Crop Factor Basics
Enter the phrase “crop factor” into your favourite search engine and you will immediately realise that the internet is awash with web pages boasting tutorials and offering advice on the topic. The rhetoric exists largely because the crop factor of a camera plays such an important role in contemporary photography. It is one of the only – if not the only – aspects of the camera that manages to influence both the image and artistic quality of a photograph. The crop factor affects the occurrence of noise in an image while at the same time affecting compositional parameters like angle of view and depth of field. So, with hesitation, let me try and unpack this issue as it pertains to landscape photography.
The first thing to know in the pursuit of crop factor enlightenment is that the entire system is based, rather arbitrarily, on the old 35 mm film standard. I say “arbitrarily” because a crop factor is simply a ratio, one that can just as easily be calculated using an 8" × 10" large-format camera or the tiny iPhone 7 camera as a reference. In fact, the whole concept of “35 mm” is itself somewhat obscure given that the 35 mm film standard features a light-sensitive area measuring 36 mm × 24 mm.
To calculate a crop factor, all one need do is take the length of one side of the 35 mm standard and divide it by the length of the corresponding side of whatever camera sensor you are attempting to calculate for. As an example, the Canon 7D Mark II has a sensor measuring 22.4 mm × 15 mm. If you take the longest edge, the crop factor will be calculated as 36 mm ÷ 22.4 mm, which approximately equals 1.6.
There are no rules governing the sizes that manufacturers can use when developing camera sensors and, as a result, there are a host of cameras with different crop factors available on the market today. Yet, despite the crop-factor-free-for-all, two sensor sizes have established themselves as somewhat of a standard in the world of crop sensor DSLR cameras, namely APS-C and Micro Four Thirds (abbreviated as MFT or 4/3).
There is nothing at all special about either of these sensor sizes. APS-C simply refers to cameras with a crop factor of 1.5 or 1.6 – depending on the camera manufacturer – and MFT refers to those with a crop factor of 2. Anyone who claims that one is better than the other is – as we will see a little later – rather missing the point of the crop factor all together.
All this raises the obvious question, Why do crop sensor cameras exist at all? and the answer is that they are cheaper to manufacture. Compared to full-frame cameras (cameras featuring sensors with the same dimensions as the 35 mm standard), a physically smaller crop sensor allows a camera’s entire optical system to be made smaller. Inside the camera body, a crop sensor camera is likely to have a smaller mirror, shutter and even viewfinder when compared to a full-frame counterpart. In the lens, the extent of the possible size reduction is even greater. A manufacturer has the option to reduce the size of almost every component of a lens should it be designed exclusively for use on a crop sensor camera.
In many respects, the crop factor itself can be misleading when you begin to think in terms of size reductions. It is normal to assume that for any component that can have its size reduced as a result of being designed exclusively for use with a crop sensor, that the magnitude of that reduction is equal to the crop factor, but this is not true. The magnitude of the reduction is, indeed, far greater; in fact, it is the square of the crop factor. A full-frame camera has a sensor area of 8.64 cm2 while the APS-C sensor found in the Canon 7D from our earlier example has an area of 3.36 cm2. This means that the surface area of the APS-C sensor is 2.57 times smaller than that of a full-frame sensor.
Theoretically, at least, this means that the optical elements inside a lens designed specifically for APS-C sensors can be 2.57 times smaller, and that translates into saving on a lot of very expensive glass. Generally speaking, crop sensor DSLR cameras cost about half that of comparable full-frame cameras and, while I’m sure that at least some of that difference can be chalked up to the manufacturer’s marketing department, much of those savings are rooted in physics.
It seems fair to say that in a world of enormously expensive photography equipment, any device that offers cost savings is going to be an attractive purchasing proposition. However, cost is not the only benefit that has given rise to the enormous popularity of crop sensor cameras. A further advantage can be found in the alternative name for the ‘crop factor’ – the ‘focal length multiplier’.
As the name ‘focal length multiplier’ suggests, attaching a crop sensor camera body to any lens effectively increases the focal length of that lens. If, for instance, one were to take a 50 mm lens and attach it to the Canon 7D body, the effective focal length of the combined body and lens will be 1.6 × 50 mm or 80 mm.
The first critical thing to understanding how such an accomplishment is even possible is to appreciate that the actual focal length of the lens has not changed at all. Focal length is defined by the distance between the primary optical element of a lens and the focal plane of the sensor. When a lens has a fixed focal length, such as is the case with the 50 mm, it is physically impossible to change that number. Instead, focal length multiplication is a by-product of cropping.
The best way to wrap your head around cropping is with an example. If, for the sake of illustration, we had two camera bodies, one with a full-frame sensor and one with an APS-C sensor, and we attached both bodies to the same lens with the same focal length and focused on the same subject, the resulting images might look as follows.
The APS-C photo clearly appears to have been zoomed in and this despite the lens in both images having the same focal length – this is the focal length multiplier in action. The only true difference between the photos is that the physically smaller APS-C sensor has sampled just a portion of the image offered up by the lens. When both photographs are viewed back at the same size, the result is the APS-C image appearing as if it were taken using a longer lens.
The focal length multiplier gives the first hint at the real-world implications of choosing to buy a crop sensor camera. Many photographers, especially those shooting wildlife, enjoy the fact that crop sensor cameras provide for extra effective focal length. Long lenses are expensive and by using a crop sensor camera, one can make a 400 mm lens appear longer than a considerably costlier 600 mm lens.
For the landscape photographer, on the other hand, the focal length multiplier can be a problem. Landscape photographers do a great deal of work with wide-angle lenses and unfortunately, the focal length multiplier applies as much to these lenses as it does to long lenses. Crop sensor cameras nullify a lot of what a good wide-angle lens can do and, even if you choose to use exceptionally short focal lengths to overcome the problem, the resulting images can often display exaggerated distortion.
In upcoming articles, I will take a closer look at the more intricate details of life with a crop sensor camera and ask the question, Is it at all possible for landscape photographers to get the same results from both crop and full-frame cameras? The answer is that it is possible but when doing so, crop sensor cameras don’t look nearly as cost effective as they do at face value.