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How to Calculate the Zoom From a Lens

Understating the actual zoom capabilities of a lens can be very confusing. When lenses were exclusively manufactured for 35 mm film SLR cameras, zoom was a straightforward set of numbers. But now that digital SLR cameras have a different size recording plane than the 35 mm film plane, the zoom lens number will not give you the same results as if you were using a 35 mm film SLR. Additionally, some digital cameras list their zoom capabilities as a magnification factor rather than a range. By calculating the zoom for each of these listings, you can determine the actual zoom capabilities of your camera or lens.

Things You'll Need

Calculator
Camera lens
Camera instruction book

Instructions

Magnification factor to focal length.
- 1
  Find the magnification factor listed on the camera (for example 7X or 10X).
- 2
  Locate the 35 mm equivalence of your zoom (most cameras are a 28 mm or 35 mm minimum range).
- 3
  Multiply the figure in Step 1 by the figure in Step 2 to calculate the maximum length of your zoom. For example, a 28 mm lens with a 10X zoom will have a maximum zoom of 280 mm.
Focal length to magnification factor.
- 4
  Examine the camera lens to find the listed minimum focal length.
- 5
  Look at the space next to the minimum focal length to see if there is a maximum focal length listed. ̵6;Prime̵7; lenses have a fixed focal length and do not zoom. A Standard lens is a 50 mm lens, which approximates the view seen by the human eye. Zoom lenses have a range over which they zoom. For example, a 28 to 80 mm zoom lens can zoom out to a 28 mm wide angle and zoom in to an 80 mm telephoto.
- 6
  Divide the higher number by the lower number to calculate the magnification factor of your lens.
Figuring telephoto lens length for a Digital Camera
- 7
  Find the multiplication factor for your digital camera (the amount that the camera increases the zoom of the lens).
- 8
  Multiply this number by the maximum focal length of your zoom lens to calculate the new zoom length. For example, if your digital SLR has a multiplication factor of 1.8, and your 35 mm film lens is a 45 mm wide angle, then on a digital SLR it becomes a 85 mm lens.
- 9
  Multiply the number by the minimum focal length of your wide angle to calculate the new wide angle length. For example, if your digital SLR has a multiplication factor of 1.5, and your 35 mm film lens is an 18 mm wide angle, then on a digital SLR it becomes a 28 mm lens.

Photography