f/16

f/16 holds everything. A stone at the camera and a mountain behind it are both sharp, which is why landscape work lives here.

The cost is uniform. Diffraction softens the entire frame rather than one part of it, so the picture gains depth and loses bite at the same time.

The band of sharpness on a wide lens runs from under a metre to the horizon, so there is nothing left to focus on in the usual sense.

Diffraction is the limit. Light bends around the edge of the iris, and the smaller the hole the more of the image it blurs. On a digital sensor this is plainly visible at f/16.

The exposure rule of thumb belongs to this stop. In bright sun, f/16 with a shutter close to one over the ISO gives a correct exposure without a meter.

Use it outdoors, in strong sun, for landscapes and for frames where a near object and a far one both have to read.

Skip it on any interior and on anything shot for detail. There is not enough light, and the softness will show.

f/11 gives almost the same depth with less diffraction, so it is the better choice for most deep frames.

The test is texture at the centre of the picture. If the sharpest part of the frame is slightly mushy you are at f/16 rather than f/11.

Landscape photographers stop down this far to hold a foreground rock and a distant ridge in one exposure. The sunny 16 rule comes from the same place: bright daylight is one of the few conditions that pays for the stop.

The model blurs the background anyway. Ban shallow depth of field, name the wide lens, and say the frame is sharp from the near foreground to the horizon.

The other failure is a clinical, over sharpened frame. Ask for the slight overall softness that comes with the stop, or the shot reads as a composite.