When we take a landscape photograph, well, any photograph, we always aim to obtain a clear and well-defined image. However, there is an invisible enemy constantly working against us: diffraction.
What is diffraction?
Diffraction is a physical phenomenon that influences the movement of light waves when they encounter an obstacle, such as when they pass through a small aperture. Essentially, when a wave passes through a hole, its path deviates according to the size of the hole.
So how does this physical phenomenon affect the world of photography?
The answer is down to a fundamental mis-match: the light that passes through your lens is a wave, and the aperture is a hole. The result then is that light travelling through the aperture will not maintain its original path, and this will result in a loss of image sharpness.
Diffraction and depth of field
In landscape photography one of our goals is always to maximise the depth of field, and to do this we have learned that the more we close the aperture, the more the depth of field increases. If diffraction did not exist, to maximise the depth of field we would use apertures equal to f/22 or even higher. Unfortunately, the more we close the aperture the more we impact the wavelength of the light, increasing the phenomenon of diffraction and therefore loss of sharpness in the image.
The optimal aperture
So what is a good compromise between aperture and depth of field so as not to suffer too much from the effects of diffraction? In landscape photography, a good reference value for the aperture is f/11. This particular aperture is optimal because if used together with the hyperfocal technique, it guarantees that you can use wide-angle lenses with a high depth of field, without being particularly affected by the negative effect of diffraction.
This does not mean that higher aperture values cannot to be used, but should only be used when it is really needed; Such as when we have a really close foreground and need to further extend the depth of field, or when the light conditions are too intense and we need to stop the light down. In any case, it is good to never exceed the aperture value of f/16, otherwise the effect of the diffraction will be too intense and we would probably compromise the quality of the final image.