There's an old photographer's rule of thumb that states the sharpest aperture on a given lens can be found about three stops from wide open. That means on a lens with a maximum aperture of ƒ/2.8, the sharpest aperture is likely to be around ƒ/8.

How do I know which aperture is sharpest?

Find the Lens' Sweet Spot

The sharpest aperture is when the overall image is at its sharpest. The sharpest aperture of your lens, known as the sweet spot, is located two to three f/stops from the widest aperture. Therefore, the sharpest aperture on my 16-35mm f/4 is between f/8 and f/11.

How do I find the best aperture?

If you care about detail in your corners – as most landscape photographers do – I recommend finding the aperture with the highest “corner” value. In the chart above, that would be f/8, although f/5.6 is close (Remember that the “corner” value only applies if you use a full-frame camera.