DSLR Filmmaking: How to Control Aperture for Effect
Your aperture setting only allows a select amount of light into the DSLR lens. This is useful when filming. Think of it as a doorman at an exclusive nightclub. When you dictate aperture selection, you control the level of focus in the scene much like the guy behind the velvet rope selects who gets in.
Aperture refers to the opening on your camera’s lens and how much light it allows onto the sensor. Under low-light conditions, the aperture, or iris (take your pick, they are essentially the same) accommodates more light into the lens by widening the opening. Conversely, brighter-lit situations require a narrow setting, altering the area of focus in the scene.
Depth of field in filmmaking
When you focus the camera on a desired point in the frame, the area of sharpness varies. That variation refers to the depth of field in the scene. Sometimes it appears as though the entire image is in focus, and other times only a narrow area escapes the blur.
This level of focus in the scene depends on several factors including subject distance, focal length, and, of course, aperture setting.
When the aperture is wide open (letting in the most light into the lens), the level of focus becomes extremely limited.
Known as a shallow depth of field, this level of focus doesn’t usually go beyond the focused area of the subject. In a portrait, the subject appears in focus, whereas the rest of the scene appears soft or even blurred. This works well for situations where emphasis is paramount. It’s also referred to as selective focus.
Stopping down the lens (letting less light in by using a higher aperture setting) provides a greater depth of field. This works best with brightly lit situations when you wish to render every part of the frame in focus.
DSLR reciprocity at work
The democratic nature of exposure provides a reciprocal balance between the amount of light that comes into the lens and duration the lens stays open. In photography terms, this refers to aperture setting and shutter speed, respectively.
Reciprocity defines the balance between shutter and aperture in regard to creating exposure. The sensor sees exposure in its purest form. But, you can alter the shutter or aperture setting to accommodate your needs. When you open the aperture of the lens, say from F5.6 to F4, you need to limit the duration by one stop, going from 1/30 to 1/60.
Thanks to reciprocity adjustment, you can adjust the aperture to open up the lens to limit depth of field, or stop it down to increase it. Of course, the shutter speed must be changed to compensate.
Be sure to have the camera in the full manual mode because automated DSLR automatic modes like Shutter Priority do not work in the movie mode. The automatic setting selects everything including the ISO. If you want to control depth of field, you’ll need to shoot in manual mode.
You can capture a low-light subject that requires a little more exposure by using a lower shutter speed like 1/30, or a fast-moving subject that requires a higher shutter speed at 1/250. However, for most situations, it’s a good idea to adhere to the 180-degree shutter rule that basically means you should strive to keep your shutter at double your frame rate.
Factors that affect DSLR depth of field
"One size fits all" does not necessarily apply to depth of field. In principle, a wide-open lens provides a shallow level of focus, whereas a stopped-down lens increases it. As for predictability, factors such as subject distance, focal length, and overall illumination play their parts in the amount of focus between the foreground, background, and subject.
Here are some things to consider about depth of field:
The closer you are to the subject, the more focus differs at foreground and background points.
Wide-angle focal lengths provide a much greater depth of field.
Telephoto lenses produce a more narrow depth of field.