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You'll find tons of detailed lens tests on the web. They are great for photo geeks.
I don't do detailed tests, but I always test a lens for sharpness - in a practical setting. For me, sharpness is the #1 determining factor when choosing a lens.
In Kenya, I tested/shot with the Canon EF 200-400mm f/1 IS USM with built-in 1.4X extender.
The result: even in a tightly cropped image, I could see a fly (dark body and white wings) in my one of my photographs. Now that's sharp.
While I am on the topic of testing sharpness, I also shot with the new Canon 100-400mm f/4.5-5.6L IS II USM lens on a recent trip to Florida.
I wanted to see if the lens was sharp enough to show the birds' individual feathers.
The non-technical answer, even when the image is cropped: Yup!
Well, those are my short and sweet lens tests.
Explore the light,
Canon Explorer of Light
Speaking of new: Check out my new, and 36th book (Kindle version for now): Creative Visualization for Photographers.