Image Capture

Instead of film, a digital camera uses a device called a CCD (charge coupled device).  The CCD is a light-sensitive chip that converts light into a digital (electrical) signal.  The CCD is made up of thousands of individual sensors that are sensitive to green, red, or blue light.  The sensors are also known as pixels (picture elements)

CCD Sensitivity

The CCD is designed to have options similar to using different film speeds (ISO settings). The higher the number or ISO setting, the greater the sensor's sensitivity to light. At an ISO of 800 or more, the camera's aperture only needs to be open for a short time in order to create an image, whereas using lower numbers means that the sensor needs more light to reach it in order to record a properly exposed shot.

 

http://micro.magnet.fsu.edu/primer/digitalimaging/concepts/images/ccdanatomyfigure2.jpg

Image Resolution Resolution is explained as the number of horizontal pixels multiplied by the number of vertical pixels.  The more pixels there are, the higher the resolution.  In addition, the higher the resolution the more space (memory) your image requires. Higher resolution numbers also mean you will get a better quality (deatil and focus) print from a certain file. Screen resolution is 72, for printing you will want to keep the resolution above 200.

 

Metering

Correct exposure happens when your photograph has the same range and intensity of tone as the scene being photographed.  Underexposure occurs when the sensor does not receive enough light, and overexposure occurs when the sensor receives too much light.  Sensors on the camera measure the amount of light reflecting back off the subject(s), then the camera uses this information to calculate the exposure – a process known as metering.

Underexposed photos will be dark because the camera did not have enough light or enough time to gather light, color and detail information.	Overexposed In traditional film overexposure creates a DARK negative that then produces a light, washed out photo . Digital photography skips the film step...

All metering systems are calibrated to average the reflected to an 18% grey. Colors similar in density (value) to 18% grey are referred to as the mid-tones, these include green foliage, brown brickwork and concrete.

Two main types of metering systems are used in digital cameras, center-weighted average metering and multi zone metering.  As the name would imply, center-weighted average meters take an average of the entire frame but gives most emphasis to the central area of the frame.  The down fall of this system is that is it has difficulty exposing correctly for very bright or very dark scenes in addition to backlit subjects.  When the frame is divided into a number of zones, as in the multi zone meters, an exposure reading is taken for each zone. It then uses the individual readings to calculate the overall exposure.  It too has difficulty with backlit subjects.  However, is more successful in coping with more situations than the center-weighted system.

Aperture and Shutter Speed

Proper exposure also depends on finding the right combination of aperture (the amount of light allowed into the camera) and shutter speed (how long the light is allowed to enter the camera).  Aperture controls the amount of light much like the iris of a human eye, widening and constricting to let in more or less light depending on exposure needs.  Apertures are shown in f/numbers.  When viewed in sequence each f-stop lets in twice (or half) the amount of light.

Aperture choice will also change the amount of the final photograph that appears to be in focus.  This is referred to as Depth of Field.

Shutter Speed is measured in fractions of a second.  For example 250 is 1/250th of a second, 2 is ½ (half) a second and 1 is a full second. Again each step is called a stop, and represents the doubling or halving of the amount of light entering the camera. Shutter Speed can be manually controlled to alter how motion (a moving subject) is captured in your photographs.