File size

Different file types use different compression methods, which affect the quality of the image in varying ways. For instance, if you save your file as a TIFF or BMP file, the image is uncompressed, every pixel is left in its place and hence the file size is big, but the quality is maximum. If you save your file as a JPG for instance, the file size is smaller, but the quality of the image suffers somewhat.

JPG uses a compression method that analyses blocks of 8x8 pixels and, depending on what the image manipulation software considers to be appropriate for the specific image, reduces the quality of pixels within a block.

For instance by merging different colored pixels whose colors are almost the same, and make them the same color, so less color information needs to be saved. If the compression is too high, the detail in the image is reduced, and since JPG uses this block method, can look "blocky" or washed out as color info is removed.

If pictures are being manipulated for on-screen use, some compression can be useful (as it reduces file size, for instance when working for the web) but is very undesirable for print as the compression artifacts are often visible in high quality prints.

Different file formats use different algorithms for compression, with varying results.

Once a file is saved compressed, it is impossible to get the original quality back, so ALWAYS, save your compressed/manipulated pictures as a copy, keeping the uncompressed original, if available.

Image capture

Digital cameras are very similar to the traditional film cameras.  The big difference between them is how they capture an image.

Film cameras record the image on film that is coated with chemicals that sense color and the intensity of the color.
Digital cameras use an electronic chip called an Image Sensor

There are two types of Image Sensors:

1. CCD or Charge-Couple Device

2. CMOS or Complementary Metal-Oxide Semiconductor.

CCD is the most common.  They generally give better image quality and have a greater dynamic range.  They also offer faster speeds.  However, CCDs tend to be more expensive and use more power.

CMOS sensors are larger than CCDs, often resulting in bulkier cameras.  CMOS sensors are getting better and some are better than CCDs.

The surface of the Image Sensor chip contains a grid of hundreds of thousands or millions of Photosensitive Diodes or Photo elements, also known as Pixels (Picture Elements).  Each pixel in the Image Sensor will capture the intensity of the light for each pixel in the photograph.

When the shutter of a digital camera opens, each pixel on the image sensor records the brightness of the light that falls on it with an electrical charge. The more light that on a pixel, the higher the charge, the less light, the smaller the charge.