Lesson 5 of 9 · 12 min
Exposure
Exposure is how bright the picture is. Four controls set it, and each one also changes the look of the picture. The skill is not only to get the brightness right. It is to choose which control to use, because of what it does to the image.
In this lesson
- Understand a stop: double or half the light.
- Use aperture, shutter angle, ISO and ND filters together.
- Know the side effect of each: focus, motion blur, noise.
Try it Expose the shot
A runner crosses the frame. The camera records 24 frames each second. Choose a place, then set the four controls until the meter needle is near 0. Watch what each control does to the runner, the background and the grain.
- Sunny street with shallow focus: f/2.8 or wider, 180°, and a correct exposure. (You will need an ND filter.)
- Night street with a correct exposure. What did it cost?
- Freeze the runner with a 45° shutter.
The name for it The stop
A stop is a doubling or a halving of light. Every exposure control is measured in stops, so you can trade one for another. Close the aperture one stop and the picture gets half as bright. Double the ISO and you get that stop back.
- Aperture f-stop
- The size of the hole in the lens. Each full stop: f/1.4, 2, 2.8, 4, 5.6, 8, 11, 16. A smaller number lets in more light. Side effect: depth of field.
- Shutter angle
- How long each frame is exposed, as part of the frame time. 180° at 24 fps is 1/48 s. Half the angle is half the light (one stop). Side effect: motion blur.
- ISO
- How much the camera amplifies the signal. Double the ISO, one stop brighter. Side effect: noise and less detail in the shadows.
- ND filter
- Neutral density: dark glass in front of the lens. It cuts light with no other effect. Every 0.3 of ND is one stop (ND 0.9 = 3 stops).
This is why cinema crews carry ND filters. In sunlight, a cinematographer who wants shallow focus at f/2 and a 180° shutter has far too much light. ND takes it away without changing the look.
A light meter tells you the correct exposure. The histogram and false colour on a camera monitor show where the bright and dark parts are, so you can see when the highlights are about to clip (burn out to pure white).
Correct exposure is a choice, not a number. A night scene can be dark on purpose. "Correct" means the parts you care about are where you want them, and nothing important is lost.
Check Four questions
You go from f/4 to f/5.6. What happens to the light?
Bigger f-number, smaller hole. One full stop is half the light.
At 24 fps with a 180° shutter, what is the shutter speed?
180° is half of each frame's time: half of 1/24 s is 1/48 s.
It is a sunny day. You want shallow focus and normal motion blur. What do you add?
ND cuts the extra light, so you can keep a wide aperture and a 180° shutter.
Which control adds noise when you raise it?
ISO amplifies the signal, and the noise with it.
Field exercise Watch the blur
Watch a fight or battle scene and a dance or romance scene. Pause during fast movement. Is the motion sharp and jerky, or soft and smeared? You are seeing the shutter angle the cinematographer chose.