A photographer has to deal with three main characters so as to create a composition. The first character is the object of interest namely – “Subject”. The second character is the ‘lens’ in use and the third character is the material to record the composition that is the film or the ‘sensor’.
If we place the three characters in a row, an inverted image of the subject is formed on the film/sensor. This inverted image is called the real image. As we vary the distance between the lens and the recording surface the real image becomes larger or shrinks. The real image becomes larger if we increase the distance and smaller if the lens is brought closer to the recording surface. The real image however can get blurred at some distances depending on the capacity and nature of the lens. So there is a particular distance between the lens and the real image which will give the desired composition. This is the required focal length.
The focal length of a lens defines the capacity of the lens to magnify the real image. A lens having a small focal length would be able to capture a subject at a smaller distance with a wider scene in view whereas a lens with a large focal length would be able to capture far away subjects but with a narrow scene in view.
The focal length to capture the exact size of the subject that is neither magnifying nor shrinking the real image is 55mm for a film camera and 35mm for a digital camera often called the normal focal length. Tele-lens is above 55mm for a film camera and above 35mm for a digital camera. Wide angle lens on the other hand is below 55mm for a film camera and above 35mm for a digital camera.
A camera can be fitted with lens having a “fixed focal length” or such which could create “variable focal lengths”. The one with a “fixed focal length” is called prime lens, and the one with a “variable focal length” capability is called zoom lens.
A zoom lens uses a combination of lens elements which can be moved back and forth to create variable real image sizes for a particular subject.
Mathematically zoom = maximum focal length / minimum focal length. Say the minimum focal length of a digital camera lens is 6mm and the maximum is 72mm then the optical zoom is 12x.
A macro lens is used for subjects which are very small or to capture a very tiny part of the subject magnified such that it fills the full frame of the film/sensor. The ratio of magnification could be 1:1 or higher. The professionals prefer prime macro lens over zoom macro lens.
Hence the selection of lens would depend on the choice of subject.