I have previously been quite happy with the sharpness of the 30mm lens, so I am curious to see how the 19mm lens performs. It makes sense to compare it with the Lumix G 20mm f/1.7, with a similar focal length.
The test images were taken with the GH3 camera at ISO 200, on a tripod, and with self timer. The focus was set on the centre of the frame.
|Lumix G 20mm @ f/2.8||Sigma 19mm @ f/2.8|
Here are 100% crops from the centre of the image frame at various apertures:
And from the lower left corner:
We see that the Lumix G 20mm f/1.7 has an impressive level of sharpness, even wide open. The Sigma 19mm, on the other hand, needs a bit of stopping down before reaching the same level of sharpness. At f/2.8, the 19mm lens is a tad bit dull, even in the centre of the image frame.
This finding is consistent with other tests I have seen. Generally, it is observed that the Sigma 19mm lens is not the sharpest at f/2.8, and improves when stopped down to f/4 and f/5.6. Stopping down beyond f/5.6 does generally not add anything to the overall sharpness, but does give you more depth of focus (DoF). If you need a deep DoF, it may still be wise to stop down to f/8 or even further, but this will give you slightly worse sharpness at pixel level.
Still, it makes sense to say that both lenses are sharp. Comparing the Sigma lens with the Lumix G 20mm f/1.7 is perhaps a bit unfair, since the latter is known to be very sharp. For most uses, the Sigma lens is certainly sharp enough.
On the positive side, the Sigma 19mm f/2.8 EX DN appears to handle flare a tad bit better. There is less flare effects around the strong highlights.
My experience also confirms that the Sigma 19mm f/2.8 EX DN has a much better autofocus performance. The focus operation is also less noisy.