Seconic 558 (and other) and Gossen Starlight, both has a dual-function retractable lumisphere adjustable for sphere of flat characteristics. When the lumisphere is retracted it resembles a flat receptor for incident light, as e.g. Minolta flashmeter V has. However when light from side strikes the flat receptor angle of light rays relative to the receptor plane is accounted. When the same side light strikes retracted sphere receptor, things change A LOT: some rays never hit the receptor but the wall around the receptor, and the similar is to the sphere area on opposite side of the light source for it is in the shade. Also some rays hit the receptor with 90 degree to the receptor surfice. What the hells math is behind it. What actually the retracted sphere is for. IT JUST CANNOT REPLACE FLAT RECEPTOR FOR IT IS NOT FLAT. Does it take some average three dimensional photographic condition as sphere does in general, and actually is for nothing for it is not accurate as photog might need. Did anyone made some tests on this matter.