Want better looking video? It's not all about the Ks, it's all about exposure. In this tutorial, we'll dig in to how a digital video sensor works.
What is Exposure?
Exposure is the amount of light allowed to hit the image sensor of your camera while taking a photo or capturing video. In the days of film cameras, there was some kind of mechanism that was physically in front of the film. When you took a photo, that mechanism would move and allow light to hit the film for a period of time. Then the mechanism would move back into place to block the light once again. You would literally expose the film to light.
Most digital photo cameras work in a similar way, like a DSLR. However, smaller digital photo cameras, and almost all video cameras (except for ultra high-end cine cams), don't have a physical mechanism in front of this sensor and instead use an electronic shutter. With an electronic shutter, the camera reads the information from the sensor. Each pixel charges up at the beginning of the exposure. At the end of the exposure the camera discharges the pixels and takes a reading of each one. Both mechanical and electronic shutters function in a similar way. Longer exposure times results in a brighter image and more motion blur. Shorter exposure times results in a darker photo. For a photo, you are dealing with one exposure or perhaps a burst of exposure at a time. Video is a constant stream of exposures (called frames) any where from 24-60/second for "normal" looking video and 60-2500+/second for high speed video (slow motion).
The amount of times per second the camera reads and records the image is the frame rate.
In any given camera system, there are things that limit the amount of light that reaches the sensor. First, is the camera's lens and its aperture. All lenses will block some light. That's just the nature of optics. The amount of light loss from the glass is not something we have to think about right away. We do need to consider your lenses' aperture.
An aperture is a hole or an opening through which light travels. Almost all lenses on modern camera systems (except smartphones and many action cameras) have variable apertures and making the aperture smaller or larger will affect the amount of light that hits the sensor.
The size of the aperture is usually specified as an f number. A lens typically has a set of marked f stops that the f number can be set to. A lower number denotes a greater aperture opening which allows more light to reach the film or image sensor. A lens that has a maximum aperture of f/1.4 will let in about twice as much light as a lens that has a maximum aperture of f/2.8. Similarly, setting a lens that has a maximum aperture of f/1.4 to f/1.8 will let in about half as much light.
Aperture also controls the depth-of-field, which is the distance between the nearest and farthest objects that appear acceptably sharp in an image. More on this later!
The next part is shutter speed sometimes called shutter angle. The shutter speed is the amount of time your image sensor is exposed to light. Or in the case of an electronic shutter, the pixels charge up from light at the start of the exposure and then they are discharged and read by the camera at the end of the exposure. Like aperture, shutter speed (also called exposure time) is variable and affects the amount of light captured by the sensor. The shutter speed also affects how much motion blur is in your image. A shutter speed that is half your frame rate is considered "normal", creating motion blur that is similar to how our eyes see blur. For example if you are shooting in 24P, which is 23.976 fps, a shutter speed of 1/50 or 1/48 sec would look very natural. Increasing the shutter speed to 1/100 sec for example, would result in an image that has less motion blur, but is also darker because the sensor has less time to expose the image.
What Else Effects Exposure?
The last thing in a camera system that affects exposure is the sensor itself. You can think about this like the volume on your stereo. In a camera system it's called gain or ISO. All image sensors have a range of sensitivity they can be set to in order to properly expose the image. Just like your stereo, the when you turn the volume up you get more noise in the audio. The more your sensor is turned up, the more noise is added to the image and somewhat decreases the detail and dynamic range.
Let's review: the 3 things that affect exposure are aperture, shutter speed, and the image gain or sensitivity (ISO). Altering any one of those will not only change the exposure, but also change how the picture looks to some degree.
So, why is this important? It's important because when you light a scene, you're not lighting for your eyeballs. You're lighting for your camera system. Lighting for how your eyes see is sort of like theater lighting. If you light a scene in theater, you light for how your eyes see it and that works because everyone's eyes work about the same. Camera systems have many variables, so you have to consider the camera settings and exposure when you are lighting.
