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Beginner's Guide to Understanding Exposure.


So you bought your first DSLR and are not sure if it was a good idea or not. Or, perhaps you bought an advanced compact camera as your photography friend told you to get something with manual exposure controls. So now what do you do?

Virtually all cameras; compact cameras, compact system cameras, and even DSLRs - except for the professional-grade DSLRs have an Auto exposure control. This allows the novice to begin taking photos right out of the box. While this will suffice for many situations, at some point, you may wish to go beyond Auto exposure, as that is where the land of creativity lies - photography speaking.

What is exposure anyway? For any given photo, the camera will capture a certain amount of light. The amount of light it recieves must be just so, or the photo will not be correct. Just like making pancakes; too much or too little water will not provide a good result. Proper exposure is like using the proper amount of water for your pancakes.

This holds true for both film and digital cameras. The film or digital sensor must receive just the right amount of light when taking a photo. For cameras with an Auto mode, this is done automatically for you, with the camera taking the reading and adjusting itself. While this works most of the time, it does not work all of the time.

Blurry photo.

Photo too dark.

Background OK, subject too dark.

We've all had photos such as this that just didn't work. In every case, it was because the exposure was not correct.

Anatomy of a Photo

As shown below, there are three things that regulate the amount of light that creates the photo. The diameter of the Aperture, the length of time the shutter is open, and the electrical sensitivity of the sensor. These three characteristics work in harmony to (hopefully) cause just the right amount of light to be used for the photo. Collectively, this is called the "exposure", and the harmony of the exposure is called the "exposure triangle".

When your camera is in auto mode, these three characteristics are controlled by the camera as to what it thinks is the best combination. However, when you take the camera out of manual mode, you have control over each setting.

A rookie mistake is to misunderstand that there are limits as to what the settings can be. You cannot arbitrarily set the three entities to anything you wish. You must not only balance the three settings, but you must also ensure they are within the bounds of what the camera is capable of.



Sensor Electrical Sensitivity: This is called ISO, and it dictates how sensitive the sensor is. The more sensitive the sensor, the more light it can detect. However, there is a downside to this - and that is noise. The more sensitive you make the sensor, the more noise the photo will have.

Low noise photo - using a low ISO setting.

High noise photo - using a high ISO setting.

A good analogy is an AM radio station. If you are listening to a local AM radio station with a strong signal, you can hear the station with little if any background noise. But if you tune in a weak station, you may have to turn up the volume, and will hear a lot of background static, and in the worst case, it may completely drown out the signal from the station. ISO noise is an optical representation of this scenario. When you have a strong source of light, you can use a low ISO and achieve a noise free photo. But when the light is weak, you may have to increase the ISO, which increases background noise in the photo.

The universal rule of thumb for setting ISO in a camera is to always use the lowest ISO you can. In auto mode, the camera will make this decision for you, and may select a high ISO that might not provide the best photo. However, when you control the ISO, you can prevent this from happening. However, as indicated, you will face limits as to what you can set your ISO, and you may have to increase it at least by some amount. The thing is - you are in control of that setting.

Different cameras have different ISO capabilities. Generally the larger the sensor, the more sensitive to light the sensor is - which means you can use a lower ISO. For example; DSLRs - having larger sensors (up to 15 times as large) have more sensitivity than smaller-sensor compact cameras. And compared to cell phone cameras, a DSLR may have a sensor that is 200 times larger.



Shutter Open Time: This is called Shutter Speed, and can range from 1/4000th of a second to 30 seconds or more. Again, in auto mode, this setting is made for you. However, you can adjust it yourself when you are out of auto mode.

Long duration shutter opening - blur due to movement.

Short duration shutter opening - "stop action" photo.

When you adjust the Shutter Speed, you instruct the camera how long to keep the shutter open. The opening and closing of the shutter is what actually takes the photo, as it allows light to enter the sensor (or film in the old days), creating the photo. The length of time the shutter remains open determines how much light falls on the shutter. It should then be obvous that the darker the scene, the longer the shutter may need to be open. Again, there is a limit to this setting as shown above. If the shutter is open too long, any movement in the scene will result in blur. If the shutter is open only a short period of time, the blur will be minimal.

Again, in auto mode, the camera makes this decision for you, which may or may not be the best setting. However, there are some creative reasons to purposely use a certain shutter speed, such as in the above example. Purposely causing the water to be blurry gives a suggestion of motion, and tells a story. When the camera is in auto mode, this technique is not possible.



Diameter of the Opening: This is called the Aperture, located in the lens. It is not unlike the eye's iris. When you are in bright daylight, your iris becomes smaller, and lets less light in. When you are in a dark room, your iris opens - or dialates. This is exactly how the lens aperture works. As you might expect, when your camera is in auto mode, it will automatically make these settings for you.

Small aperture diameter - results in wide in-focus range.

Large aperture diameter - results in narrow in-focus range." photo.

Again, there are limits and trade-offs when selecting an aperture. Using a wide opening lets in more light, but it results in a narrow in-focus area. By reducing the aperture, the in-focus area is expanded, but at the expense of letting less light in. You can try this by squinting; especially if you are near sighted. With your eye-glasses off, look at something blurry. Then squint. You may just notice that the blurry object is now in-focus, and the scene is probably darker as well.

You may wish to control the aperture for various situations. For instance, a portrait is usually better when the background is out of focus, as it keeps the photo from having a distracting background, and directs your attention to the subject. I know of some portrait photographers that have even shot portraits in a parking lot full of cars. By restricting the depth of what is in focus, the cars in the parking lot are blurred out of recognition.

Conversely, if you are taking a landscape photo of a beautiful mountain range, you want pretty much everything from the foreground to background in focus.