Wednesday 1 June 2011

What is acutance?

Acutance is how sharp an edge is.

Low acutance is a blurry edge that merges into the next thing in the photo.

High acutance is a sharp edge that is well-defined and clear.

What affects acutance?
Acutance is dependent on the quality of your lens, and on post-processing techniques (e.g. sharpening tools and blurring/ unsharpening tools).

What is a "bit" in photography?

In computing, a "bit" is an abbreviation for a "binary digit", which is smallest unit of data (in binary code) in which computers store information.

What has this got to do with photography?

Digital cameras store photos in bits too. The colours of the shot are interpreted by the computer chips in terms of binary code of 0s and 1s. The "bit depth" of a picture describes how many colours can be stored as information for a photo, or more usually, it describes how many colours can be stored as information for one pixel of a photo.

Each pixel of colour is made up of different percentages of the primary colours: red, green and blue. Each one of these primary colours within a pixel is called a "colour channel". The information for the intensity of each channel is stored in bits (bits per channel), and the information that records the combination of the 3 colour channels together is also stored in bits (bits per pixel, bpp).

Usually each channel has 8 bits available for information storage. i.e. a computer reads it as a series of 8 figures of zeros and/or ones. This means that each of the three channels can have up to 256 different permutations of zeros and ones, and 256 different shades of their primary colour. Since there are three of these channels, if you calculate the total number of permutations with all 3 channels together it comes out to 16,777,216 different possible colours you can obtain.

The number of bits per pixel varies depending on the settings of your camera and digital equipment.
  • For black and white images, or monochrome (ie a range between two colours, like black and white, or dark red to white), you only need 1 bit per pixel.
  • For 4 colours (called CGA), you need 2 bits per pixel
  • For 256 colours, (VGA) you need 16 bits per pixel
  • For 16,777,216 colours (SVGA or "true colour") you need 24 bits per pixel. This is more than the eye can detect because the eye can only perceive about 10,000,000 colours so for most purposes 24bpp is as high as you'd need.
References
http://www.cambridgeincolour.com/tutorials/bit-depth.htm