I’ve been disappointed by the image quality of my landscape photographs from time to time, where the images viewed at full size on my computer screen at home are lacking in detail and very soft. However, I’ve also noted that the results vary from shoot to shoot and occasionally from shot to shot. Having reviewed the images, I found that the poor results were all shot in bright light, when I used a small aperture in order to attain maximum depth of field and detail in the image.
In the example of the image accompanying this blog post, one would expect that a smaller aperture like f/22 would be the right one to use; ensuring that everything is sharp, detailed and in focus from the snow in the foreground to the mountains in the distance. However, I decided to take a range of shots between f/4.5 (the widest possible using the Sigma 10-20mm at the zoom level I chose) and f/25 (the smallest aperture). As you can see from the following comparison, which shows a crop at 100% of the same view photographed using f/5.6 and f/25, there is a very wide difference in sharpness.
My test has shown that for the Sigma 10-20mm EX DC HSM used here, an aperture of between f/4 and f/8 produces the best results. Combined with the extreme wide-angle view, and consequent exaggerated depth of field, this only presents a problem when I need to gain an unusually larger depth of field in a landscape. (For example a foreground rock a couple of metres away and a distant mountain range.)
I’d recommend that if you’re using an SLR – whether digital or not – you do the same tests with each of your lenses in turn. If image sharpness is important to you, try to use the optimal aperture for that lens whenever you can.