Lenses have an f-stop setting at which they will take their sharpest image. To see which f-stop is the sharpest, take 4 or 5 shots of the exact same subject. Start at the widest aperture (smallest f-stop number) and decrease the aperture by one f-stop with each shot. In your editing software, zoom in 200%-300% and look at a sharp line in the image. The photo in which the line is the sharpest will be the optimal f-stop number for the sharpness of that lens. This is not the same as depth of field.