Megapixels

When you choose a camera you will need to decide the size of the largest picture you will want to print. As a general rule of thumb, you can determine the largest print size by the megapixel of your camera.

Here is how to do the math:
One megapixel is approximately 1 million pixels. A true photo quality print will be printed using 300 DPI (dots per inch).

What this means is that if you want an 8 x 10 inch print, you will need to multiply

8 x 300 = 2,400 pixels
and
10 x 300 = 3,000 pixels

This gives you the pixel length and width of your photo, which are 2,400 x 3,000 pixels for an 8 x 10 inch print. Now you need to calculate the number of pixels in the 8 x 10 rectangle. Calculate the density, in this case the "square pixels" by multiplying:

2,400 x 3,000 = 7,200,000 pixels 

Note that a megapixel is approximately 1 million pixels. Now, divide the "square pixels" by a million pixels to see how many megapixels your picture will be:

7,200,000/1,000,000 = 7.2 megapixels

This means that you will need a 7.2 megapixel camera to print an 8 x 10 inch photo in true photo quality.

Do also note that this calculation is just an approximation. In actual practice, you may be able to achieve an 8 x 10 photo quality print using a 4 or 5 megapixel camera provided that you have a good sensor or use good photo editing software afterwards. Also, shooting in RAW format instead of JPEG can give you better results especially if you have software on your computer that can render the raw image, edit, and print it.

Image processing

When you take a picture with your digital camera, the scene is converted into an array containing millions of uniformly spaced coloured dots. Those dots or picture elements (pixels) are stored on the memory card in your camera until you download them into your computer.

Figure 3.3 Image capture

 

An image that is stored in this way is defined by the width and height of the array of pixels along with the number of bits that are used to define the colour. Up to a point, the more pixels that the camera produces to represent a given field of view, the better the image. Similarly, the more bits that are used to store the colour, the better the overall quality of the image. This is particularly important in terms of subtle shades of colour.

The number of pixels per unit area is commonly referred to as resolution. For example, the display monitor that I am currently using displays an array of 1,280 x 1,024 pixels in a rectangular area with a diagonal measurement of 19 inches. (This is not a particularly high resolution monitor.) The number of bits used to represent the colour of a pixel is commonly referred to as the colour depth. Most modern computers routinely use a colour depth of 32 bits. Note however, that some file formats used for the storage and transmission of bitmap graphics data use fewer than 32 bits for the representation of each pixel in an image.

 

Figure 3.4 Image on the left has a higher pixel count
(higher resolution) than the one to the right

Different file formats are commonly used to store and transmit image data. In most cases, it is usually desirable to reduce the size of the file required to store a given image as much as possible while maintaining the quality of the image. Different formats use different compression algorithms to reduce the size of the file. This often results in a tradeoff between file size and image quality. Three of the more popular file formats are GIF, JPEG and PNG.

The Graphics Interchange Format (GIF)

GIF is a format that is often used to store low quality images in very small files. The format can store a maximum of 256 different colours and can designate one of those colours to represent a fully transparent pixel. The GIF format would not be very satisfactory for images produced by your digital camera, but it is fine for many purposes such as screen icons where high colour quality is not an important consideration.

The Joint Photographic Experts Group format (JPEG)

This image format uses a lossy compression algorithm to allow 24-bit colour depth with a small file size. Lossy compression means that what comes out of the compressed file is not identical to what went in. The loss in picture quality is often acceptable, however, given that the format allows for different degrees of lossiness which is inversely related to the size of the compressed file. Many of the digital cameras in the market produce JPEG files as the standard output and some of them allow the user to select the degree of compression and hence the degree of lossiness. The JPEG format does not support alpha transparency^[1] and, therefore, it is not suitable as a file format for transmitting images with alpha data between computers.

The Portable Network Graphics format (PNG)

This format was produced as an open-source alternative to the GIF file format. The PNG format supports at least sixteen million colours and uses lossless compression. The PNG format also supports alpha transparency allowing for up to 256 levels of transparency in a compressed format.

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^[1] In some cases, the pixel also contains another value referred to as the alpha value that represents the transparency of the pixel.