What Is Crop Factor and Why Does it Matter? Video 101

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Crop factor might sound like some sort of agricultural term, but it's actually an important topic to keep in mind when selecting both cameras and lenses. Crop factor is just a number really, but it has an important effect on how your camera and lenses interact. I know, I know, that's kind of vague. Read on, my camera-loving comrades!

In a previous lesson, we reviewed what image sensors are and talked about some of the more common sensor sizes found in cameras. These varying sensor sizes have a big impact on image quality, and they also interact with lenses differently. Most, but not all lenses have a focal length based on being paired with what's called a full frame sensor.

The term, "full frame sensor" is derived from the era of film-based cameras. Some of you might remember loading 35mm film into your cameras. The dimensions of a 35mm negative are now used as a reference size for modern DSLR sensors. When a sensor fills that original 35mm film size, it's known as full frame. Those sensors produce great image quality, but they are more expensive than smaller sensors. In mid-level and lower end DSLRs, you'll see smaller sensors, with the most common being the APS-C sensor. APS stands for Advanced Photo System, but you don't need to worry about that right now. Just keep in mind that these are the type of sensors typically found in most entry and even mid level DSLR cameras. So, what does all this tech talk mean? Check out this video below for some knowledge:

Now that you've watched the video, everything is crystal clear to you now, right? No? Okay, let's dig into the details a bit.

At a basic level, the main idea here is that the same lens will capture more light or less light, depending on the camera body it is attached to. Take a look at the diagram below to see what I mean!

Notice that light enters the lens and forms a circular image, but the image sensor is rectangular, so some of that light is not captured for recording. On a less expansive DSLR, the sensor is usually smaller and it captures less of the light entering the lens. Essentially, the image is being cropped. Let's take a look at another example like the image below.

Keep in mind that we were using the same lens, with the camera at the same distance and position, but we swapped between a full frame camera body and an APS-C sensor camera body. Our 50mm lens gave us a significantly larger field of view on our full frame camera. So, what do you do if you want to know what sort of focal length you're actually getting? That's where some simple math comes into play, but don't worry, math is our friend in this case. With a bit of multiplication, we can figure out the actual field of view we'll get.

Math Time!
Crop factor itself is a number, generally in the range of 1.3 to 2.0. All that number tells you is how much smaller the sensor is versus a full frame sensor. The closer the number is to 1.0, the less image cropping will occur. In the image above we had 50mm lens on our APS-C based camera, which has a crop factor of 1.6. If we multiply our lens focal length by our crop factor (50mm X 1.6), we get 80mm.

It's as easy as that! To find out what your cameras' crop factor is, you'll have to look it up in the manual or online. As a general guide, most lower level Canon cameras have a crop factor of 1.6, while with Nikon's entry level DSLRs, 1.5 is the crop factor. Again, this number will vary with different cameras and manufacturers, so make sure to check before you start calculating.

Now that your brain is swimming (but hopefully not scrambled!) in numbers, I'm going to have to qualify all this knowledge. Sometimes, you don't need to know the crop factor. It depends on the lens.

Yup, as if this topic wasn't confusing enough, certain lenses are actually made for APS-C sensors. In that scenario, the listed focal length on the lens itself will, when placed on your APS-C camera, give you a field of view that's true to the lenses specifications. For example, a 35mm lens that's been made for an APS-C based camera will give you 35mm field of view when mounted to an APS-C based camera. It sounds more confusing than it actually is, but keep this in mind when you're on the lookout for new lenses.

Why Crop Factor Matters
By this point, I know you might be wondering, "Why does this even matter? Can't I just move the camera further from the subject to get a wider field of view?"

The answer is yes, but sometimes your setting will constrain your movements too much. This comes up when shooting indoors or in close quarters, like a car. Unless you buy wide angle lenses made for APS-C sensors, this will limit your shooting abilities.

I don't want to give the impression that full frame cameras are always better. First off, they are expensive, usually a lot more expensive than APS-C based cameras. It's also important to note that crop factor can sometimes help, rather than hinder your shooting. If you're trying to get a closer shot of your subject but physically can't move closer, the crop factor will act as a sort of focal length multiplier. For instance, an 85mm lens on an APS-C based camera (with a 1.6 crop factor) will give a field of view of 136mm.

If you're still scratching your head, don't fret! This lesson is here for you to read and reread as many times as you need to. Remember, Vimeo Video School is here to help. Classes are in session and the tuition is free! Soon enough, you'll be impressing friends and strangers alike with your vast crop factor knowledge. Onward!

Ready for more? Learn the difference between zoom and camera movement or view more lessons at the Vimeo Video School.

Guest Post's article first appeared on OnlineVideo.net