Photography

Aperture & Shutter

The amount of light that enters to the camera's imaging sensor (exposure level) plays a large role in the quality of the image. The exposure can be controlled mainly by two parts of a camera: the Aperture and the Shutter. A perfect balance between the settings of these two parts are essential to achieve the desired quality and effect.

Aperture

The aperture is the opening of the lens where light enters before it goes to the imaging chip. It acts like the pupil of a human eye that contracts to make the opening larger and allowing more light to enter or expands making the opening smaller thus reducing the amount of light that enters to the eye. In the camera the Aperture Opening is controlled by the Iris which is composed of mechanically interlinked overlapping blades that opens or closes as needed.

Aperture opening is expressed in F-values called F-stops. F-value is equal to the focal length of a lens divided by the aperture's diameter. A lens with a focal length of 100mm for example with an aperture opening diameter of 36mm has an F-value of 100/36 = 2.8. The aperture value will be expressed as f/2.8 or F2.8. The same way, if the diameter of the opening of the same lens is reduced to 25mm then the F-value is 100/25 = 4, so the F-value is f/4.0 or F4.0. This shows that the larger F-value means a smaller aperture opening.

Conventional F-values are based on √2 or 1.414. Common F-values used by camera lenses today are the following: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, etc. The values are rounded off to these numbers to make it easier to remember. Each step of these values is called a "stop". Each stop higher reduces the opening area by half, and as a result allows only half as much light to pass through. f/4 for example is one stop higher than f/2.8 so it offers only half as much light than f/2.8.

There is no ideal aperture opening as different aperture values offer varied results to different situations so a camera which offers a wide range of aperture values would be a good basis. If the camera's aperture is not fixed then you need to check the specifications at what range of F-values the camera can operate on. It is expressed in either of three possible ways:

1. Max Aperture, ex Max Aperture = F2.8;
2. Aperture Range, ex Aperture = F2.8-F8.0;
3. Max Wide-Angle/Telephoto Aperture, ex Aperture = F2.8(W), F3.5(T);

A camera with high speed lenses (F2.8 or lower F-values) are ideal for action photography or night photography. Higher aperture values (F8, F11 or higher) are useful to achieve certain effects like correcting chromatic aberration or increasing the depth of field. More about this will be discussed in succeeding topics.

Shutter

The shutter is the part of the camera that controls the amount of time the imaging sensor is exposed to the light coming from the lens. It is normally closed and only opens momentarily when the shutter button is pressed. The time it takes for the shutter to stay open is referred to as the Shutter Speed and is measured in seconds or fractions thereof.

There are different types of shutter but the common ones that are still being used today are the diaphragm shutter (leaf shutter) and the focal plane shutter. A diaphragm shutter is made up of a set of plates that swings towards the center to close and swings outward to open similar to the camera's Iris. The Focal-plane shutter used by most SLR cameras is made up of two plates or curtains that slides horizontally. The first curtain covers the sensor on standby mode. When the shutter button is pressed the first curtain slides sideways exposing the sensor to the light. After the pre-determined time the second curtain slides in the same direction to cover the sensor.

With a fast shutter speed, the sensor will only receive light in a very short time allowing it to see the image very quickly. This results in a very sharp image assuming of course that the lighting is generally good and the object is focused properly. If the lighting is poor however, the resulting image will be dark or underexposed and many of the details specially in the dark areas will be lost. In this case a slower shutter speed will be useful as it will allow the sensor to receive light at a longer time giving it more time to record the image. With a slow shutter speed, care must be taken that the subject and the camera are both perfectly still to avoid blurring. A very slight movement while the shutter is open will cause the recorded image to appear blurred since the movement will be recorded as well.

Most cameras have shutter speeds ranging from 1 second to 1/1000 second with intervals of about half of the value of the previous step, i.e.: 1 sec, 1/2 sec, 1/4 sec, 1/8 sec, 1/15 sec, 1/30 sec, 1/60 sec, 1/125 sec, etc. each step letting in half the amount of light as the previous step. Some newer cameras and most SLRs are equipped with much wider range of shutter speeds from 30 seconds to 1/8000 second. Some also have additional options, the B setting (for Bulb) which allows the shutter to stay open as long as the shutter button is pressed, and the T (for Time) which keeps the shutter open after pressing the shutter button and only closes when the shutter button is pressed again.

These extra shutter speeds and options may not be useful in your everyday photography purposes but it's good to know that you have them when the time comes when you decide to explore deeper into these features.