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.
View The Full Video Transcript:
Hey I’m Stephen Schweickart with this episode of ReelSEO’s ReelRebel. We’ve given you a brief rundown of how to crush that depth of field, now let’s focus on one of those elements, the F-stop.
Here’s a lot of words for you so be sure to take notes. On a lens, the f-stop number affects the size of the lens’ aperture by controlling the iris. Got it? No. OK here’s a simpler explanation. Imagine your eye is the lens and the pupil is the aperture, or size of the opening letting light into your eye. Your iris changes size in different situations to make sure your retinas (I have no idea where those are so I can’t point to them) don’t get fried from too much sunlight. On your eye, the iris controls itself automatically, but on a camera it’s controlled on a scale of numbers called the F-Stop scale. And it’s very important to understand how the scale works in order to keep control over your image.
So let’s take a look at it, shall we? The scale starts down here at one and can go all the way up to 32 on a normal scale, with the amount of light decreasing as the number rises. So an f/2.0 will let in more light than an f/11. That part is pretty straight forward. Smaller numbers equals more light, which you know from our last depth of field video equals shallower depth of field. You may think the numbers on this scale are arbitrary, but they actually represent the ratio between the size of the aperture, and the focal length of the lens, which is the distance from the aperture to the film plane where the image is captured.
Using a DSLR to shoot gives you a lot of simple options for maximizing your aperture so you keep that depth of field nice and slim. First, put the camera on Manual. If Terminator taught us anything it’s that you can’t trust machines and they may try to murder you in order to keep your unborn son from leading the rebellion in the near future. Not sure what that has to do with video, but nonetheless putting the camera on Auto is the first step towards that future, do you really want that?
Anyway, putting the camera to manual let’s you first control the f-stop, which for now we’ll assume you want as wide open as the lens will allow, letting in as much light as possible. From here, DSLRs have a handy dandy feature that let’s you select what ISO or sensitivity you want to shoot at. Now pay attention, this can get tricky. While the lower the f-stop the more light you get, the HIGHER the ISO the more light you get. So with the f-stop all the way open, you’ll likely need to lower the ISO to keep your shots from being blown out. It’s a simple process, but it will take some tweaking.
PHEW! That was a lot of information, but once you process all of that mess you’ll have a better understanding of f-stop, aperture, and iris, and ultimately will achieve ultimate control over your image. We’re talking about your camera image, not your personal image... that one, not salvageable.
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