This series of articles is excerpted from Rocky Nook’s Enthusiast’s Guide to Exposure by John Greengo covers the controls — shutter, aperture and ISO. The week, John tells how the aperture is fundamental to choosing a lens.

Maximum aperture explained

The maximum aperture describes a lens’s light-gathering ability when the aperture is fully open. This is a key feature to know when selecting a lens. It’s a sign of how well the lens will perform under low-light conditions. It can also affect the focus performance, size, weight, and price of the lens.

Apertures for zoom and prime lenses

Prime lenses (non-zooms) will have a single number indicating the maximum opening (e.g., f/1.4 or 2.8). Zoom lenses will have either a variable maximum aperture (e.g., f/2.8-4.0) or a fixed maximum aperture (e.g., f/2.8). The lower the number, the better the lens will do in low-light conditions. All lenses will have apertures that can close down, so the minimum aperture isn’t nearly as important at the maximum aperture. Zoom lenses usually have slower apertures (f/2.8 to f/5.6) when compared with fixed or prime lenses (f/1.4 to f/4). Budget zoom lenses will often have the slowest apertures (f/3.5–5.6) and thus a limited ability to shoot images with a shallow depth of field.

Fast lens = more light

Faster lenses are available, but typically they are larger, heavier, and more expensive. How fast a lens do you need? This depends on what you hope to do with your lens and your budget. The standard kit lens, often an f/3.5–5.6 zoom lens, will be perfectly suitable for beginners.

Apertures Rocky Nook Enthusiasts' Guide to Exposure
Images with shallow depth of field are hard to achieve with cameras that have small sensors. This is a unique look best achieved with larger sensors and faster lenses.

Longer and faster for shallow depth of field

For shooting with a shallow depth of field, you’ll likely want a faster lens as well as one with a longer focal length. You can get a fast zoom, but their maximum aperture is typically around f/2.8. If you want or need something even faster, you’ll need to go with a prime lens, in which the maximum aperture may be as fast as f/2 or f/1.4. The least expensive and easiest way to shoot with shallow depth of field is with a 50mm f/1.8 or 85mm f/1.8 lens. These lenses are very common and relatively inexpensive. These normal to short telephotos with fast apertures are best known as portrait lenses but they are useful for much more. These are among the most common lenses in use today, and you should be able to easily find new and used options from a variety of manufacturers.

Opening photo: ©2020 Kevin Ames

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