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Whether it’s a snapshot or fine art photography, exposure of a picture is depends on how much light registers on the media (film or digital camera sensor) that gathers that light. The correct exposure of a picture is a product of the three factors that control how much light is gathered: aperture, Iso, and shutter speed. When these three variables are in the proper ratios, the proper exposure of pictures will result.

ISO is a measure of how sensitive the film or digital sensor is that is collecting the light. The lower the ISO number, the lower the sensitivity. The higher the ISO, the higher the sensitivity.

The diaphragm in the lens of the camera is adjustable and the hole that allows light to pass through it is called the aperture. It operates very much like a pupil. When the aperture is wide open, more light can pass through this larger opening and land on the light collecting media. When the opening is narrowed to create a smaller aperture, less light is able travel to the sensor. The ratio of focal length of the lens to the diameter of the lens opening is expressed as f/(aperture value) and is commonly known as the f stop. An aperture of f/2.8 tells us that the focal length of the lens is 2.8 times the opening inside the lens. Because of this expression of ratio, smaller numbers in the f stop such as f/1.8 means that the aperture is larger as opposed to larger numbers like f/22 which indicate a smaller opening inside the lens.

Shutter speed is the easiest of the three variables to understand. It is as simple as it sounds. This is the amount of time that the shutter is open to collect light. The longer the shutter is opened, the more light is allowed to pass through the lens to be collected.

All three of these factors come with other benefits or drawbacks depending on how you look at it. These side effects can be put to creative use When fully understood and properly utilized.

Knowledgeable photographers understand the causes and effects of these factors and will set them before ever taking a picture. Usually, a photographer is concerned first with ISO. Higher ISO settings result in grainier images. For this reason most photographers normally keep ISO settings as low as possible. Even though noise and film grain are usually unwanted, sometimes they are used to achieve this effect in the image.

Changing the opening in the diameter of the diaphragm changes how much light passes through the lens and onto the light catching media whether it is film or a digital sensor. When changing the aperture the focus in the image captured is also changed. Smaller apertures such as f/8 and f/16 will result in more area in focus between foreground and background. Called depth of field, this is very important in the capturing of an image. Bigger apertures like f/1.8 and f2.8 narrow the depth of field so that most everything in the final picture will out of focus. This helps to highlight the subject in a dramatic way by leaving the subject in focus while distracting foreground and backgrounds remain out of focus.

Shutter speed regulates how much light passes through the lens by varying how long the shutter is open. Obviously, the longer the shutter is open the more light will pass through the lens on it’s way to the film or sensor and vice versa. The side effect of shutter speed is motion blur or lack of. The longer the shutter is open the more the apparent moving parts of the image will blur. Blurring moving subjects or even the entire image using slow shutter speeds can be put to creative use and it often is. Other times a fast shutter speed is required when “freezing” a subject is the desired result.

When the values for these three variables are correctly determined and set accordingly, a well exposed image will be produced. After selecting these settngs, a change in any one of these variables calls for a compensating change in at least one or both of the others. Making a change in only one of the variables changes the amount of light that was will be captured by the sensor or film. If you choose to use a higher ISO film or setting then you will be increasing the sensitivity of that media which will result in an overexposer unless you reduce the amount of light that enters the lens by reducing the aperture or shutter speed. Using a faster shutter speed to freeze a passing race car will give you an underexposed image unless you increase sensitivity of the light collecting media by boosting the ISO or unless you open up the aperture to allow more light through the lens to compensate for the shorter duration that the shutter is open.

These three variables determine the exposure of the final photograph. And because of the side effects associated with these variables, a knowledgeable photographer knows what his desired result is and will determine each of these settings prior to pressing the shutter release button.

This entry was posted on Sunday, May 18th, 2008 at 7:51 am and is filed under Interior Decorating. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.