Beginner’s Corner – Crop Sensors
By Charles MacPherson–
So what in the world is a “crop sensor” anyway?
There are two kinds of sensors in DSLRs – full frame and crop.
Full-frame sensors are exactly the size of 35mm film (24mm X 36mm) Crop sensors are smaller and come in different sizes.
The reason that the sensor size is important is that it changes the way a lens behaves. To explain that, we first have to define a couple of things about lenses, specifically their focal length.
The focal length of DSLR lenses are spoken of in millimeters. You’ll hear about a 50mm lens, a 600mm lens, a 24mm lens and many others. That “mm” specification tells us how much the lens magnifies the scene we’re looking at.
50-55mm is considered “normal”. That means that looking at the scene through the camera gives you the same view as you see with your eye.
Anything above that gets into the telephoto category. So a 200mm lens is like looking through a telescope. a 600mm lens is like looking through a bigger telescope.
Anything less than 50mm is like looking through the wrong end of the binoculars – everything looks further away, and shows you a wider angle of view. So looking through a 35mm lens makes things look farther away than it is. A 24mm lens, even more so.
Now with that out of the way, let’s talk about how the size of the sensor figures into all of this.
Crop sensor cameras have a “crop factor”. Most Nikons are 1.5. Most Canons are 1.6, except for some of the 1D series, which are 1.3.
These sensors have the effect of multiplying the focal length by the crop factor, thereby increasing their effective focal length.
Here’s what the difference looks like:
Full Frame. Canon 5D MkII.
600mm lens, 600mm effective focal length
1.3 Crop factor. Canon 1D MkIV.
600mm lens, 780mm effective focal length.
1.6 Crop factor. Canon 7D
600mm lens, 960mm effective focal length
The crop factor works in your favor when you want a longer focal length. For example, in wildlife and sports photography.
But it works against you when you want the widest possible focal length, as you might for landscapes, by making your nice wide-angle into less of one. For example, my Canon 16-35mm L II zoom acts like a 25.6mm-56mm on my Canon 7D camera (1.6 crop factor).
In general, full-frame sensors exhibit lower digital noise and will help you to create a better shallow depth-of-field. Cameras with crop sensors cost less. Those – and the effect on focal length are the facts to keep in mind when you’re buying or using a DSLR.
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