The Kostinsky effect is a localised chemical development phenomenon that causes dense, heavily exposed image points in close proximity to each other to appear to migrate apart in the developed image, while lightly exposed image points in close proximity appear to move closer together. The effect is named after the Russian astrophotographer Sergei Kostinsky, who first described and documented it in the early twentieth century in the context of astronomical photography.
The underlying cause of the Kostinsky effect lies in the uneven distribution and rapid local exhaustion of developer chemistry around heavily exposed areas of the emulsion during development. When two densely exposed image points are situated close together, the developer solution between them becomes rapidly depleted as it is consumed by the intense chemical activity required to reduce the large concentration of exposed silver halides in both areas simultaneously. The exhausted developer between the two points is less effective at continuing development, while fresh developer approaching from the outer edges of each point continues to develop normally. The result is that development is relatively retarded on the inner facing sides of the two dense points and continues more vigorously on their outer sides, causing each point to develop asymmetrically and appear to shift slightly away from the other in the final image.
The opposite effect occurs with lightly exposed image points in close proximity. In this case, the relatively low demand for developer between the two points means that fresh, active developer accumulates in the space between them more readily than at their outer edges, producing slightly more vigorous development on the inner facing sides and causing the two points to appear to shift towards each other.
The Kostinsky effect has particular significance in scientific and astronomical photography, where precise positional measurements of star images and other point sources of light are made from photographic plates, and where even small positional errors introduced by development effects can have meaningful consequences for the accuracy of measurements. It is one of several adjacency and development effects - alongside edge effects, the Eberhard effect, and the Mackie line - that result from the localised diffusion and exhaustion of developer chemistry within the emulsion during processing.