Photography News | Issue 34 | absolutephoto.com
25
Review
All about focal length
Format diagonal (standard focal length)
Name
Format size
Crop factor
Every lens has a focal length expressed in millimetres or mm. The focal length is, very basically, the distance from the optical centre of the lens to the sensor/film plane when focused at infinity. The focal length is not a measure of the lens’s physical size. The focal length tells you the whether it’s a wide- angle, standard or telephoto, but this depends on the image format, ie. the size of the camera’s sensor. As a guide, the standard focal length for an image format is the diagonal distance across the format, so with 35mm full-frame, the diagonal is 43.2mm so that is the focal length that gives pictures with a normal perspective similar to that of the human eye. Focal lengths that are shorter than this give a broader view and lower magnification, and are classed as wide-angles. While lenses longer than this give a narrower view and a higher magnification, and are called telephotos. Defining a fixed focal length lens is easy, but classifying zooms is more tricky because there are so many options. Typically, a standard zoom covers 28- 85mm, a wide-angle zoom has a range of 16-35mm and a telephoto zoom around 70-200mm. We also have lenses that start from 28mm and go as long as 300mm, covering wide-angle, standard and
telephoto focal lengths. These convenient ‘all-in-one’ lenses are referred to as superzooms. Thus far we have only discussed focal lengths in relation to the 35mm format. This long-established format remains very popular. But there are many more formats on the market, especially smaller ones. However, the 35mm format’s history and popularity means it’s treated as an industry standard, so when focal lengths are quoted for other formats they’re often shown with the 35mm format equivalent figure too, just to give an idea of the effect. Of course, this is not much help if you’ve never used a 35mm format camera. The key thing to remember is that the same focal length has a different effect across formats. This is often summed up as the crop factor, and uses the 35mm format as the starting point. So a 50mm lens on a 35mm format camera behaves as a 50mm lens, ie. a crop factor of 1x. But with a 50mm lens on an APS-C Canon camera the crop factor is 1.6x, so the optical effect is as if an 80mm lens is used (50mm x 1.6 = 80mm). Similarly, a 50mm focal length lens on a Micro Four Thirds camera (2x crop) gives the effect of 100mm on a 35mm format camera (50mm x 2 = 100mm). See the table to the right.
35mm full-frame
36x24mm
1x
43.2mm
APS-C (Canon)
22.3x14.9mm
1.6x
26.8mm
APS-C (Fujifilm, Nikon, Pentax) Micro Four Thirds (MFT) 1 Nikon Series 1 and 1in sensor
23.5x15.7mm
1.5x
28.2mm
13x7mm
2x
21.4mm
13.2x8.8mm
2.7x
15.8mm
Note – actual sensor sizes can vary fractionally from the figures quoted above. For example, the Nikon full-frame FX format is 35.9x24mm and Fujifilm’s X-series sensor measures 23.6x15.6mm.
Images These shots were taken from the same position to show the effect of different focal lengths. They were taken with a full-frame (FF) 35mmDSLR at the quoted focal lengths, and the equivalent focal lengths needed for a Micro Four Thirds (MFT) or an APS-C 1.5x camera are also supplied.
FF 14mm MFT 7mm APS-C 1.5x 10mm
FF 20mm MFT 10mm APS-C 1.5x 13mm
FF 28mm MFT 14mm APS-C 1.5x 19mm
FF 50mm MFT 25mm APS-C 1.5x 33mm
FF 85mm MFT 43mm APS-C 1.5x 57mm
FF 100mm MFT 50mm APS-C 1.5x 67mm
FF 135mm MFT 67mm APS-C 1.5x 90mm
FF 200mm MFT 100mm APS-C 1.5x 133mm
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