Understand your lens true dimensions

Unless you ve paid for a high end dSLR, or a professional camera such as the Leica M9 , your pocket snapper s sensor will almost certainly be smaller than a frame of 35mm film, the standard point of reference against which all focal lengths are measured.The 35mm in a frame s name actually relates to the space between the top and the bottom of the film strip, which as well as the frame itself also contains some border areas and the sprocket holes used to move the film through the camera. A 35mm frame is positioned lengthwise on this strip, with its shortest dimension top to bottom perpendicular to the film s direction of motion. As such, neither the height nor the width of the frame measures 35mm, but instead 24x36mm.

To understand how the stated focal length on any lens will affect the shot captured by your camera, you need to factor in the multiplier effect, which converts the size of your sensor to the size of that 35mm piece of film. The multiplier is often between 1.5 and 1.7 but varies between manufacturers and models.So, if you re buying a lens for the Canon EOS 600D with its 22.3x14.9mm sensor you d need to multiply the stated focal length of the lens by 1.6. This would make a 50mm lens, commonly used in portrait photography, act like an 80mm lens, thus increasing the effective zoom and narrowing the amount of the scene seen in each frame. On a Nikon D5100 , which has a slightly larger sensor (23.6x15.6mm) you d need to multiply the lens measurements by 1.5, in which case an equivalent 50mm lens would act as though it were a 75mm unit.