Jun

Zoom lens, what’s that and what does it do?

As a photographer I get a whole lot of questions about a whole lot of photography related things. Some are easy to answer others need to be demonstrated. This one needs to be demonstrated…

I will not go into great technical details, however understand that a zoom lens usually allows for more than one “ZOOM” factor and it has moving glass in addition to focus. In fact it is usually safe to assume that the further away the glass elements are, the longer the lens is, the higher the zoom factor.

So what do we mean by zoom factor? It is the magnification that a lens can produce in relation to the human eye. In photography, it is generally accepted that a 1x zoom factor is equal to a 50mm lens on a 35mm film/digital format camera, I am not going into more details than this as it is not relevant to this post. So assuming that 50mm is 1x what would be 400mm ?

The equation is:

In a Canon EOS 5D the answer would be (400×1)/50 or 8x magnification. However with a crop sensor camera such as the Canon 7D, it would be (400×1.6)/50 and thus would result in a 12.8 magnification. But what about a 16mm wide angle lens? Well (16×1.6)/50 results in a 0.512 magnification which is a theorical negative magnification. So what are the actual result in a photo? Here are a few photos of commonly used zoom factors to illustrate, all taken at the same place with a Canon EOS 7D with a 1.6 crop factor.

16 mm (0.51x)	24 mm (0.77x)	35 mm (1.12x)	50 mm (1.60x)
70mm (2.24x)	85 mm (2.72x)	100 mm (3.20x)	135 mm (4.32x)
150mm (4.80x)	200 mm (6.40x)	300mm (9.60x)	400 mm (12.8x)
500 mm (16x)	600 mm (19.2x)	700 mm (22.4)	800 mm (25.6x)

Click on photo to enlarge

Magnification table
(for a 1.6 crop factor)

Lens focal length	Magnification factor
10	0.32
16	0.512
24	0.768
35	1.12
50	1.6
70	2.24
85	2.72
100	3.2
135	4.32
150	4.8
200	6.4
300	9.6
400	12.8
500	16
600	19.2
700	22.4
800	25.6
1200	38.4