Photograph an impasto canvas straight on, in flat light, and you lose the thing that makes it worth painting. The relief disappears and what is left is a slightly clumsy image.
Turn a lamp so the light crosses the surface at a shallow angle and the painting reappears. Ridges cast small hard shadows. The direction of every stroke becomes visible. You can see, quite literally, the order the painting was made in — which passages went down first, where the artist stopped and reloaded, where a stroke was laid over something still soft.
This is why the works here are photographed with a raking key light rather than an even one, and why they look different again on your own wall. A canvas hung near a window changes character between morning and evening. That is not a flaw in the photograph. It is the point of the medium.