Remember, a shallow depth of field is often desirable as it can produce a more interesting, film-like result, whereby the subject of the frame is in clear focus vs. the background - allowing the viewers' eye to more easily focus in on the primary subject. The first step in learning to control the depth of field is understanding aperture and determining what f-stop is appropriate for your shot.
F-Stop, Aperture, and the Lens Iris - Explained:
So, what is an f-stop? Simple, the term f-number or f-stop is used to define the ratio of the focal length (the distance from the sensor to the rear of the lens) to the diameter of the aperture, as controlled by lens' iris. OK, that may not sound simple, but in all honesty, it's basic photography 101 and it is critical to understanding how an image is exposed. We'll break it down for you.
Inside of your DSLR camera's lens, is a mechanism called the Iris. The Iris has an adjustable opening that controls the amount of light which can enter the frame and be processed by the camera's sensor. That opening itself is called the aperture. Imagine your eye is the lens and the pupil is the aperture. Your eyes' iris changes size automatically in different situations to prevent your retinas from becoming over exposed to sunlight. In a similar way, you can adjust your camera lens' iris to allow for more or less light to enter through the aperture.
The aperture is adjustable in increments, known as stops, f-stops, f-numbers, f-ratio and each f-stop allows for half as much light as the previous one (going right to left) on the F-Stop scale.
The standard f-stop scale starts at 1 (not seen in the illustration above) and can go all the way up to 32 on a normal scale, with the amount of light decreasing as the number rises.
If you remember anything, remember this.
- A larger f-stop value will mean a smaller aperture, and will result in less light entering the lens.
- A smaller f-stop value will result in a larger aperture, which will allow for more light.
So, the larger the f-stop, the more light and vise-versa. An f/2.0 will let in 10 times more light than an f/11.
How Does F-Stop Affect Depth-of-Field?
As the size of the aperture changes, the angle of light hitting the sensor also changes and it is this that affects the depth of field. The simplest way to think about it is that if you are looking for a large depth of field, whereby more of the image is in focus (example - landscape shot), you will want to close down the iris, which will give you a smaller opening/aperture (larger f-stop) and allow for less light to enter. If you are looking for a shallow depth of field, you'll want to open that lens wide to allow for as much light as possible.
- Large F-Stop = Small Aperture = Large DOF
- Small F-Stop = Large Aperture = Small DOF
Using a DSLR to shoot video gives you a lot of simple options for maximizing your aperture so as to keep that depth of field nice and slim. First, you'll want to put your DSLR into manual mode in order to control the aperture f-stop. If you want it as wide open as the lens will allow (smallest f-stop), you will let in as much light as possible. From here, you'll need to determine the proper shutter speed and ISO sensitivity (which is another post in itself). Suffice to say, if you open the f-stop all the way, you may need to either lower the ISO or increase the shutter speed to keep your shots from being blown out. It’s a fairly simple process, but it may take some tweaking. As with most things, practice makes perfect.