Most amateurs photographers think that zoom is how many times closer a lens can bring the object they are shooting. That is not right.
The Zoom in lenses is the ratio of the longest focal lenght to the shortest focal lentgh of a single lens that has variable focal length.
Professional and experienced amateur don’t view the Zoom characteristic of a lens. They take notice of the 35mm equivalent focal length of a lenses. Lenses with focal lenght above 85mm (equivalent to 35mm sensor) are considered telephoto. Commercial telephoto lenses can reach 600mm.
But even the focal lenght alone isn’t enough. What really brings the object closer is the field of view which is relative to the focal length and the size of image sensor.
The image sensor plays a crucial role to the zoom of a lense. A small sensor permits smaller lenses to deliver bigger zoom with a cost of quality. All the super zoom compact digital cameras have small sensors. They have tremendous zooms but they take tremendously bad photos under low light. Small sensors suffers from low lighting.
The common size of an image sensor in professional photography is the full frame image sensor which has the size of the classic 35mm film. It is common because old lenses for 35mm film cameras are compatible with full frame digital cameras. During the recent passage from film photography to digital photography the professionals managed to rescue their old expensive 35mm lenses.
A lens designed for a full frame sensor behaves as a lens with longer focal length when it is attached to a smaller sensor camera. The difference is called crop factor. APS sized sensors have around 1.5 crop factor. Micro 4/3 have a 2 crop factor. Crop factor is the ratio of the sensors diagonal lenght.
What is the effect of the crop factor? A 100mm lens for a full frame sensor in an APS sensor behaves like a (100mm x 1.5 =150mm) 150mm lens. The same lense on a micro 4/3 sensor behaves like a (100mm x 2 = 200mm) 200mm lens. It seems smart to use a smaller sensor and bring the object you shot closer but there is a quality toll.
Lenses for full frame sensor 35mm are called EF by Canon and AF-S by Nikon. Cheaper lenses only proper for APS sensor are called EF-S by Canon, AF-S DX by Nikon and DT by Sony.
The focal lenght effects the field of view. Try to see from inside a tube. From a short tube you see more things than a long one. The field of view is what the sensor receives. The small field of view sends more details of a distant object to the sensor. That is perceived as Zoom. The smaller field of view the less amount of light reaches the sensor. Less amount of light demands longer exposure, tripod and higher ISO.
That’s why quality telephoto lenses parallel to their big length have also big diameter in order to accept more light than cheaper narrower lenses. These quality telephoto lenses are called bright telephoto lenses because they receive more light and have less noise from lower ISO. Thery are also called fast telephoto lenses because they need shorter exposure in order to bring good results. .