I contacted B+W, the filter company, about 3 weeks ago and was told, there should be no problem with TTL metering using b/w filters at high attitude. ( I gave an F2AS to a friend going to nepal ).(posted 9029 days ago)Regarding external meters, e.g. a Gossen Variosix, it might depend on the spectral sensitivity. Gossen was kind enough to provide me with a sensitivity versus wavelength graph, which shows highest sensitity around 580 nm, dropping in a gauss curve to almost zero at 380 and 780 nm. So the effect should be rather small for this meter.