Understanding Lens Focal Length and Aperture for New DSLR and Hybrid Camera Users

New Digital SLR and Hybrid Mirrorless Interchangeable Lens Camera owners take note: you are possessing a precision-engineered optical imaging device that gives you sharp focus on any object that you so choose. To master the use of these optical devices, you need to understand what the numbers on the lenses stand for and what they mean to you. After all, the purpose of owning these cameras is to be able to change lenses, but if you don't know what kind of lenses do what, then you wouldn't know what lenses you need to buy.

This article will not go into the technical details, but it should allow the novice to have a flavour of lenses. And if that triggers your curiosity, you can do more detailed research over the Internet or attend photography courses.

The 2 important attributes that make a lens are focal length and maximum aperture.

1. Focal Length
If you are a point-and-shoot compact camera user, you would probably just be concerned about the zoom factor (3x/5x/10x zoom). More camera makers also try to market the wide-angle lens feature like 24mm. With a wider-angle lens, you are able to see wider view without having to move too far back.

On the other extreme, when the focal length is very long, like 200mm, you can zoom-in to the subject without having to go too close-in. Great for shooting wildlife, or become a paparazzi. A handful of compact cameras have powerful zooms, to like 400mm. That would cost thousands of dollars on an equivalent DSLR lens.

The below is a table to describe the common uses for lenses of different focal lengths.

Do take note that the focal length described in all interchangeable lenses are the 35mm full-frame equivalent. On a Four-thirds system, you need to multiply by 2. On cameras using APS-C sensor, you need to multiply by 1.5. Therefore, an Olympus or Lumix lens with 20mm focal length will give you 40mm field of view, while the Samsung lens with 30mm focal length will give you 45mm field of view.

2. Aperture
Aperture determines the "brightness" of the lens, or commonly used to refer to the "speed" of the lens. The smaller the f-number, the brighter the lens. Here's a diagram to explain it all.

To determine what the biggest aperture size, look for the value next to "F" (e.g. F2.8), or next to "1:" (e.g. 1:2.8). If there are 2 values separated by a dash (e.g. 1:3.5-5.6), it means the aperture varies from the minimum focal length to the maximum focal length for zoom lenses. This varying aperture characteristic happens to most low-end lenses and compact cameras, simply because when the lens zooms in, the lens barrel extends and less light reaches the sensor.

From a price point, a large aperture size (i.e. small f-number) instantly translates to an expensive lens, with the exception of 50mm f/1.8, easily the cheapest lens for the aperture size. Even so, prices may vary for the same lens type among brands due to the lens quality.

Here's a simplified table showing the common aperture sizes you might find in different types of lenses.

Other lens information that are just as important:

- lens diamater (front): denoted by a letter o with a stroke  (⌀) followed by the number. This information is useful when you want to purchase lens filters. Buy the correct size so that you can screw the filter to the lens.

- lens mount (back): each lens is built with a lens mount for a specific camera brand or range of models. If in doubt, ask before buying. Third-party lens brands like Tokina or Tamron often manufactures different lens mounts for the same lens model (e.g. 28-70mm f/2.8), so be sure to buy the correct lens mount. Some mounts are specified by the type instead of the brand because the mount is used by several brands. For instance, Four-thirds, Micro Four-thirds, K-mount.

- choosing and testing lens (quality control during production): even with the same lens model, the sharpness may vary during factory assembly. As I mentioned, lens is a precision optic, so any misalignment will result in a less-than-stellar outcome. Some customers would test several lenses to select the sharpest. If you want to do the same, please go to those popular camera shops that lets you do that without qualms.

- lens care: you are recommended to at least buy a neutral filter (e.g. UV filter) to protect the front lens element so that the special lens coating would not get scratched by abrasions. Do not drop the lens, especially if the lens has a built-in image-stabilizer function, because the mechanism is highly sensitive. Do not get so uptight when you spot dust inside the lens, and there is no way to prevent this from happening over time, because the lens is not sealed (unless you never remove the lens from the camera body, which then defeats the purpose of an interchangeable lens system). Dust will NOT affect overall image quality.

Take heart that while you may upgrade your camera body to higher megapixels or improved high ISO quality, the function of the lens remains unchanged. Getting great lenses will last your for decades (and may even cost more than the camera body), so treat your lenses well. A word of caution: hybrid camera lenses are generally less durable than DSLR lenses, cost less, and are made of lighter plastic materials, so try not to man-handle them during shooting.


I hope this write-up provides you some useful info when determining what lenses you should get for what purpose (generally), why there are price differences among lenses, and how to take care of your lenses.

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