DLSR photography allows you to prioritize what you want your photo to show: whether that be action, focusing in on highly specific areas of the image that you're composing while blurring out everything else (depth of field), freezing action, etc.: almost anything is possible. Before learning how to prioritize all of those things though, it's necessary to learn about the importance of the amount of light coming into the camera, how that's controlled, and what the limitations are.
All DSLRs have built-in light meters which measure the light that the camera sees through the lens (TTL). This helps determine the shutter speed and aperture needed for a proper exposure after you choose your ISO (choosing your ISO is covered here).
NOTE: Most DSLRs have light meters which can evaluate light in several different ways: for example, 'Evaluative, Spot, Center-Weighted, etc.' - please see your instruction manual for the differences between these settings. The different kinds of light meter settings tell the light meter which part(s) of the subject matter you're photographing to prioritize: 'Evaluative' takes into account the entire frame, whereas a 'Spot' looks at a very tiny part of the frame - other settings prioritize differently. I personally keep my camera on Evaluative about 97% of the time but you should do what you find best for you. Once you're shooting, play around with this to see what results you like best: this is the huge advantage of digital - you can see your results right away. It's important to take advantage of instant results as a learning tool - if you don't like what you're seeing, try a different setting.
The main three adjustments that need to be made to the camera are:
1. ISO (adjusts the sensor's sensitivity to light)
2. Shutter Speed (how fast the shutter fires)
3. Aperture (the size of the opening in the lens when the shutter fires)
Each of these have a section devoted to them, but this overview is necessary first as all of these tie in together.
To make this tie-in, I'm going to make an analogy with an hourglass. This analogy has multiple parts: as you read, only move on once you've understood the section you just read.
The Hour Glass
All of us know how an hourglass works: you flip it over, and the sand drains to the bottom of the hourglass - simple. In the analogy below,
- The sand in the hourglass respresents the light that needs to travel to the camera's sensor to make the perfect exposure
- The top of the hourglass when it's flipped (so that all the sand is in the top) represents the subject of the photograph
- The bottom of the hourglass represents the camera as it receives the light (here represented as sand) to make the perfect exposure.
Once you have that understood, we move to our primary three adjustments themselves and how they're represented in the analogy.
1. ISO - the size of the hour glass
2. Shutter speed - how long it takes all the sand to get down to the bottom
3. Aperture - how big the opening is between the top and the bottom of the hour glass
ISO is selected on the camera first, and ISO is the simplest part of the analogy:
The smaller the hour glass is - the less sand that needs to drop to fill the bottom
The higher the ISO number is - the less light you need to get a proper exposure
Once you've choosen your hour glass size (ISO) then you need to get the sand (light) from the top to the bottom. Depending on how wide the opening is (aperture) between the top and the bottom will determine how much time (shutter speed) the sand needs to go from being completely in the top to completely in the bottom. In the same way, if you want the sand to take a certain amount of time (shutter speed) to fall to the bottom you'll need to adjust the size of the opening (aperture).
KEY POINT: It's important to notice how one directly affect the other: shutter speed and aperture are bound together - if one changes, the other must change to to make the same exposure.
With a DSLR, either aperture or shutter speed can be prioritized. That is, if a certain aperture is more important to you you can change the shutter speed to allow for your preferred aperture (more on why you'd want to do this in the aperture section here). If a certain shutter speed is more important to you, you can change the aperture to allow for your preferred shutter speed (more on why you'd want to do this in the shutter section here). It can be thought of as a math problem too. If you need your sum to be 10, you can either add 2+8 or 5+5 or 9+1 - they all equal ten. With shutter and aperture, when you adjust one the other one must be adjusted as well to continue to get the same exposure. This will be looked at in detail in their respective sections, but what is important to grasp now is their relationship to each other: when one changes, the other must change too to continue to get the same exposure. This relationship stays the same no matter what ISO is used. To use the math analogy again, if the sum needs to be 5 instead of 10, you can use 1+4, 3+2, or 4+1 - they all equal 5. The math analogy summarized: pick the final number you need (ISO) then figure out the two numbers (aperture + shutter speed) that you need to get the exposure based on the ISO you've chosen.
1. First choose your ISO based on how much available light you have (just like the size of the hourglass will determine how much sand you have to work with)
2. Then decide if aperture or shutter speed is the priority for the photo you're taking and make that adjustment
3. Finally, based on if you prioritized aperture or shutter speed setting, select the other setting in relation to your prioritized setting.