Exposure basics: Shutter speed

- Over the next three movies we're going to cover the very basics of photographic exposure theory. If you're new to photography it is critical that you watch these movies. If you're already experienced with exposure theory then you should skip ahead to the movie "How is Exposure Different on a Smart Phone?" At a fundamental level all cameras are basically the same. They are lightproof boxes that contain a material that is sensitive to light. In addition they have a way of controlling when that material is exposed to light and for how long. Your smartphone is also a lightproof box that contains a piece of light sensitive material alongside a massive super computer, a GPS radio, a cell radio, a Wi-Fi radio, a Bluetooth radio, some accelerometers, possibly a barometer, a battery with a charging system, and so on. But, when it comes to the camera part of your smartphone, it is fundamentally the same as cameras going back 150 years. The light sensitive material in your smart phone is just like the light sensitive material in any other digital camera. It's a chip covered with a bunch of tiny pieces of a special type of metal. That metal releases electrons when it's exposed to light by measuring how many electrons are released across the surface of the chip the onboard computer can build up an image. Both digital sensors and film have something in common. The longer that you expose them to light the brighter the resulting image becomes. And as the image gets brighter the brightest areas begin to lose detail. Your eye works the same way. If I shine a really bright light in your eye, your eye overexposes and you lose the ability to perceive detail in the brightest parts of your field of view. Now you probably don't notice that because while it's happening you also experience tremendous pain and so you slam your eyes shut. We call an image overexposed if it is so bright that we have lost detail that we would like to have in the brightest areas of the image. Conversely, if we don't expose the sensor for long enough then the resulting image will be too dark and we will be unable to see details in the darkest parts of the image. Somewhere inside most normal cameras there is a mechanism called a shutter. Basically a door that opens and closes to expose the sensor to light. The longer the shutter is open, the more light gets through to the sensor and the brighter the resulting image becomes. We refer to that time that shutter is open as the shutter speed and we measure it in seconds. In a well-lit room or outside on a bright day, a typical shutter speed will fall between 1/60 and 1/250 of a second. On this iPhone 11 Pro the shutter speed can go as fast as 1/125,000 of a second on the front-facing camera and 1/45,000 of a second on the rear camera. Or on either camera as slow as one second. Shutter speeds vary from phone model to phone model. By changing shutter speed we can control how bright or dark the image is. But shutter speed also has a secondary effect. If the shutter is open for a while and something in your scene moves that thing will be blurry. Even if the shutter is only open for a fraction of a second, if something in your scene is moving very quickly it will blur. But perhaps more importantly if the shutter is open and you shake the camera there's a chance that you're entire image will be blurry. So in addition to controlling brightness, shutter speed also gives us the power to stop or blur motion within the frame. With a fast shutter speed we can freeze say a moving athlete. With a slow shutter speed we can blur a speeding race car to make it look like it's moving quickly. But even if our creative goals have no need for motion control we still need to pay attention to shutter speed to ensure that camera shake doesn't wreck our image by making it too blurry. Now I'll be honest, because it lacks a telephoto lens, a smartphone camera is usually not a great tool for sports or wildlife photography so it's rare that you're going to end up shooting with your phone in a situation where you need very precise shutter speed control. That said, shooting kids or pets can often require some shutter speed thought. While your phone will usually do a good job of figuring out an appropriate shutter speed on its own, having an understanding of the concept of shutter speed will give you more creative options as well as help you shoot sharper images especially in low light. If you'd like to know more about shutter speed and exposure in general check out my course, "Photography Foundations: Exposure."