This question arises from time to time, folk are taken aback at the cost of additional lenses and seek a less expensive option.
For the purpose of this test, we used a Nikon 18-70mm, Cosina 19-35mm, Tokina 12-24mm and a Neewer wide angle adapter (this adapter is just one of several available) on a Nikon D90.
Our reason for choosing to test the Cosina lens in addition to the Nikon was due to the fact we were aware of the fall off on the edges on the Nikon with the adapter fitted.
The Cosina lens it is worth mentioning was originally designed for 35mm film cameras, whereas both the Nikon and the Tokina lenses are designed to only cover the smaller image circle of the APSC digital sensor.
The Neewer adapter is a 2 element lens, it screws into the filter of the main lens.
The camera was set on aperture priority, the lens was set at f8, at its widest focal length and used on autofocus.
Lenses and adapter used in test.
From left to right: Neewer adapter, Tokina 12-24, Cosina 19-35, Nikon 18-70
Whole frame images
Nikon 18-70
Nikon 18-70 plus adapter
Cosina 19-35
Tokina 12-24
Centre of image
Nikon 18-70
Nikon 18-70 plus adapter
Cosina 19-35 plus adapter
Tokina 12-24
Edge of image
Nikon 18-70
Nikon 18-70 plus adapter
Cosina 19-35 plus adapter
Tokina 12-24
Conclusion
What a mixed bag of results, we used the Nikon 18-70 without adapter as our benchmark.
Widest angle by a narrow margin is the Tokina 12-24, the Nikon plus adapter is close, but take a look at the fall off in the corners, crop this out and the Cosina 19-35 plus adapter takes the 2nd place instead.
Best centre sharpness is obviously the Nikon 18-70, the Cosina takes second place, followed closely by the Nikon with adapter, surprising is the poor center sharpness of the Tokina.
Edge sharpness honours go to the Nikon 18-70, followed by the Tokina, Cosina plus adapter in third place and in last spot a fair way behind is the Nikon 18-70 with adapter.
If only producing small prints, then either of the lenses with adapter or the Tokina 12-24 will be more than satisfactory. For larger images the choice is more straight forward, it is the Tokina 12-24, followed by the Cosina 19-35 plus adapter.