First of all: The Digital Picture.com has an excellent page where you can compare the sharpness of different lenses and different apertures.

But recently I did some quick tests with my 50 mm f1.4 myself. It’s a decent lens and regarded excellent for the price range. And it’s pretty fast, featuring an aperture of 1.4. But lenses loose sharpness and contrast at low apertures, and I wanted to know exactly how much. In this lens the difference is huge. Here are two 100% crops:

50 mm @ f5.6

50 mm @ f1.4

Yes. That’s the same lens, the same camera and the same conditions. Only a different aperture value. But will you see the difference if you snap an image at full resolution and reduce the size down to what you’ll need on a web page? Let’s have a look. 21 megapixels from my Canon 5D MarkII reduced down to an image that is 500 pixels wide.

50 mm @ f5.6

50 mm @ f1.4

And I guess you’ll need a slightly trained eye to see the difference clearly. First of all you see the vignetting: darker corners. Then you see that the DOF is more shallow and that the cars at the bottom of the image in the f1.4-version is not completely focused. This is how it should be. A shallow depth of field is one of the beauties of a low aperture value.

But if you look closely you’ll see that the overall sharpness and sense of depth in general in the f5.6-image is better. Even when the image is reduced down to this size.

So, if you want the ultimate in technical quality you should plan your aperture. And buy very nice lenses…