal quality that can be difficult to achieve through paper grade selection alone.
Soft developers are particularly valued in fine art black and white printing, where the subtle nuances of tonal gradation and the relationship between shadow detail and highlight luminosity are primary aesthetic concerns that justify the additional complexity and care required to exploit the full contrast controlling potential of soft developer formulations and two bath development techniques. The ability to fine-tune contrast and tonal gradation through developer selection and development technique, in combination with paper grade and exposure adjustments, gives the skilled darkroom printer an exceptionally rich set of tools for interpreting the tonal content of a negative and translating it into a finished print of precisely the desired character and quality.