Digital image consists of squares, called pixels, each pixel having a value or a color. So, the number of pixels defining an image is larger, we have more information and details will be smoother. Same thing happens with tonal and color range. An image resolution measures the number of pixels per unit length (eg inches or centimeters) and is a quality parameter of an image.
How to calculate the number of pixels or an image resolution in digital photography?

Number of pixels forming a digital image is calculated by multiplying the number of pixels per row, with the one obtained from the columns. Take for example a digital image of a picture with 2000×3000 pixels (2000 pixels per row and 3000 pixels per column). This image is defined by six million pixels.

This image size in inches will be given by the image resolution. If resolution digital image of a photo is 100 pixels per inch, then the image size is 20×30 inches.

To get good quality printing, the image must be prepared at a resolution of 300 pixels per inch.

With some programs it is possible to increase the number of pixels in a digital image by interpolation. Thus, we increase the size or resolution of a digital image. New image pixels are created based on information from “neighboring” pixels and are inserted between them. Have to be careful, because these artificial pixels sometimes leading to loss of image quality.