Let's Look at Some Examples:
The settings that I'm at right now on my camera system are f/2.0, 1/50 sec and ISO of 160.
Let's imagine I want to increase the depth-of-field to get more of this image in focus. To this I would change the aperture from f/2, down to f/2.8 and then to f/4. That is a change of two full stops. By closing the aperture I took away half the light, then half the light again, leaving one quarter of the original amount of light passing through the lens. (Note: often lenses can change in 1/2 stop, 1/3 stop, or continuously variable, but I will be using full stop changes because geometric counting—what we're doing now—is hard enough as it is and it's far easier to follow the math with full stops.)
Let's repeat: the light has been cut in half by going from f/2 to f/2.8, then cut in half again by going from f/2.8 to f/4. The result is 1/4 of the original brightness I had before. To make sure that I have the same exposure without adding more light, I need to increase the ISO.
In the previous image the ISO was set at 160. In this image, the left side has been set to ISO 320 which will get us halfway there, and the right side has been doubled again to ISO 640, which will be the same exposure that I had before. Now more of the image is in focus, because the camera lens is stopped down, which increased our depth-of-field.
So let's go down even further with the aperture to try and get a little more depth-of-field. Let's take the aperture from f/4, all the way down to f/8. That's another change of two full stops. To get the exposure back to where it was before, I've increased the ISO again two full stops, by doubling it to ISO 1250, and then doubling it again to ISO 2500. Although a lot more of the fan is in focus, there is also a lot more noise in the image.
Let's reset the exposure to f/2, 1/50 sec and ISO 160.
What I want to do now is increase the shutter speed (ie. the exposure time) to reduce some of the motion blur in the fan. I'm going to reduce the exposure time from 1/50 of a second to 1/200 of a second. That's effectively reducing the exposure to 1/4 of the brightness.
To compensate, I need to bump the ISO up again. I've changed from ISO 160, to ISO 320, and then to ISO 640, (that's two full stops) because we lost two full stops of light by increasing our shutter speed.
Need even less motion blur? Here, I reduce the exposure time (shutter speed) from 1/200 down to 1/800 sec. That is effectively a two stop reduction in exposure. So again, I need to increase the ISO two more stops from ISO 640 to ISO 1250, and then from ISO 1250 or ISO 2500.
The Relationship Between Aperture, Shutter Speed and Image Sensitivity
Let's reset the image one more time to f/2.0, 1/50 sec, and ISO 160.
For this final example, I will mix up the adjustments to the exposure by changing the shutter speed from 1/50 to 1/100 and going from f/2 to f4. The change in aperture from f/2 to f/2.8 to f/4 is a two stop reduction in light. Changing the exposure time from 1/50 to 1/100 is a one stop reduction. Therefore, to compensate, I need to increase the ISO by thee stops. My starting point is ISO160 so a three stop increase will be ISO1250.
If you make a change to aperture and you didn't want to compensate with shutter speed, and/or ISO, you would have to make a change to the amount of light light in the scene to maintain the same exposure. For example, if you changed your aperture from f/2 to f/4, you will have to increase the light by two stops. That means doubling the light intensity and then doubling it again! Very easy to do with a flash or strobe (photographers have it easy), not always easy with constant lighting sources.
Understanding this relationship and how your camera works is critical for getting a proper exposure. Some of you may be thinking, "These new camera systems have excellent ISO sensitivity with ever decreasing noise." Well yes, this is true. And it's definitely a lifesaver when you can't light or you don't have time to light something. But in general, noise is not a great thing. The less noise you have the more detail you have in your image.
Now Go Set Up Your Camera and Experiment!
To make these concepts stick, set up a similar experiment at home. Find a fan, or something that moves quickly (with a consistent speed) and run through these types of issues. Adjust your aperture and then compensate with shutter speed and/or ISO. Do the same for shutter speed and compensate with aperture (if you can) and ISO. This will help you master the concept of exposure and lighting!