|Sent on:||Wednesday, June 6, 2012 8:04 PM|
Great article sent along from John Mariana – thanks John!
So often I hear discussions of a lens on a full sensor camera and that when put on the cropped sensor camera it is a longer lens. I think this is ridiculous. A lens cannot change it's focal length simply by placing it on another camera. .
When a 300mm lens is used on a full sensor camera it will produce an image at the "image plane" of the sensor. When the same lens is placed on a cropped sensor camera it will produce the same image at the image plane but the smaller sensor makes it seem larger because it is cropping the image. This is better explained in the following article that appeared in Outdoor Photographer.
Attached is a PDF with this explanation and a very good chart of a variety of lenses.
– By the Editors
Cropped Sensor vs Full Sensor
Magnification is a function of the subject's size, its distance from the camera and the focal length of the lens used to make the photo. Using a longer lens, moving closer to the subject or choosing a larger subject will make the subject appear larger in the image frame.
A given focal length focused at a given distance produces a given magnification at the image plane. If you put a 300mm lens on a full-frame camera and focus on a moose at a certain distance, the moose's image will be 0.5 inches high at the image plane. Since a full-frame sensor measures approximately 1x1.5 inches, the moose's image will take up half the height of the frame.
If you then put the same lens on an APS-C (1.5x) DSLR and take a photo with that camera from the same spot, the moose's image will still be 0.5 inches high at the image plane, but the APS-C sensor measures approximately 0.6x0.9 inches, so that the 0.5-inch-high moose image will occupy nearly the entire height of the photo (0.5 of the 0.6 height).
So it is said that putting the 300mm lens on the APS-C camera effectively turns it into a 450mm, but really it doesn't—the APS-C sensor just crops the image produced by the lens at the image plane more tightly.
So that on a full sensor you would need a 450mm lens to make the moose at the image plane be nearly the entire height of the full sensor (0.8 or 0.9 of the 1 inch height of the 1x1.5 inch sensor).
It's commonly believed that long lenses compress perspective and short ones expand it. We think of "telephoto compression" because we generally use long lenses at great distance—and shooting at great distance does indeed flatten perspective. And we think of "wide-angle expansion" because we generally use short lenses at close shooting distances—and shooting at close range does indeed expand perspective.
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