What does it mean when someone tells you that a camera has a “1.6x crop factor”? Well, first off, “crop factor” can refer to both sensor size as well as lenses, because it generally affects both.
Check out this quick article about sensor size, for some background information. Digital cameras can use very differently sized sensors, but many times will accept lenses from the same manufacturer. For example, a Canon EOS Rebel T1i has an APS-C sized sensor, but the Canon 6D has a “full-frame” or “35mm” sized sensor. But many lenses will work for both cameras – how does that work, when you’re recording the image on a much bigger or smaller sensor?
Well, lenses project images in a circle. The circle can be very large, or quite small, depending on the lens’ design. Here’s a crude example of how it looks:
But sensors come in many different sizes. If you use the same lens, but a camera with a smaller sensor, it looks like this:
You can see that the bigger sensor records everything the smaller sensor does, plus more. Since the image is being projected, a smaller sensor only “sees” a zoomed-in version of the whole picture. For example, the below picture was taken with a “full frame” sensor. A smaller sensor, with the same lens, may have captured something close to the area outlined in red:
This “zoomed in” effect is what people are talking about when they say crop factor. A smaller sensor appears more “zoomed in” because it’s only saving part of the whole image.
Since [focal length] is how we measure the amount a lens is “zoomed”, a crop factor gives you the amount by which a smaller sensor is “zoomed” – when compared to a full frame sensor. This makes sense – smaller sensors are always “zoomed in,” so the same lens on a smaller-sensor camera would appear to be similar to a more “zoomed-in” lens.
So if I told you a sensor has a 2x crop factor, and it had a 50mm lens on it, it would take pictures that look like a 100mm lens on a full frame sensor. We would say that the 50mm lens has the perspective of a 100mm lens, when on the 2x “cropped” sensor.
Of course, when you look though the viewfinder of a camera, you see what the picture would look like. So for cameras with smaller sensors, the viewfinders tend to be smaller as well! Image quality aside, this is one of the major advantages to a “full frame’ sensor.
But remember, full frame is just one of many different sizes. There are digital sensors much larger than “full frame.”
One last quick point – you may have noticed FX/DX lenses for Nikon, or EF/EF-S lenses for Canon. What is that about? Well, the former are “full frame” lenses that work for all cameras in that brand. The later are lenses that only work with smaller, “cropped” sensors. Why don’t the lenses work with full-frame cameras? Because the light is projected in a smaller circle! It’s sized perfectly for APS-C sized sensors, but not full frame sensors:
Let me know if you have any questions in the comments, and I’ll be glad to answer!