Tips on How to Choose a Lens for Your Canon EOS 70D
With a dSLR (digital single lens reflex) camera, like the Canon EOS 70D, you can change lenses to suit different photographic needs. To decide which lens is the best partner for your camera, start by considering these factors:
Lens compatibility: Your camera accepts two categories of Canon lenses: those with an EF-S design and those with a plain old EF design.
The EF stands for electro focus; the S stands for short back focus. And that simply means the rear element of the lens is closer to the sensor than with an EF lens. And no, you don't need to remember what the abbreviation stands for. Just make sure if you buy a Canon lens other than one of the two sold as a bundle with the camera, that it carries either the EF or EF-S specification. If you want to buy a non-Canon lens, check the lens manufacturer's website to find out which lenses work with your camera.
Two other lens acronyms to note: First, the 18–55mm and 18–135mm lenses that you can buy as part of a 70D kit are IS lenses, which means that they offer image stabilization. Second, they also carry the designation STM. That abbreviation refers to the fact that the autofocusing system uses stepping motor technology, which is designed to provide smoother, quieter autofocusing.
Finally, be aware that some lenses can't take full advantage of the Dual Pixel CMOS (see-moss) autofocusing system that's used during Live View and Movie recording. Don't worry about what the name means — the important point is that it produces faster, more accurate autofocusing. If you're interested in learning more, go to the 70D product page at the Canon USA website, which has a link to a section that explains the technology and lists lenses that support it.
Focal length and the crop factor: The focal length of a lens, stated in millimeters, determines the angle of view that the camera can capture and the spatial relationship of objects in the frame. Focal length also affects depth of field, or the distance over which focus appears acceptably sharp.
You can loosely categorize lenses by focal length as follows:
Wide-angle: Lenses with short focal lengths — generally, anything under 35mm — are known as wide-angle lenses. A wide-angle lens has the visual effect of pushing the subject away from you and making it appear smaller. As a result, you can fit more of the scene into the frame without moving back. Additionally, a wide-angle lens has a large depth of field, which means that both the subject and background objects appear sharp. These characteristics make wide-angle lenses ideal for landscape photography.
Telephoto: Lenses with focal lengths longer than about 70mm are telephoto lenses. These lenses create the illusion of bringing the subject closer to you, increase the subject's size in the frame, and produce a short depth of field so that the subject is sharply focused but distant objects are blurry. Telephoto lenses are great for capturing wildlife and other subjects that don't permit up-close shooting.
Normal: A focal length in the neighborhood of 35mm to 70mm is considered "normal" — that is, somewhere between a wide-angle and telephoto. This focal length produces the angle of view and depth of field that are appropriate for the kinds of snapshots that most people take.
The following figure offers an illustration of the difference that focal length makes, showing the same scene captured at 42mm (left image) and 112mm (right image). Of course, the illustration shows just two of countless possibilities, and the question of which focal length best captures a scene depends on your creative goals.
Why select a lens that offers a single focal length when a zoom lens offers a range of focal lengths? In a word, quality. You typically see some reduction in picture quality at certain points in the range of a zoom lens. On the flip side, a zoom lens is more convenient than carting around a bag of prime lenses, and many zoom lenses today offer very good image quality.
Aperture range: The aperture is an adjustable diaphragm in a lens. By adjusting the aperture size, you can control the amount of light that enters through the lens and strikes the image sensor, thereby controlling exposure. The aperture setting also affects depth of field: A wide-open aperture produces a short depth of field, so the subject is sharply focused but distant objects appear blurry; a narrow aperture produces a long depth of field so that both the subject and distant objects appear sharp.
For the purposes of lens shopping, you need to know just a few things.
Every lens has a specific range of aperture settings. Obviously, the larger that range, the more control you have over exposure and depth of field.
The larger the maximum aperture, the "faster" the lens. Aperture settings are stated in f-stops, with a lower number meaning a larger aperture. For example, a setting of f/2 results in a more open aperture than f/4. And if you have one lens with a maximum aperture of f/2 and another with a maximum aperture of f/4, the f/2 lens is said to be faster because you can open the aperture wider, thereby allowing more light into the camera and permitting the image to be captured in less time. This not only benefits you in low-light situations but also when photographing action, which requires a fast shutter speed (short exposure time). So, all other things being equal, a faster lens is better.
With some zoom lenses, the maximum and minimum aperture change as you zoom the lens. For example, when you zoom to a telephoto focal length, you usually can't open the aperture as much as you can at a wide-angle setting. You can buy lenses that maintain the same maximum and minimum aperture throughout the whole zoom lens, but you pay more for this feature.
After studying these issues and narrowing down your choices, finding the right lens in the category you want is just a matter of doing some homework. Study lens reviews in photography magazines and online photography sites to find the best performing lens in your price range.