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For more than 70 years, 35mm film was the dominant photographic medium. Then digital photography came along. 35mm film is still used for some still photography and used a lot for motion picture photography. Like a digital sensor, a piece of 35mm film crops a specific sized rectangular image out of the image circle projected by a lens. After 70 years of using 35mm film people got used to specific focal lengths having specific fields of view. So photographers became accustomed to the idea that a 50mm lens was a normal lens, because when used with 35mm film a 50mm lens has roughly the same field of view as the human eye.
Anything longer than 50mm is telephoto. Anything shorter is wide-angle. With 35mm film a 28mm lens is pretty wide. A 16 or 24 millimeter lens is ultra-wide. Conversely, 300mm has a good amount of telephoto power and 600mm puts you in the realm of serious surveillance. Most digital cameras had image sensors that are smaller than a piece of 35mm film. That means they crop a narrower rectangle from the circle projected by the lens. So if you take the 50mm lens from your 35mm camera, which yields a normal field of view, and put it on a digital camera which has a smaller sensor, you're going to get an image with a narrower field of view.
Let's look at a specific example. This is a Canon 7D. It has an image sensor that's the size of a piece of APS film. That's a little smaller than 35mm film. The Canon Rebels and 10 series cameras also use the same size sensor. With this sensor, any lens that I put on the camera has a 1.6x crop factor for its field of view. So if I put this 50mm lens on this camera, which is a normal lens on a 35mm film camera, I end up with an equivalent field of view of 80mm, or 50 times 1.6.
This means that the lens is now more telephoto than normal. 50mm and 35mm terms is normal. 80 is a little telephoto. So after converting this to 35mm terms, we realize that 80mm is no longer normal. It is though a great portrait lens. Many Nikon SLRs also use the sensor that's smaller than a 35mm piece of film and they have a multiplication factor of 1.5. If your camera has a crop sensor, then your manual should list the appropriate multiplication factor.
If you tend to shoot with long lenses then a crop sensor is great, because your lens is end up with the field of view of a much longer lens. For example, if I put this big 800mm lens on my 7D, I end up with an equivalent field of view of 1280mm. A tremendous amount of telephoto power. The downside is if I want to shoot wide-angle. Here is a 28mm lens which is a nice wide-angle lens on a 35mm camera. On my crop sensor 7D though we'll end up having an equivalent field of view of a 44mm lens.
Means I've gone from wide- angle to roughly normal. Fortunately, as you'll see later most vendors manufacture lenses specifically designed for their crop sensor cameras. Here is an Nikon D700 and a Canon 5D Mark II. Both of these cameras have what are called full-frame image sensors. That is their sensors are the same size as a piece of 35mm film. With a full-frame camera there is no multiplication factor. So right now I've got a 24 to 70 millimeter lens on this camera and that's equivalent to a 24 to 70 millimeter lens. It's just like it would be on a 35mm camera.
This is a micro four thirds camera. We're going to talking more about these later. They're cool, in-between a point-and-shoot camera and an SLR. The sensor is bigger than what you'll find in a typical point-and-shoot, but it's smaller than either the APS sized sensor and a crop sensor SLR and much smaller than the full-frame sensor and a full-frame SLR. Micro four thirds cameras have a multiplication factor of 2. So right now I've got a 20mm lens on here. That's going to make it equivalent to a 40mm lens or roughly a normal lens. I take it off though. It's one of the beautiful things about micro four thirds cameras is like an SLR they have interchangeable lenses and I can put on this 14 to 42 millimeter lens.
2x multiplication factor means this is equivalent to a 28 to 84 millimeter lens. It's a good walk-around range. Here is a point-and-shoot camera kind of a typical small little camera. This is a Canon S95. I'm going to turn it on and you'll see the lens come out. So you can see physically this is a very small lens. The actual focal length of this lens is 4.9mm, which is tiny, to 22 1/2 millimeters, but in 35mm equivalence, because the image sensor in this camera is so tiny, this lens has an equivalent focal length range of 28 to a 105 millimeters.
Now you might be thinking, "I don't shoot 35mm film, I never have, so why should I care what my digital camera is equivalent to?" As you get more serious about photography you'll probably get more cameras and they might have different size sensors. Having a single reference point for what is normal, what is telephoto, what is wide-angle can make it easier to understand what you can expect from different lenses on your different cameras. And 35mm equivalency is the general reference point that all manufacturers use. So take a look at your lenses and your camera manual and try to determine the focal length equivalency of your different cameras and lenses.
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