都列的出品牌這個還要教?

"A lens in which three wavelengths are brought to a common focus is called an apochromat. Often this term also implies that the spherical aberration is corrected for two wavelengths as well"

是不是APO要看ray tracing結果.
剛好Pentax早期的ED就不能算是APO.
不過多了field flattener,
所以少了field curvature.
光是ED Glass就分FPL51,FPL52,FPL53,只有FPL53(?)的性質才接近螢石.