The Crop Factor Explained
What is the crop factor and why is it important? And why do some cameras have smaller sensors in them the their more expensive full frame cousins? I’m an avid reader of science books. Admittedly whenever an equation is thrown into a sentence I feel a bit lightheaded. However, it’s a small price to pay for a better understanding of the natural world.
One of my favourite authors is the palaeontologist Stephen Jay Gould (the late Stephen Jay Gould unfortunately, it’s a sobering thought that I’ve now read all his books and will never see another new one). My favourite book by Gould has the intriguing title of ‘The Panda’s Thumb’. If you’ve not read it you should.
The theme of the book is the idea that the evolution of life is constrained by what’s gone before. Humans will never evolve a pair of wings because there’s no part of our body plan that could be co-opted to the task. One of the analogies that Gould uses to explain his idea is the QWERTY keyboard. The QWERTY keyboard isn’t very efficient. It was originally developed to actually slow typists down. In the days of the mechanical typewriter if you typed too fast you’d jam the mechanism. Unfortunately for everyone who uses a keyboard QWERTY stuck and became a standard.
Photography has a standard that’s stuck and it’s called full-frame (a full-frame digital camera has a sensor which has exactly the same dimensions as a 35mm film frame). The focal length of lenses is invariably described in relation to that of a lens on a full frame-frame camera. I’ve a Canon G10, which has a lens with a focal length range of 6.1 to 30.5 mm. However most people refer to the lens as the equivalent of a 28-140mm full-frame lens. This is largely because 6.1mm means nothing to most people, whereas 28mm, why that’s a wide-angle lens.
This has led to the use of another term. Crop factor. .
A full-frame sensor is a big sensor. Big sensors are more expensive to produce than smaller sensors because fewer of them can be fitted onto a wafer of silicon. (sensors are manufactured on standard-sized circular wafers of silicon. The more sensors you can fit onto the wafer, the cheaper each individual sensor will be). In the Palaeolithic era of digital photography (i.e. ten years ago) camera manufacturers like Canon and Nikon started to produce DSLRs with sensors that were smaller than full-frame. These sensors (nicknamed cropped sensors) were the same size as a now largely forgotten film standard called APS-C. This meant that DSLRs could be made more cheaply, to the benefit of everyone.
Unfortunately there was one problem. The angle of view of a lens is different when fitted to APS-C sensor camera than a full-frame camera. The angle of view of a lens is a measurement – in degrees (°) – of the proportion of a scene that’s projected by the lens onto the sensor. Because an APS-C sensor is smaller than a full-frame sensor the image projected by the lens is essentially cropped more tightly. The practical upshot of this is that a 28mm lens captures more of a scene when fitted to a full-frame sensor camera than when fitted to an APS-C sensor camera (this is a disadvantage if you like to use wide-angle lenses, but and advantage if you like to use telephoto lenses – a telephoto lens will appear ‘longer’ on an APS-C sensor camera).
The crop factor figure of an APS-C camera allows you to calculate what lens you’d need that would give you the same angle of view as one fitted to a full-frame camera. The crop factor of most APS-C cameras is 1.5x (Canon buck the trend by using slightly smaller chip so the crop factor is 1.6x on their cameras). Say you want to know what the equivalent to a full-frame 28mm lens is. You’d simply divide 28 by 1.5 to get 18. So an 18mm lens on an APS-C camera will have the same angle of view as a 28mm lens on full-frame camera.
Simple. Though actually I’ve just realised that the paragraph above comes very close to including an equation. I’ll stop there before I get too light headed and faint onto my frustratingly inefficient keyboard.