Understanding Aperture

Understanding Light: Photography is not about selecting buttons and switches on a highly sophisticated piece of electronic and optical equipment; photography is about seeing the light. Without light, there is no photograph; end of story. Light controls what you see in a photograph and how you see it. Aperture, which is measured by f/stops, controls the volume of light passing through your lens. Controlling the intensity of light as it passes through the lens (opening) and onto the camera’s electronic sensor (or film) is what aperture is all about. There are only two other mechanisms used for controlling light, filters and shutter speed. The chart above shows how the amount of light is decreased as the f/stops increase in "full" f/stops. Increasing the f/stop numbers to reduce the amount of light passing through the lens is known as “stopping down” the lens. In contrast, decreasing the f/stop numbers to double the amount of light that is passing through the lens, is known as "opening up" the lens.

Lens Aperture & Identification: Every lens has a limit on how large or how small the aperture can get. The maximum aperture of the lens is much more important than the minimum, because it shows the “speed” of the lens. The f/number is the focal length of the lens divided by virtual aperture (the circle you see when looking through the back end of a lens. Every lens is designated by its LARGEST opening (smallest f-number) e.g. 50mm f1/4. BIGGER numbers (f/16, f/22) specify a SMALL aperture (lens opening) that admits LESS light. SMALLER numbers (f/2, f/2.8) indicate a LARGER aperture (lens opening) that admits MORE light.

The f/stop Scale: The first thing you need to know the f-stop scale. The scale is as follows: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22. The thing to know is that, from each number to the next, the aperture decreases to half its size and allows 50% less light through the lens. This is because the numbers come from the equation used to work out the size of the aperture from the focal length. You’ll notice, on modern day cameras, that there are apertures in between those listed above. These are1/3 stops, so between f/2.8 and f/4 for example, you’ll also get f/3.2 and f/3.5. These are just here to increase the control that you have over your settings.

The zoom problem: Unless buying the “body only”, almost every camera, no matter how good the camera,  comes with an inexpensive zoom lens. As you zoom the lens to a longer focal length, but maintain the same maximum aperture size, you end up with a larger f-number that results in a dimmer image. Zoom lenses that do this are called "variable-aperture zooms" and are designated with a hyphenated maximum aperture, e.g. an 18-55mm f/3.5-5.6. This means that you get f/3.5 when set to the maximum aperture of 18mm, but only f/5.6 when zoomed to 55mm. A "constant-aperture zoom lens” on the other hand will maintain the same f-number at all focal lengths when set manually. Such zooms, e.g.24-70mm f/2.8 Zoom Lens, are optically much more complex than variable-aperture zooms, resulting in a bulkier, heavier and more expensive lens. Because this type of lens allows you to capture better pictures in low light situations, and usually has more aperture blades that provide better “bokeh” (the wider the aperture, the narrower the depth of field), it is definitely worth the extra money.

Setting the f/stop (aperture): Simply put, aperture is the hole within a lens, through which light travels into the camera body. The lower the f/stop number, the wider, or bigger, the aperture.  Changing the aperture for a particular evenly-exposed shot means you need to shoot faster to compensate (or increase the ISO). This is because shooting faster reduces the amount of light going through the lens. This is the relationship between aperture and shutter speed. It is recommended that you only shoot in the Auto mode (where the camera makes all the decisions on settings) when circumstances dictate quick, responsive, shooting. Turning your camera’s Mode Dial to A or Av (which stands for Aperture value) means that you set the aperture you want and the camera compensates by making the decisions on shutter speed. This is a particularly good way of ensuring you achieve the DOF you want from your image, the other reason why a faster lens is desirable. Bokeh. The wider the aperture, the narrower the depth of field.

What is Depth of Field?  The one important thing to remember is that the size of the aperture has a direct impact on the depth of field, which is the area of the image that appears sharp. A large f-number such as f/32, (which means a smaller aperture) will bring all foreground and background objects in focus, while a small f-number such as f/1.4 will isolate the foreground from the background by making the foreground objects sharp and the background blurry, creating what is commonly referred to as “bokeh”.

Setting Exposure: The optimum exposure for any given scene is comprised of four elements; light, shutter speed, aperture and ISO, the last three under your control are know as the "Exposure Triangle". There are two parts involved in exposing a digital sensor to light. One is the intensity of the light and the other is the length of time the light is allowed to strike the sensor. Exposure is the measure of how much light (the AMOUNT OF LIGHT controlled by the aperture) that is captured over a SPECIFIC AMOUNT OF TIME (controlled by the shutter speed), e.g. Exposure = Intensity x Time. Understanding the relationship between aperture and shutter speed and how to control them will take your photography to another level. Aperture and f/stop are indeed references to the same thing. The aperture is the lens mechanism and f/stop is the measure of mechanism’s engagement. Think of it this way; a properly exposed photo might need 1,000 "units" of light to hit the sensor. Your camera can do one of two things to ensure that 1,000 units hit the sensor: (1) it can open the shutter for more time - this is called the shutter speed. If opening the shutter for half a second lets in 500 units of light, doubling that to one second will let in the 1,000 units you need for a well exposed photo, (2) your camera can allow those 1,000 units in is to make the hole at the front of the camera wider (this hole is the aperture). The shutter speed can stay at half a second, but make the hole at the front of the camera twice as large and 1,000 units of light can enter in that same half a second. The longer the shutter is open, the longer the light has to expose the film, the shorter the shutter is open, the less time there is to exposed the film. Just remember that each full f/stop either halves or doubles the amount of light entering the camera and each full shutter speed stop either halves or doubles the amount of time of the exposure. It is possible to have the same exposure with a variety of different f/stops and shutter speeds depending on what effect you want to achieve. If you are in aperture priority and change the f stop the shutter speed automatically changes for a proper exposure; if you are in shutter speed priority and change the shutter speed the f/stop automatically changes for a proper exposure. Since you don’t have to manually change both factors of an exposure in these modes, many new photographers have a hard time understanding this relationship. Then there is ISO. While shutter speed and aperture determine how much light reaches the sensor, ISO relates to how the sensor reacts to that light. The higher the ISO, the more sensitivity and the greater the sensor's light gathering capacity; meaning that less light is needed to create the desired exposure. So I know you are now thinking, if my camera can  automatically do this for me why should I care what my f/stop or shutter speed is?  Because a knowledge of these concepts will allow you to determine the sharpness or depth of field given to a given area. If you let the camera do everything for you, you will only get average images. In Summary - Practice, Practice, Practice:  The best way to improve your photography is to practice as often as you can. Always try to have your camera with you. Even when that is not possible, continue to look at your surroundings as if you were trying to create a photograph. Compose hypothetical shots in your mind by thinking about light and what settings you might use to achieve various effects.