Photography 101: Fixed Aperture Explained

Written by: Tiffany Joyce

A reader recently sent me an e-mail, asking me to explain how fixed aperture lenses work. She was suffering under the misconception that a fixed aperture on a lens means that’s the only aperture that lens is capable of shooting.

Apertures tend to be a subject that can be confusing to a beginning photographer. Primarily, it’s the larger number/smaller opening, smaller number/larger opening concept that tends to throw folks off at first. Aperture is a calculation of the lens opening with respect to the lens focal length, with each full f-stop increment representing an opening in the iris that has twice as much area as the stop below it. When a person buys their very first digital SLR, the kit lens it comes with tends to be a variable aperture lens. For example, when I bought my Canon Digital Rebel XTi back in 2007 (which has now been replaced by the Canon EOS Rebel T1i), it came with an EF-S 18-55mm f/3.5-5.6 II Lens.

Let’s break that down. The 18-55mm means that the lens shoots at 18mm at its widest point, and 55mm at its longest point. This range is what defines it as a “zoom” lens. The f/3.5-5.6 means that the largest available aperture is f/3.5 when shooting at its widest point (18mm), and the largest available aperture is f/5.6 when shooting at its longest point (55mm). In less expensive lenses such as this one, the aperture functions in tandem with the lens focal length, which is why they have variable apertures. This is usually demonstrated by a lens barrel that extends and contracts as it is zoomed in and out.

Now, take my favorite walking-around lens, the EF-S 17-55mm f/2.8 IS. Again the 17-55mm makes this lens’ widest point 17mm, and longest point 55mm. However, the single aperture number indicated, f/2.8, means that the largest available aperture is f/2.8 throughout the entire focal length. I can shoot at f/2.8 at 17mm, at 55mm, and at every point in between. In the construction of a lens of this type, the aperture functions independently of the focal length, which results in lens of heavier (and more expensive) construction. Often, the barrels on fixed aperture lenses do not extend and contract as they are zoomed in and out (though the 17-55 does).

A fixed aperture lens is desirable because photographers are able to expose the photograph equally at all focal lengths (and of course, the larger the available aperture, the more light is captured and the “faster” the lens). In a variable aperture lens, photographers need to be cognizant of the fact that properly exposing the shot will require decreasing the shutter speed and/or increasing the ISO at longer focal lengths in order to achieve the same level of exposure as at the shorter focal lengths. There are also various implications to be aware of when using a flash. However, accommodating for that, variable aperture lenses are perfectly acceptable tools in your arsenal of gear.

Now, just because a lens has a fixed aperture certainly does not mean that it ONLY shoots at that aperture. An f/2.8 fixed aperture lens will shoot at every other available aperture (typically up to f/22 or f/32). An f/3.5-5.6 variable aperture lens will also shoot up to f/22 or f/32 at either end of the focal range. This applies to prime lenses as well – for example, a Nikon 50mm f/1.4D AF Nikkor shoots the full range of apertures. In all cases, the aperture indicator on the lens only refers to the largest that the iris of the lens is able to open.

I hope I’ve been able to provide some clarification to this somewhat confusing topic. If you have any questions or input, feel free to let us know in the comments!

Photo credit: “Camera lens and aperture” by Nayukim on Flickr Creative Commons.

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