What is Zoom in lenses;

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.


The canon EF 1200mm f/5.6 - f/32 L USM.
The canon EF 1200mm f/5.6 – f/32 L USM. This incredible and unique lens is not a zoom lens. This doesn’t mean that it can’t shoot object at extreme distanses. Its aperture is great relative to the focal length. It weights 16.5 Kgr. Certainly not a handheld lens. It has 13 elements including two synthetical fluorite elements. No more than 100 have been made.
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 Canon EF 600mm f/4 USM lens.
The Canon EF 600mm f/4 USM lens. The 600mm is the limit of commercial and affordable super telephoto lenses. The specific lens is again not a zoon lenses despite been made to capture clear photo from long distances. Lately with the used of synthetic fluorite elements their weight is low.
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.


The sony SEL18200 f/3.5 - f6.3 is designed for crop sencor APS-C mirrorless  cameras.
The sony SEL18200 18mm-200mm f/3.5 – f6.3 is designed for crop sencor APS-C mirrorless cameras. The 35mm equivalent lenght of this zoom lens is multiplied by the crop factor. The crop factor is 1.43 so the equivalent full frame 35mm focal lenghts are 25mm-285mm. Zoom lenses have variable focal lenghts and most of them variable apettures.
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.


Sigma 200-500 f/2.8 EX DG.
Sigma 200-500 f/2.8 EX DG. This beast super telephoto zoom lens combines everything. Great range of focal lenghts and incredible apperture. It weights 16 Kgr and has 17 elements. It costs 25.000.000 dollars. Not an affordable choice.
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. . 

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