For people wanting to see the
side effects of diffraction, when decreasing aperture to increase depth of
field, I attached also a detail of a
Roman (? do you think it's authentic?) die taken with aperture F/4 and F/32 (I exaggerated a
bit in using extreme aperture values in order to highlight the problems). My explanations may be wrong, so experienced photographers are welcome to correct me
As you see in this
Roman die, with "optimal" (at least given a few experiments I did and the natural light I
had when taking the picture) aperture (F/4) and shutter time, only a very limited proportion of the object may be in focus at very high magnification (the die was almost completely filling the frame).
A relatively fast and easy way increases the DOF is to increase the f-number (reducing the aperture
diameter), if your camera allows you do so (as any reflex camera). However, reducing the aperture also reduces the amount of light arriving to your film or sensor, so that the
quality of the image and of the colors may not be optimal.
You may try to compensate this using more light to illuminate your subject and/or you may try to process the picture with an image editing tool later on, but it is not always possible to obtain a satisfying result (see the die at F/32) and – in any case – you will loose some details (is it so? Yes, because of diffraction, that is increased when light is passing through a smaller hole. “Diffraction is the name given to the observation that when light squeezes through a small opening it tends to spread out”)
In the big image to the left (F/32) the depth of
field is very high, but (as you see in the zoomed image, above
part) with created some problems in term of diffraction (which are visible only at high magnification, but
still problematic if we want to create a poster of our
Roman die!).