Luminous Landscapes have generated an open letter here in regarding T-stops: An Open Letter To The Major Camera Manufacturers Basically, the author is discussing the issues with sensors and sensor design in relation to fast lenses. What is really being discussed here is pixel/microlens issues. Basically, the faster the lens, the greater the angles of light rays hitting them. Depending on the sensor/pixel design, some cannot handle the extreme angles and the pixel doesn't register all the available light. The author's beef is that camera manufactures know this and "secretly" boost ISO a little to compensate. This is a different issue than I really wish to talk about. We might have opinions one way or another on this. I think a more elegant explaination is made by Andy Westlake from dpreview: Fast lenses, pixel vignetting and blur circles: Open Talk Forum: Digital Photography Review He made some calculations using a Noctilux on a GF-1. As you can see from his results, the full aperature isn't fully used. I think this is something the legacy lens users might want to know.