Most low-end lenses are variable aperture, operating with a different minimum aperture depending on the focal length.

One of the more common issues I see popping up online is understanding the difference between fixed aperture and variable aperture lenses. This is one of the most important aspects to consider when looking at lenses for your digital SLR, and there is a noticable difference between the two types. Here’s how to tell which category your lens falls under.

A fixed-aperture lens is a lens with a constant minimum aperture throughout the zoom range. This means that the aperture functions independently of the lens’s focal length, i.e. a 70-200mm f/2.8 lens can operate at f/2.8 at both 70mm and 200mm. This lens is preferable, but it’s also heavier and more expensive than its variable cousin.

Most kit and lower-end lenses for digital SLRs are variable-aperture lenses. You can tell this by the way the lens is described: for example, the kit lens that comes with the Canon Rebel XTi is a 18-55mm f/3.5-5.6 lens. While the lens is capable of operating at f/3.5, it can only do so at toward the 18mm end of the zoom range, jumping up to a decidedly-smaller f/5.6 as the focal length increases.

The kit lens example illustrates the crucial issue with variable-aperture lenses - as the focal length increases on a variable-aperture lens, the range of available apertures becomes more narrow. Less light is entering the lens, forcing the camera to choose a slower shutter speed or a higher ISO to still take a good exposure. Photographers operating their camera in manual mode may get an unpleasant surprise as their available light suddenly decreases by a few stops simply because they zoomed in a bit more.

This is an important concept for those without digital SLRs to understand as well. With very few exceptions, the lenses on point-and-shoot digital cameras are of the variable type.

Should you still buy a variable-aperture lens? Sure; there are plenty of great lenses that are a lot cheaper because of their variable design. Just keep in mind that it introduces one more thing to consider when trying to figure out how to properly expose an image.

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