D-76 used at full strength is a fine grain “solvent” developer. This is because it contains relatively large amounts of sodium sulfite that dissolve the edges of the grains to reduce the “apparent” grain in the negative. (Sodium sulfite has other benefits such as acting as a preservative). However, the “solvent” effect on the grains reduces the apparent sharpness or “edge affect” in the negative, which some (many?) find objectionable.(posted 8180 days ago)Diluting D-76 (1:1 or 1:3) strikes a more even balance between grain and sharpness, because the percentage of sodium sulfite is reduced as the dilution is increased (although this is somewhat offset by the increased development time for D-76 when diluted).