Understanding Exposure Part 1

Getting the right exposure is one of the key elements to creating quality video. Exposure refers to the amount of light that is allowed to fall on the film, or in this case, the camera’s sensor. Use Goldilocks’s method of choosing, and make sure you don’t let too much light in (overexposure) or let too little light in (underexposure), and instead make sure the exposure is just right.

How to get the proper exposure?

Well first I need to explain the three components that make up exposure:
1. Aperture – the adjustable size of the lens opening
2. Shutter speed – the length of time the lens shutter is open
3. ISO – light sensitivity

These three elements are usually referred to as the exposure triangle. Imagine a stable and balanced triangle as the proper exposure. Now say someone comes along and moves one of the legs of the triangle. The other two legs must adjust their angles/lengths in order to make the triangle stable again. Similarly, if you make a change to one of the three exposure elements (aperture, shutter speed, ISO) you need to adjust the other elements to regain the correct exposure.
exposuretriangle


Two metaphors to understand aperture, shutter speed, & ISO

Metaphor 1: The window

Picture by Mr. Wabu

Picture by Mr. Wabu

Suppose it’s a nice sunny day outside, but you’re trapped in a dark cold room with only a window with its shutters closed. You’re cold. You can’t see a thing. You want some light! In this scenario, you’d be the camera’s sensor. Now say someone opens the window’s shutters for a few moments and then closes them again so that you were exposed to the light ever so briefly. The amount of time the window shutters were opened would be the shutter speed. The longer the shutter speed, the more light that you as the sensor are exposed to.

Now say that the size of the window was actually so small that only a pinhole amount of light is able to get through. That wouldn’t be enough light for you to see anything even when the shutters were open. On the other hand, if the window was of the floor to ceiling type, once the shutters are opened, you’d be exposed to way more light than the pinhole sized window. The size of the window is like the lens’s aperture.

Finally, let’s say there was tint on that window. The darker the tint, the less light will go through and the less sensitive you will be to the heat of the sun. Lighter tint is comparative to higher ISO. Less tint means a higher sensitivity to the sunlight and heat you will feel. This metaphor is not perfect, since exposure has nothing to do with heat. So let’s try another metaphor.


Metaphor 2: An eye

Using the eye as a metaphor, consider the aperture as when you squint as compared to when your eyes are wide open. When you’re squinting, you’re making your eyes smaller so to speak so less light is coming into your eye. As an Asian, I’m stuck with a permanently small aperture.

Continuing on with the eye metaphor, shutter speed would be considered the length of time in between eye blinks. The longer you can go without blinking, the more light is exposed to your eye. Mind you, I don’t recommend seeing how long you can go without blinking.

Lastly, ISO would be your eye sensitivity. For example, when you first wake up in the morning your eyes are very sensitive to the harsh light demanding that you wake up. This would be like a high ISO. Your eyes are very sensitive to the light and everything just seems so much brighter. But you look up to see your significant other standing over you and glaring at you for sleeping in whereas he or she has been up for a couple of hours. His/her eyes would be like having a lower ISO. Their eyes have been desensitized and are not so affected by that same sunlight that you are finding so harsh.


Alright enough with the metaphors, time to get down to the technical. As you can see, the aperture, shutter speed, and ISO all contribute to “the amount of light that is allowed to fall on the sensor” (ie exposure). With all that said, it’s not always easy to achieve the correct exposure especially if you’re just going off the back of the tiny camera LCD. Secondly, cinematography is an art not a science, so there may not even be such a thing as perfect exposure. It depends on what aesthetic you’re trying to achieve. There may not be a “perfect” exposure but there is a correct exposure so that everything you want to be visible is visible and not too dark or blown out. However if you’re unsure, it’s said that it’s better to be underexposed rather than over exposed for video. This is the opposite of photography in which the lessor of two evils is to overexpose. In video post processing, it’s easier to bring footage back up from underexposure as opposed
to bringing it down from overexposure.

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