Resolution: MTF
Modulation Transfer Function (MTF) addresses the relationship between simple resolving power of the lens and contrast at the highest resolution, a relationship that defines most accurately how “crisp” and sharp a lens will appear. Not only are we looking for a well-defined line, but we’re looking for an accurate contrast break between light and dark.

In these tests, we’re seeing some interesting results.

Canon EF 85mm f/1.2L II USM @ f/1.2

Canon EF 85mm f/1.2L II USM @ f/1.8

Canon EF f/1.8 USM @ f/1.8

Curiously, as we see here, at f/1.8 the two lenses start out at very similar levels.  Where the f1.8 levels off as it’s stopped down, the f1.2 continues to improve in MTF results.  At around f2.5, both lenses seem to level off, the f1.8 actually loses some ground, however.   Coincidentally when we get back to the point of our earlier testing, at f7.1, the lenses again converge and track identically to their respective minimum apertures. 

The rule of thumb stating that most lenses perform at their best when set to two stops down from full open seems to be somewhat dispelled here, since we’re seeing both of the lenses reaching their high points in the middle of their range, and holding those levels for several stops.  It’s also interesting to see that the f1.2 continues to improve slightly right up to almost the exact middle of it’s aperture range, and then proceeds on a gradual, yet steady, decline.

Looking at the full range of both optics in the graph, we see that although the f/1.2 lens fares pretty well at f/1.8 but drops as it’s opened up to it’s maximum, it does afford a slight bonus in performance as it’s stopped down, giving at least a slight justification for it’s price.  This certainly isn’t grounds on it’s own for purchasing the lens, but it’s nice to know that the speed of the lens isn’t the only core benefit. 

Resolution: SQF
Subjective Quality Factor (SQF) will give us an indication of how all of the resolution-related components contribute to what we are able to finally see in the image. SQF takes into account the lens performance, but also the performance of the human eye, mathematically, and gives us a tangible result.

Canon EF 85mm f/1.2L II USM @ f/1.2

Canon EF 85mm f/1.2L II USM @ f/1.8

Canon EF f/1.8 USM @ f/1.8

At wide-open, the f/1.2 is showing a very slight drop in performance, that is recovered immediately when stopped down to f/1.8. The f/1.8 lens shot at f/1.8 is giving us identical readings to it’s big brother at f1.8. (Keep in mind the units on the charts, we’re once again seeing identical values.)

At f/7.1 it’s a slightly different story, with the f/1.2 lens losing a little ground.

Canon EF 85mm f/1.2L II USM @ f/7.1

Canon EF f/1.8 USM @ f/7.1

For the price difference you could either conclude that the f1.8 is a remarkable performer for the money, of that the f1.2 is not performing up to it’s price tag. At the very best, we could say that the lens holds it’s own considering it’s additional full stop of speed, not sacrificing image quality across the board. It certainly does not justify its price (or size and weight, for that matter) in terms of pure resolution, at anything in the middle of it’s aperture range.