I have just shot a long outdoor trip using the Sigma 17-50mm F2.8 EX DC OS HSM on a Canon 70D. I've read excellent reviews of the Sigma lens but the photos didn't turn out that great. Neither the image quality nor the sharpness is great. I mostly shoot in aperture priority mode.
I am not an expert photographer but I have been using a DSLR for a couple of years. Previously, I used the Nikon D3200 mostly with a 35mm f/1.8G lens and was quite happy with the pictures I got.
I think something is wrong with the 70D, or the Sigma lens, or the combination. I bought the lens used. I also get quite nice pictures with the Canon EF 50mm f/1.8 STM.
A landscape jpeg straight out of camera can be found here. Single point AF on the central pillar.
Exif: 20 mm, 1/40 sec, f/16, ISO 100, -0.33 EV
In response to the issue of diffraction, I post another image. Single point AF on the red flowers in front.
Exif: 18 mm, 1/80 sec, f/10, ISO 100
Can anyone suggest how to check the camera and the lens combo?