Aperture

Aperture, measured in f/stops, is the most important quality of a lens. Lenses with lower f/stops are heavier and cost more, but they focus faster, blur the background better, and let you handhold the camera in less light. To understand the cost difference, compare Canon’s three commonly used 50mm lenses. Each lens is one f/stop faster than the previous, passing twice as much light to the sensor:

  • Canon 50mm f/1.8: $100
  • Canon 50mm f/1.4: $350
  • Canon 50mm f/1.2: $1,500

As you can see, doubling the light roughly quadruples the cost. Size and weight also increase, especially with telephoto lenses. These three lenses are each one f/stop faster than the one previous:

  • Canon 400mm f/5.6: $1200, 2.8 lbs.
  • Canon 400mm f/4: $5,800, 4.3 lbs.
  • Canon 400mm f/2.8: $7,200, 11.8 lbs.

For professionals, the extra cost and weight is worth it. If you’re an amateur, I’d recommend starting with an inexpensive lens at the focal length you need and upgrading only when you’re frustrated with the maximum aperture—you can usually sell lenses for close to their original cost, so the risk is minimal.

For information about how to use aperture creatively, read Chapter 4 in Stunning Digital Photography.

Variable Apertures

Most consumer zoom lenses have a variable aperture, which means the maximum aperture when zoomed in is smaller than the maximum aperture when zoomed out. You can recognize variable aperture zoom lenses because their name has two f/stop numbers listed, such as “f/4.0-5.6.”

For example, the Canon PowerShot SD950 P&S camera has a zoom lens with a focal length of 7.7-28.5mm and a maximum aperture of f/2.8-f/5.8. That means at its widest angle (7.7mm) the maximum aperture is a respectable f/2.8. However, when zoomed in to 28.5mm, the maximum aperture is f/5.8—requiring more than four times more light than f/2.8.

The smaller aperture when zoomed in means your camera will have a harder time focusing and shutter speeds will be much slower. Handholding telephoto lenses requires faster shutter speeds, meaning many of your telephoto pictures will be shaky.

So, should you always avoid lenses with variable apertures? Not necessarily, but you should understand the limitations. Variable aperture lenses are much less expensive to make than constant aperture lenses. For example, the variable aperture Canon 28-135mm f/3.5-f/5.6 lens above is about $430, but the constant aperture Canon 24-105mm f/4.0 lens costs more than twice that.

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