From the course: The Practicing Photographer

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Why equivalent lenses don't always meter the same

Why equivalent lenses don't always meter the same

From the course: The Practicing Photographer

Why equivalent lenses don't always meter the same

- There's some focal length overlap in my lens collection. For example, this is an 18 to 55 millimeter lens, and this is a 10 to 24 millimeter lens. But here's something interesting: if I set them both to 20 millimeters in aperture priority mode, and dial in an aperture of F4, the camera recommends one-tenth of a second as a shutter speed when I'm using the 18 to 55. But, only one-sixth of a second when I'm using the 10 to 24. But, how can that be? Don't two different 20 millimeter lenses at F4 transmit the same amount of light? No, they don't, and this week on The Practicing Photographer, we're going to look at why. Focal length is, of course, the distance from the center of the lens to the focal plane of your camera. That's where the sensor sits. Inside your lens is an aperture of a given size. The size can vary, of course, but at any given time, the aperture will have a certain diameter. F-stop is the ratio of the focal length of your lens to it's aperture size. In other words…

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