Canon EF 40mm f/2.8 STM lens review by Ken Rockwell: "The Canon 40mm is insanely sharp at every setting throughout the entire full-frame on the 22 MP 5D Mark III"

"This Canon EF EOS 40mm f/2.8 STM works perfectly with every Canon EOS camera ever made, meaning every Canon DSLR and every Canon autofocus 35mm camera made since 1987. Of course it works great on today's 5D Mark III and Canon 7D, but it also works great on my original 1987 Canon EOS 650! The only oddness I noted on my 1987 EOS 650 is that while auto and manual focus and depth-of-field preview and everything work great, manual focus override doesn't work: you have to set the lens to manual first. Manual-focus override works flawlessly on my 35mm EOS Rebel G from 1996 and EOS 3 from 1998, so I'm not worrying about it."

"The Canon 40mm is insanely sharp at every setting throughout the entire full-frame on the 22 MP 5D Mark III.Likewise, it's always super-sharp on 1.6x cameras like the 1.6x Rebel T4i. However, at full 18 MP resolution on a small-format DSLR, the corners were slightly less sharp wide-open improving as stopped down to f/8. If softens from diffraction by f/8, meaning that it's pretty much a perfect lens at f/5.6!"

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