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In a red-free photograph taken through a green filter, veins and arteries appear black. This is due to the way that blood vessels absorb and reflect light. The green filter effectively removes red wavelengths of light, which are primarily reflected by the red blood cells in the arteries and veins. Since veins contain deoxygenated blood, which appears darker, they absorb more of the green light and thus appear black against the surrounding background when viewed through this type of filter.

Arteries, which carry oxygenated blood and typically appear lighter under normal conditions, also reflect less light in this situation. As the red light is filtered out, the overall contrast between the blood vessels and their surroundings increases, resulting in both veins and arteries appearing black in the image. This characteristic is particularly useful for examining the vascular structures in the retina, as it enhances the visibility of blood vessels and helps in diagnosing various ocular conditions.