Or more specifically, how sharp is your lens wide open at the center of the frame under controlled conditions ?

A side benefit of using LensAlign (see other blog entry) is that it can help me quickly determine how sharp a lens is at its largest aperture where most lenses are at their weakest in contrast and sharpness. Since all variables are fixed and strictly controlled, the use of LensAlign helps me get a decent glimpse of what the lens is capable of at that aperture.

Keep in mind that many lenses will yield much sharper images stopped down even by as little as half a stop. For some lenses the changes are dramatic when going from f/2.8 to f/3.2 or f/4 for example. Also this test only shows the performance of a lens at the exact center of the frame. 

Results in the field on actual targets will vary because of the use of VR or IS, lighting, focus mode, general camera settings, subject distance, shutter speed vs weight of lens when hand holding and tripod use or lack thereof.

The following images are shot with the same camera and same settings with same lighting setup. The camera and lenses are on a beefy tripod and a remote shutter with exposure delay was used for each shot. Lenses are perfectly aligned to the LensAlign target which is also on a tripod at the same height as the lenses. From all the shots I took while fine tuning each of these two lenses, I kept only the one that was sharpest to my eye.  All crops are taken at 100% with no size reduction and no post processing sharpness applied. All images shot at the lenses largest respective aperture of f/2.8 and f/1.4.

The sharpness difference is quite obvious between these two images and therefore one would probably conclude that the lens that produced the image to the left would be hardly usable wide open. In the field on real subjects, this is usually not the case. The image output from the lens on the right at f/1.4 is clearly more defined than the image on the left taken with a zoom lens at f/2.8 but I've used each at their widest aperture and have been satisfied with the output of both. In fact I always thought my zoom lens was great at all focal lengths and all aperture settings before I did this test. As mentioned, in real life situations on 3D subjects I like to photograph, images from both lenses seem fine. Images coming from the zoom lens will need a higher increase in post processing sharpness and added contrast but that's about it. You just can't expect all your lenses to perform equally wide open. My LensAlign fine tune tests have clearly shown me that most lenses are definitely sharper when stopped down but that shouldn't stop you from using them wide open for better subject isolation.