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…
eirikso 
Great post, learning something new every day about photography ❤
Well, what this tells you is “Don’t use f1.4 for landscape shots unless you mean it” 🙂 Almost any lens when shot wide open will not be as sharp as when stepped down. f1.4 is for those situations where you want to isolate the subject or there just isn’t any light. Try taking a face shot, focus on the eyes and try this again…
I can think of a million situations where a low aperture setting is the best choice. Situations where you really want a shallow depth of field and situations where you don’t have enough light to step down. A couple of examples here:
http://www.flickr.com/photos/eirikso//sets/72157616368191052
One example:
But with the 50 mm I’m talking about here I would try to step down to 1.8 or 2.0 if possible. No matter what situation you’re in this lens is not very sharp at 1.4. Not even in the center of the image.
Still, when does it matter? The situation, target, light, composition and timing… Much more important than pixel perfect sharpness. 🙂
wow, thats a big difference. I like this lense and use it often, but I never compared two pictures with two different apertures. Thank you for this useful information.
I always assumed there would be a difference in the shot depending on the aperture setting, but I never would have guessed that It would be so noticeable. Thanks for taking the time to experiment!
[…] I’ve just scratched the surface of this program. It does all kinds of advanced corrections. Here is my old test image from the 50mm f1.4. […]
All lenses are designed with a ‘sweet’ spot where it is the sharpest resolution: 5.6 or about there.
Optic designers know there is no perfect range on lenses, especially zooms and it’s a compromise in lens design.
For many years I was thought that the smaller the apperture the sharper the image.
WRONG!
Actually at f:16 is much worse.
I tested images and came to the same conclusion of using f:5.6 is about sharpest you’ll get the image with most lenses.
I shoot with 5.6 all the time and it works for me… including video.
There are many info on this online from optic designers.
You are right. F16 and f22 is not the best place to be either. And on the 50mm I’m talking about here f22 is not good at all.
I’ve tested this lens quite a bit in real life. And I’ve found that as long as the subject your taking a shot of isn’t more than about 2-2,5 meters away from you it is ok to use 1.4, anything longer away you need to stop it down. Also, it misses focus a worrying amount of the time. (at least with my 5D mk1.)