Good point about the meaninglessness of a “no filter” concept. The article is very interesting in regard to lightness mapping, and there a another, related subj

Good point about the meaninglessness of a “no filter” concept. The article is very interesting in regard to lightness mapping, and there a another, related subject of hue mapping. This is a good part of the reason why pictures from different camera brands look different. Your description of reciprocity failure is not quite right. The idea is that if you double the time the shutter is open and also decrease the aperture by one stop, you should not change the amount of light hitting the film (you will change other things, of course). The overall brightness should be the same when you make these “reciprocal” adjustments. This does in fact hold pretty well within a certain range of shutter speeds. Reciprocity failure occurs at longer or smaller speeds, where the reciprocal relationship doesn’t quite work.