Put three small objects along a diagonal on a table, with one near the camera, one in the middle and one farther away. Labels, textured cups or folded paper shapes work well because their edges are easy to compare. Keep enough space between them to see differences in sharpness, but make sure all three remain inside the frame.
Support the camera and focus on the middle object. Take photographs at three apertures, such as f/2.8, f/5.6 and f/11 if your lens provides them. Keep camera position, focal length and focus point unchanged. Adjust shutter speed to maintain similar brightness and use a self-timer if the longer exposures could be disturbed by pressing the release.
What the comparison actually shows
The middle object should remain the point of best focus, while a smaller aperture makes a larger region around it look acceptably sharp. It does not create a sharply bounded slab in space. Sharpness falls away gradually, and whether you notice that fall depends on image size, viewing distance and how much fine detail the objects contain.
At the smallest apertures, diffraction can reduce fine detail throughout the image. This means that the frame with the greatest depth of field is not necessarily the one with the crispest central texture. Compare the whole arrangement at a normal viewing size before inspecting individual pixels. A useful photograph may trade a little peak sharpness for clearer front and rear objects.
Now move the camera closer and repeat the middle aperture, refocusing on the same object. The tighter view will generally make the limited depth more obvious. Do not confuse this with simply cropping the original image: changing the camera position changes the relationship between near and far objects as well as the framing. Record the position of the camera so that you can repeat either setup.
A useful second arrangement is a row of objects running straight away from the lens. Put a clear mark on the object you focus on, then examine how softness develops in front of and behind it. Do not expect equal visible distances on both sides in every setup. The relationship changes with focus distance and the criteria used to call something sharp. This is why a fixed rule about placing focus one third into any scene can mislead.
Repeat one frame with the middle object turned so that a flat labelled face points directly towards the camera. Then turn it obliquely without moving its centre. The label may now extend beyond the sharp region even though focus has not moved. For photographing documents, products or a collection of small items, alignment can therefore solve a problem that looked like a need for more expensive equipment.
If the camera offers depth-of-field preview, compare that view with the captured file, but allow for a dimmer preview at small apertures. The finished image remains the reference. Store both the scene photograph and a short note describing which variable changed; otherwise similar-looking tests can become difficult to interpret later.
Finally, arrange the three objects at roughly the same distance from the camera. Even a relatively wide aperture may now hold all three recognisably sharp. This is why moving a subject can be more effective than closing the lens as far as possible. For a product arrangement or several faces, bringing important details closer to a common focus plane can simplify the exposure.
Keep the comparison files together and label them with aperture and setup, rather than judging them from memory. The exercise is most useful when you can see which change produced which result. It also gives you a practical reference for your own lens and output size, without pretending that a single aperture number guarantees sufficient depth in every photograph.
