Photography is all about light. Just like our eyes, our camera uses the light that bounces off our environment to create an image. Controlling that light is one of the keys to controlling the images you capture.
When it's too bright- we humans squint our eyelids to keep excess light from entering our eyes. If we hold our eyes wide open in bright daylight, the image we see is overexposed; too bright to make out the details. Our cameras have something called a diaphragm, and it works like our eyelids to control the opening that allows light through our camera lens.
This opening is called an aperture, and we measure this aperture in a series of numbers referred to as f/stops. The lower the number, the bigger the opening (confusing, right?). That's OK, we'll explain more below.
As it turns out, the size of the opening, or aperture, affects more than just the levels of light entering your lens. Aperture also has varying effects on depth of field, image sharpness, and helps determine what shutter speeds we will use.
DEPTH OF FIELD-
Depth of field is the area of your scene which will appear in focus (sharp), versus out of focus (blurred). Generally, we have a subject which we want to be in focus, and a background and/or foreground which will be out of focus. The aperture setting gives us total creative freedom over our depth of field, and every photographer needs to understand this dynamic. The rule of thumb for this is as follows:
A lower f/stop (larger opening in the lens) gives us a shallow depth of field. The subject will be in focus, and the background will be blurry.
A higher f/stop (smaller opening in the lens) gives us a wider depth of field. The subject and the background can both be in focus.
Control the f/stop, and you control the depth of your image. A key tool in a photographer's skillset.
After we decide on how we want to present our depth of field, we can move on to shutter speed. Since a low f/stop lets in a lot of light, and visa-versa, we need to factor this in.
When the aperture is open (low f/stop), a ton of light enters the camera. To adjust for this, we generally use a fast shutter in combination.
When the aperture is tight (high f/stop), less light will enter our camera, so we keep the shutter open for longer to compensate.
TOOLS OF THE TRADE-
But sometimes we want even more control. What if it is very bright outside, but we want to use a slow shutter speed to blur the motion of a stream? For this, we use a special tool called a Neutral Density filter. This simple filter, available for any camera lens, is like a pair of shades for our camera. It will block some of the light from entering your lens, but it does so without affecting the color or tone of the image.