What is Crop Factor for Digital Cameras?

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:

Light from a lens projects in a circle. A sensor sits inside the projected light and records only part of the image.

Light from a lens projects in a circle. A sensor sits inside the projected light and records some (most) of the image. Sensors (and their images) are almost always rectangles, so the lens and sensor are designed to fit about as big a rectangle as you can into the projected circle.

But sensors come in many different sizes. If you use the same lens, but a camera with a smaller sensor, it looks like this:

A smaller sensor, compared to a larger sensor. The lens always projects the same sized circle, so a smaller sensor just records a smaller portion of the whole image.

A smaller sensor, compared to a larger sensor. The lens always projects the same sized circle, so a smaller sensor just records a smaller portion of the whole image.

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:

A smaller sensor might only capture the area of this picture outlined in red. When using the same lens, the smaller sensor would appear to be "zoomed in" because it only includes the center part of the image.

A smaller sensor might only capture the area of this picture outlined in red. When using the same lens, the smaller sensor would appear to be “zoomed in” because it only includes the center part of the image.

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:

EF-S or DX lenses project an image circle that is perfectly sized for smaller sensors. But when used on a larger sensor, much of the image may be missing! While you technically can use some of these lenses on a full-frame camera, the results won't be good - and sometimes, the lens' optics may be in the way of your mirrorbox.

EF-S or DX lenses project an image circle that is perfectly sized for smaller sensors. But when used on a larger sensor, much of the image may be missing! While you technically can use some of these lenses on a full-frame camera, the results won’t be good – and sometimes, the lens’ optics may be in the way of your mirrorbox.

Let me know if you have any questions in the comments, and I’ll be glad to answer!

 

 

 

 

Author: Luke

Luke runs this site as well as several others. You can see more of his work at www.imadethisphoto.com - and feel free to comment with questions!

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