A Quick Guide to Understanding Exposure
I’ve seen a lot of new photographers arise in the recent months with DSLRs becoming cheaper and more popular. However, I see many of them stuck in the “green box,” also known as full-auto. Why buy a DSLR if you can’t control it?
Semi-auto modes (shutter priority and aperture priority) have their purposes, but under a controlled environment one should always be shooting manual. This prevents the camera from thinking for you and between shots and ensures consistent exposures from shot to shot to shot. In semi- or full-auto, the camera will take a new metering every time you press the shutter button half way down. Even though nothing in the frame has changed, aperture or shutter speed values may vary, causing inconsistent exposures. For example, if I’m taking a portrait in aperture priority where my aperture is set to f/2.8, the camera will try to meter and choose the correct shutter speed for me. However, while the aperture will remain constant (as that’s what I’ve set it to), the shutter speed may vary between 1/400 and 1/500 (arbitrary values, not important). The result is two exposures between shots. If f/2.8 * 1/500 gives me a correct exposure, why would I want the next shot to be f/2.8 * 1/400 (slightly overexposed)? Putting the camera in full manual will take the camera’s thinking out of the equation and give you full control. This may sound scary, but this is a good thing.
Two Ways to Control Light
1) The shutter controls the duration of the exposure. A fast shutter speed (1/250 of a second) allows less light to reach the sensor than a slow shutter speed (1/4 of a second). The decision to use a fast or slow shutter speed depends upon the subject matter (i.e. if you’re trying to freeze motion, use a fast shutter speed).
It should be noted that your camera will often display shutter speeds as whole numbers, even though they represent fractions of a second. For example, if you’re camera displays 250, it is actually 1/250 of a second.
2) The aperture controls the intensity of light reaching the sensor. A small aperture (f/22) allows less light to reach the sensor than a large aperture (f/2). The decision to use a small or large aperture depends on the available light for exposure and the decision to use a great or small depth of field.
Shutter Speeds (in whole stops)
1 | 2 | 4 | 8 | 16 | 30 | 60 | 125 | 250 | 500 | 1000
slow shutter < more light
fast shutter > less light
Aperture Values (in whole stops)
1.4 | 2 | 2.8 | 4 | 5.6 | 8 | 11 | 16 | 22
large aperture > more light
small aperture < less light
Opening up & Stopping down
A stop is equal to one unit of exposure; it is equal to halving or doubling an exposure.
1/60 of a second allows twice as much light to reach the sensor as 1/125 of a second – this is one stop. Going from 1/125 to 1/60 is opening up one stop.
1/500 of a second allows half as much light to reach the sensor as 1/250 of a second – this is one stop. Going from 1/250to 1/500 is stopping down one stop.
f/5.6 allows twice as much light to enter the camera as f/8 – this is one stop. Going from f/8 to f/5.6 is opening up one stop.
f/16 allows half as much light to enter the camera as f/11 – this is one stop. Going from f/11 to f/16 is stopping down one stop.
Law of Reciprocity
Exposure = Intensity * Time
Recall that aperture controls the intensity of light and shutter speed controls the duration of the exposure.
Let’s say that f/16 * 1/250 gives you a correct exposure. Any change in aperture or shutter speed will give you an incorrect exposure. If you change the aperture to f/11 and keep the shutter at /125, then the resulting image will be overexposed by one stop. The Law of Reciprocity says you need to reciprocate by changing the shutter speed to 1/500 (one stop) to compensate. Remember that going from f/16 to f/11 you are opening up one stop (more light), so you need to go from stop down from 1/250 to 1/500 (one stop). This will maintain the balance and keep the correct exposure.
Doesn’t look like I’ll be taking advantage of Canon’s Instant Rebates.
