Canon EF 24-70mm f/4 L IS USM lens review by Ken Rockwell: The Canon 24-70mm f/4 L is excellent. Its sharpness varies very little from center to edge and stays about the same with aperture changes.

The lens was tested with a Canon 5D Mark III Dslr body:

"So long as you're in focus, sharpness doesn't vary much from perfection, except by f/11, where diffraction softens the image. Hey, sorry to spare you endless boring charts, but with a lens this good, there's nothing to show other than sharp pictures under all conditions. With a lens profile on my Canon 5D Mark III or similar camera, falloff is never visible at any setting. Flare and ghosts are very well controlled, but if you push it, you will get a green dot or two.

With its 9-bladed diaphragm, this Canon 24-70mm f/4 L IS can make 18-pointed sunstars on brilliant points of light. I'm very pleasantly surprised: even though Canon claims "rounded," which usually kills any sunstars, I see sunstars at moderate and smaller apertures under the right conditions. They tend not to be that obvious (this is an unusually clear example), and they aren't that symmetrical.

 Sunstars help an image tell the viewer just how bright the points of light are, since a photo can't otherwise reproduce the effect of a light or brilliant reflection shined in your face.

I'm really excited about the sunstars. They usually have sharp points, not the soft muted ones of today's wimpy "rounded" diaphragms, and they can be seen at normal apertures like f/8 and f/11, not needing to squint at f/22."

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