Resolution
at f5.6
Nikon
15mm f3.5 AIS v Sigma 12-24mm f3.5-4.5
100%
Crops (f5.6) Centre image circle
 |
 |
| Nikon
15mm / f5.6 |
Sigma
12-24mm / 15mm
/ f5.6 |
Despite
the immediately obvious difference in colour - neither are neutral
- centre frame the Sigma zoom is clearly resolving better than the
Nikon prime. However, recent testing of Nikon's
14mm made me wary of trusting a centre frame crop alone. Wide
open, the 14mm has a weird toroidal focal pattern that surrounds a
soft centre frame with a ring of higher resolution
before gradating back to poorly resolved corners. So it proved to be
with the 15mm.
100%
Crops (f5.6) c17mm from image circle centre
 |
 |
| Nikon
15mm / f5.6 |
Sigma
12-24mm / 15mm / f5.6 |
17mm
from the image circle centre, the Nikon looks much sharper, entirely
reversing the centre frame results. The Sigma behaves as we might expect:
resolution and light falling off in proportion to its distance from
the centre of the image circle. The Nikon is playing by different rules
altogether. As we move out to the extreme corners, however, we see
the intial results reprised:
100%
crops (f5.6) extreme upper right corner
 |
 |
| Nikon
15mm / f5.6 |
Sigma
12-24mm / 15mm / f5.6 |
Allowing
for the fact
that the Sigma 12-24mm is inherently not a very high resolution optic,
a healthy one turns in commendably consistent corners even at wide
apertures. Or is it just that it looks good next to the Nikon 15mm?