In my post “256 Shades of Gray,” you learned how the digital image sensor in your camera works. Exposure is a measure of the amount of light captured by your camera’s image sensor when you take a picture. Too much light, and your picture be too bright and you will lose detail in the highlights. Not enough light, and your picture will be too dark with a loss of detail in the shadows. In this post, we will explore how to get it not too light, not too dark, but just right.
Before we get into the details of achieving perfect exposure, let’s review some of the basic concepts of how exposure is controlled.
There are three settings in your camera that control the amount of light captured by the image sensor when you take a picture, and collectively they comprise the exposure triangle. The settings in this triangle are shutter speed, aperture, and ISO and they are interdependent, in that if you change one setting you have to change one or both of the others to maintain that same exposure.
The shutter is like the door that stands between the lens of your camera and the image sensor. The amount of time that it remains open when you take a picture (shutter speed), affects the amount of light that reaches the sensor. Shutter speeds are notated as fractions of a second, and 1/250 seconds lets in twice as much light 1/500 seconds, and 1/500 seconds lets in twice as much light as 1/1000 seconds, and so on.
The aperture is like the pupil of your eye. When you are in a darkened room, your pupil gets large to allow more light into your eye so that you can see better. When you step out into bright sunlight, your pupil contracts to lessen the amount of light entering your eye. Your camera’s aperture is a part of the lens and the range of adjustment varies with each lens. Aperture size has a weird notation because it is the ratio of the focal length of the lens to the diameter of the aperture (and you have my permission to immediately forget that useless fact) and is called an f-stop. Just remember that f/8.0 lets in twice as much light as f/11, and f/11 lets in twice as much light as f/16, and so on.
Finally, there is the ISO, which is a measurement of the sensitivity of the image sensor to light. The higher the ISO the more sensitive it is to light, e.g. ISO 400 is twice as sensitive to light as ISO 200, and ISO 200 is twice as sensitive to light as ISO 100, and so on.
In the chart below, I have listed some common values you might see listed on your camera. The settings in each column cuts in half the amount of light exposure as you follow the columns from top to bottom (these increments are called stops).
By now, you should see the interrelationship of the three elements of the exposure triangle and understand that there are many combinations of “equivalent exposures”. For example, setting your camera controls to ISO 100, with an aperture of f/16 and a shutter speed of 1/125 sec, results in exactly the same exposure as ISO 100, f/22, 1/60 sec. That is, if the ISO remains the same and you change the settings to let in half as much light with the aperture and twice as might light with the shutter, it is the same exposure.
What is the perfect exposure? The answer is that it depends on how you want your picture to look and how much light is available. You see, even with equal exposures that are perfect for the available light, your photo will look vastly different depending on which settings you use because there are trade-offs with everything. Sorry, that’s life.¯\_(ツ)_/ However, I will take you through a real world example that I believe will help you to understand what’s going on here.
There is a rule of thumb in photography (Sunny 16) that says that in bright sunlight at an aperture of f/16, the shutter speed will be equal to the ISO at the correct exposure. The trade-off with ISO is that the higher the number (more sensitive) the more noise and grain in the picture, so I always shoot at ISO 100 unless there is some reason I cannot. Thus the perfect exposure on a bright sunny day will be ISO 100, f/16, 1/125 sec (actually 1/100, but I am rounding to the nearest standard setting for clarity).
Say you are taking a picture of your significant other on a busy afternoon street. However, the cars and buildings make for a distracting background. The trade-off with aperture size is that the larger the aperture (smaller the f/number) the shorter the depth of field e.g. the area in focus behind and in front of the focal point. Using that to your advantage, stand close to and focus on your subject, open that aperture to f/5.6, and you will have a nice blurry (out of focus) background in your picture. Since f/5.6 will let eight times more light (+ three stops) get to the image sensor you need to change the shutter speed to 1/1000 sec (- three stops) to compensate and maintain an equivalent exposure.
The trade-off with shutter speed is that the slower the speed the more motion can blur the image (either movement in the picture or camera movement). In the example above, you might lower the shutter speed to 1/60 sec (and compensate with an aperture of f/22) and make the cars in the street blurry imparting a sense of motion to the picture.
What will you do if the day is not bright and sunny? The same rules apply, but chose a wider aperture as your baseline for the adjustments. Like an “Overcast 8” or a “Shady 4.” Actually, bright and sunny is one of the hardest conditions for taking good photos and you will find that you have a lot more equivalent exposure options available to you in softer, diffused light.
Why should you go through all this trouble when you are using your camera in automatic mode and getting good pictures? Your camera is looking at your picture scene, measuring the average light, and making a guess as to what setting will result in the best picture. It has taken over creative control and does not always get it right. If you accept the camera calculations on the average, you will get average pictures. For great pictures, you need to work in manual mode.
In practice, when you switch to “manual mode” your camera will display a light meter thereby eliminating “rule of thumb” calculations. Set the ISO, set the shutter speed, and adjust the aperture (or set the aperture and adjust the shutter speed) until the meter centers and your exposure will be close to where it needs to be. From there, make slight adjustments until you get your exposure just right.