Photography Step By Step

This feature can be found in all digital cameras. In autofocus (AF) mode the camera will automatically focus on the subject in the focus area in the center of the LCD or viewfinder. Many digital cameras allows the user to also select other autofocus regions that will be indicated on the LCD/viewfinder.

In “single AF” mode, the camera will focus when the shutter release button is pressed halfway. Some cameras offer “continuous AF” mode whereby the camera focuses continuously until you press the shutter release button halfway. This shortens the lag time, but reduces battery life. Normally a focus confirmation light will stop blinking once the subject in focus. Autofocus is usually based on detecting contrast and therefore works best on contrasty subjects and less well in low light conditions, in which case the use of an AF assist lamp is very useful. Some cameras also feature manual focus.

This system allows the camera to maintain focus on subjects that are moving. It is a very useful feature for sports or wildlife photography and it is normally found on digital DSLRs.

Autofocus Servo is normally triggered by switching focus to “AI Servo” on Canon, or “Continuous” on Nikon, followed by half pressing the shutter release. The camera will continue to focus on the subject as long as the shutter release will be half pressed or fully depressed (actually taking shots). The Autofocus Servo usually also puts the camera into “release priority” mode so that the camera will take the shot when the shutter release is depressed, regardless of the current AF status.

This is a lamp which is used to illuminate  the subject you are focusing on when shooting in low light conditions. The lamp is usually located beside or above the lens barrel. These lamps are not very powerful and they usually only work over a short range, up to about 4 meters. The light that they make is usually visible light, but there are lamps which uses infrared light instead. This is better for candid shots where you don’t want to startle the subject. Notable higher end external flash systems feature their own focus assist lamps with far greater range.

The picture above is a hologram AF found on some Sony cameras. This works by projecting a crossed laser pattern onto the subject. This bright laser pattern helps the camera’s contrast detect AF system to lock on to the subject. The system works well as long as the subject is large enough to be covered by several laser lines.

Sensors from your digital camera consists of millions of pixels with photodiodes which convert the energy from the light (actually from the incoming phototns) into an electrical charge. That electrical charge is then converted to a voltage which is amplified to a higher level so that it can be further processed by the ADC (Analog to Digital Converter). In accordance with the voltage value, the ADC classifies it into a number of discrete levels of brightness and assigns each level a binary label (a number in binary code, consisting of 0 and 1). The ADCs are classified by the number bits that this labels are formed of. So a one bit ADC will classify the voltage as either 0 (black) or 1 (white). A two bit ADC would categorize it into four (2^2) groups (black-00; white-11; and two levels in between-01 and 10). Most consumer digital cameras use 8 bit ADCs, allowing up to 256 (2^8) distinct values for the brightness of a single pixel.

The minimum resolution (bit rate) of an ADC is determined by the dynamic range (accuracy) of the sensor. For example if the dynamic range of the sensor is 1000:1 (or 60dB) the ADC must be at least 10 bits which is 2^10 = 1024 discrete levels. This will avoid loss of information. Theoretically a 10 bit ADC is the perfect fit for this dynamic range (1000:1), a 12 or 14 bit ADC will not generate additional tonal information other than noise. However, in practice it makes sense to overspecify the ADC to 12 bits to allow for some margin of error on the ADC.  It is also useful to have extra bits available to minimize posterization or banding when applying the tonal curve to the linear data.
Often, marketing material advertises the bit rate of the ADC to suggest the digital camera or scanner is able to output images with a high dynamic and tonal range. From the above it is easy to understand that this is only true if the sensor itself has sufficient dynamic range. The tonal range and dynamic range can never be larger than the dynamic range of the sensor.
Digital SLR cameras have sensors with a higher dynamic range and are usually equipped with 10 or 12 bit ADCs. Normally such cameras offer the option to save the 10 or 12 bits of data per pixel in RAW because JPEG only allows 8 bits of data per channel.