Depth of field is a massively misunderstood “side effect” of iris size, and an ultimately useful storytelling tool (when used properly).
DSLR photography has spawned a large group of cinematographers who are dealing with largely light-insensitive camera sensors (usually producing fairly unacceptable noise when used at greater sensitivies than ISO 800). This has created a need for “fast” lenses, which are lenses which have very large maximum apertures, featuring maximum f-stop ratings like f/2.0, f/1.8, and even f/1.4 (there are even some f/0.7 lenses out there, but I’m sticking to the realm of affordable DSLR cinematography, at the moment). Using firmware like Magic Lantern allows us to use focus peaking to exploit manual-focus and non-EF mount lenses (for those Canon-philes among us), to bring down the effective cost of shooting “fast” lenses. I have some M42 “Pentax screw-mount” lenses which open to f/2.8 – acquired for less than 10 USD each, plus a 7 USD M42-to-EF adapter ring. If you’re interested in more information on how to use focus peaking, check out how it works.
So, you’ve beaten high-cost DSLR and cinema camera body manufacturers by using “fast” lenses, right? Well, not so fast, there… You have to consider the side effect of fast lenses and wide apertures: the shallow depth of field. It’s both a blessing and a curse; we tend to associate shallow depth of field images and video with expensive equipment (rightfully so, in most cases) and a more personal type of image, but those lenses are not universally usable “wide open”. Many shots come off looking amateurish and ill-composed, when half of the subjects are out of focus, because they’ve exceeded the edges of the “sharp” area in the focus range.
(There’s also lack of lens sharpness and chromatic abberation to consider, which are generally present, to some degree, in the widest aperture settings available on most lenses. That’s a very intensely technical discussion, however, and will probably be reserved for a future posting.)
When considering the depth of field you want, consider the subject (or literal focus) of the shot, and figure out how much of the shot needs to be in focus to properly tell your story. The depth of field is very important for exposing the mise-en-scène in the way which best represents both the explicit visuals and external representation of internal character development and exposition. If your character needs to “pop” out of the scenery, a narrow DOF would be perfect – but if they are to appear as a figurative cog in the machinery of the world, you’re probably going to be looking for the widest DOF (approaching infinity) as you can get. When widening the DOF like this, you’ll most likely have to compensate by boosting the ambient lighting, or adding additional lighting, to adjust for the decreased light hitting your camera sensor.
There are a number of tools for figuring out the proper DOF for a certain shot, based on the camera/sensor, lens focal length, and distance to primary focal point.
There are more of them, but if you specify the variables, you can adjust the aperture f-stop to the widest setting which properly fits the scene and visuals which you’re attempting to achieve.
Like anything else, this isn’t strictly an instructional A-to-B guide on how to achieve stunning visuals; it’s simply an attempt to share some of what I’ve learned, and to help share the knowledge and tools to exploit your own inner artistic vision. Good luck shooting!