Focal Length – The degree of magnification & field of view, displayed in millimetres, mm

Prime Lenses - These lenses have one fixed focal length. If you want a different field of view, you have to physically move closer or further away from your subject. Prime lenses are usually of higher quality, and often sporting wider apertures. 

Zoom Lenses - These lenses have a variable focal length, and therefor the field of view can be adjusted without the need to move closer or further from your subject. Zoom lenses 

Short focal lengths, around 8-30mm have a wider field of view allowing you to take in more of the scene. 

Medium focal lengths, around 35-75mm have a moderate field of view similar to human sight. 

Long focal lengths, 80-200mm have more zoom and takes you closer to your subject. 

(The field of view available for each focal length varies with different camera sensors, often referred to as crop factor)

Shorter focal lengths, often referred to as Wide-Angle, bend a large amount of the scene onto the sensor an as a result can have an ‘aspherical’ or ‘fisheye’ effect on the subject to varying degrees. Additionally, it increases the distance between the subject and the background and increases the overall sharpness of the image. They can also make scenes or landscapes appear more dramatic. They are most useful for capturing wide scenes, when you are close to your subject or when you are in a small space and still need a wide shot.

In contrast, longer focal lengths, or telephoto, have the effect of pulling the scene across the sensor and can make the subjects appear fuller or wider, hence the phrase ‘the camera adds 10lb’. They also have the additional effect of pulling the background closer to the subject and softening the image. They are most useful for capturing close up details when you are far away from your subject. 

 Medium focal lengths around 35-75mm provide a more natural representation of form and can be used when minimal distortion is required, at a moderate distance. I recommend a medium focal length for the most realistic and flattering representation of your subjects and their surroundings.