The Subjective Quality Factor (SQF) of the lens is a mathematically-derived rating that aims to characterise at how the image will appear to the eye, taking into account all the characteristics of human vision. Very simply, it is a rating to indicate how the optics will appear, even “feel” to the viewer at certain viewing distances.

Once again, we’re seeing the Canon 17-40mm f/4 lens take the slight edge on the 16-35mm f/2.8 at wide-open. 

Canon 16-35mm II USM @ f/2.8

Canon 17-40mm @ f/4

At two stops down, the results are even more dramatic. The EF 17-40mm f/4 appears sharper at

Canon 16-35mm II USM @ f/4.5

Canon 17-40mm @ f/6.3