We explain how crop factor works in our ‘What Is?’ video series
Crop factor allows you to work out what kind of effect a particular camera will have on the focal length of a lens. It’s become necessary to state this as cameras today use a variety of different sensor sizes and lenses are marketed by their actual focal length rather than the effective focal length when used against a camera with a particular sensor. In other words, using the same lens on different cameras can give you a different effective focal length to work with, so the crop factor helps you to make sense of this, which in turn helps you to decide which kind of lens is suitable for a particular subject.
Given that focal lengths are often translated into 35mm terms, crop factors are applicable to cameras whose sensors as smaller than the standard measurement here which is full frame. Full-frame cameras have sensors roughly the same dimensions as a frame of 35mm film, and so they apply no crop factor to any mounted lenses. So, a 50mm lens used on a full-frame camera will maintain its 50mm focal length and its natural angle of view, just as if you were using it on a 35mm film camera.
APS-C sensors, which are smaller than full frame, are found in the majority of DSLRs and many Compact System Cameras, Their smaller size results in a crop factor of around 1.5-1.6x, the reason being that the smaller sensor only covers a central part of the lens, and so the peripheries of the lens are left unused. This has a similar effect to using a longer focal length, and you can work this focal length out by simply multiplying the focal length by this crop factor.
So, that same 50mm lens on a camera with a crop factor of 1.5 would result in an effective focal length of 75mm, as this is what you get when you multiply 50 by 1.5. APS-C sensors aren’t all the same size, however. The sensors in Canon’s DSLRs are slightly smaller than those inside Pentax, Nikon and Sony’s SLT models, and so this increases slightly to a factor of 1.6. So, that same 50mm lens will be closer to 80mm lens than 75mm.
Cameras with the smaller Micro Four Thirds sensors have a crop factor of 2, and so the effective focal length is easier to calculate here as all you need to do is double the focal length of your lens. A 50mm lens, for example, will behave more like a 100mm lens. It’s no coincidence that some lenses developed for this system match the focal ranges of popular 35mm system optics – for example 12-35mm lenses which offer effective focal ranges of 24-70mm and 35-100mm lenses which behave more like 70-200mm lenses.
Sensors that are smaller than these such as the 1in sensors in Nikon’s compact system cameras and the 1/1.7in sensors inside some Pentax’s models increase this even further. For this reason, you’ll notice the kit lenses supplied with these bodies have even shorter focal lengths than those designed for cameras with larger sensors. For example, the 5-15mm kit lens that ones with certain Pentax models seems wide on paper, using it with the Q-S1 model shows that what you actually end up with a lens more like a 23.5-70.5mm lens given the crop factor of 4.7x.