On MacOS + Intel Iris Graphics 6100 (may affect other config too), the texelFetch operation bypass the base mip setting of the texture object. Using textureLod with lod = 0.0 ensure the lowest (after clamping) mip will always be selected. Also disable the texture filtering for this sampler to avoid unecessary fetches. This should fix T78653 Blender 2.83 broken Depth of Field in Viewport