Depth of field is a very important tool in photography for creative choices of composition. For example, a landscape image may need sharpness of details from near to far whereas a portrait may only show the person’s face in sharp focus and everything in front and behind blurred. Since the eye goes to the sharpest parts of an image first, the photographer can guide the viewer through the image by placing sharpest areas where they want the viewer to pay most attention.
Any casual reader of photography books will at some point learn that depth of field is a function of lens aperture; i.e., F stops. Smaller f/numbers (f/2.8, f/4.0) will give narrow depths of field whereas larger apertures (f/16, f/22) will give wider depths. When 35mm film was the basic medium for SLR photography depth of field was engraved on the lens, enabling a photographer to focus, set aperture, and look at the scale to know how close and far objects would be in focus.
Digital camera sensors now range in size from 35mm film equivalent to tiny sensors in point-and-shoot cameras. What are the depths of field for all these sensor sizes? It turns out for equivalent lens focal lengths and distance to subject there is a relationship of depth of field to sensor size. This, along with other aspects of the subject, are covered in an article by Zeiss, the German lens design company.
Here’s the summary for this particular aspect. This table shows f/numbers for equivalent depths of field (same focal length, same distance to subject) based on sensor size:
Now I understand why I’ve been getting more depth of field in my Olympus digital lenses than I got with my film ones. I usually set my aperture to f/8 since it gives the sharpest images but the depth of field always seems too wide. Makes sense since it’s like shooting at f/16! This also explains why APS and 4/3rds users keep hooking older 35mm film lenses to their cameras – they can get shorter depths of field. A f/2 film lens on a 4/3rds sensor is like having a f/1 lens, which doesn’t exist yet for these cameras.
For a landscape photographer who wants everything in focus this is not of much interest just set the aperture at a high f/number and shoot. For portrait photographers or anyone interested in creative uses of depth of field, keep in mind your sensor size will affect your composition and adjust aperture accordingly.