Since the difference between f-number and T-stop with those lenses often varies widely from one focal length to another, the difference between f-number and T-stop can not be fully explained by transmission loss. It seems that the actual entrance pupil size does not grow at the same rate with the 28-300mm from, for example, 70mm to 100mm as it does from 35mm to 70mm.
View attachment 179591
As with all variable aperture zoom lenses, the size of the entrance pupil, as magnified by the optical elements between the aperture diaphragm and the front of the lens, is not keeping up with the focal length as it is zoomed to longer focal lengths. Yet at certain points in the zoom range of these lenses they seem to "make up" some of the loss. If it was all transmission loss one would expect the difference between f-number and T-stop to be constant and the above graph would be flat lines for each of the lenses. Link to lens comparison at DxO Mark. As always, ignore the "number scores" and go to the actual measured data by clicking on 'Measurements' and then 'Transmission'.
That’s all very interesting, but my inquiry was to the application of lens speed (introduced by the term “slower”) as it pertains to specified compatibility with the line sensors and their spacing.
Better stated, when you say canon has broken the rule and various zoom lenses are “slower” than their specified maximum, do you mean that the exit diameter as viewed by the PDAF unit is smaller than f/5.6 would imply because it doesn’t benefit from the magnification of the zoomed geometry? Or did you really mean “slower” to mean that although the light is falling in the right location on the sensors, there isn’t enough of it?
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