A lot of people assume that prime lenses are sharper than zooms so I wanted to see how this relatively inexpensive prime lens fared against this huge professional zoom. I chose to only show the corners since the center sharpness was pretty much even across the board.
One reason I did this test was because I use the 28-70mm / D700 combo for most of my concert shots. I noticed that I only shoot at the extreme ends of this lens. All of my shots are at either 28mm or 70mm and rarely anything in between. I was thinking about using the 28mm f/2.8D prime on the D700 and the 50mm f/1.4G on the D300 using two cameras with smaller lenses rather than one camera with a cumbersome zoom lens.
- Vignette. The first thing I noticed was that the 28mm f/2.8D had severe vignetting. At f/2.8 the vignetting was so bad that it was more like an overall underexposure than a vignette. The corners are about 1 1/3 stops darker than the center of the image. At f/2.8 the 28-70mm showed minor vignetting. At f/4 the prime lens was still vignetting pretty badly, the zoom lens however showed very slight darkening of the corners. At f/5.6 the vignetting was gone from the 28-70mm, but the prime still showed a little darkening. At the rest of the apertures the vignetting was completely gone.
- Sharpness. At f/2.8 I was surprised by the softness of the prime lens. It was very soft. At f/4 the prime wasn't much better and only started to improve when stopped down to f/5.6. At f/8 the prime was good and at f/11 and f/16 it was sharp, with f/16 being the sharpest. Which is odd considering that by f/16 you should see some softening due to diffraction. The zoom lens was as sharp at f/2.8 as the prime at f/8 and only continued to get sharper. The 28-70mm f/2.8D is an amazingly sharp lens at all apertures.
- Distortion. The prime lens showed a surprising amount of distortion in the corners. With an ultra-wide lens you expect to see this type of distortion, which looks like the corners are being stretched out, but with a moderate wide angle like the 28mm you shouldn't see it so much. With the zoom lens the distortion is controlled better and it looks exactly as it should. Both lenses show moderate barrel distortion, but unless you're shooting brick walls or a lot of straight lines, it's not really gonna show. You can fix it pretty easily in Photoshop using about a +2.5 - 3.0 correction on the lens distortion filter.
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This lens test was performed on a full-frame D700. On a DX camera such as the D90 or the D300 the corner performance will be much better and the distortion almost undetectable. Vignetting on the 28mm prime will be evident at f/2.8-4, but you won't notice any vignetting with the 28-70mm.
The 28-70mm f/2.8D is an extraordinary lens. It's almost as sharp wide open as it is stopped down. The 28mm f2.8D isn't very sharp wide open and has pretty bad vignetting, but it also costs less than 1/4 of the price and weighs almost nothing.