In order to choose your lenses wisely, you should know the basic aspects of lenses and features to look for when choosing. Remember, you can change camera bodies in the coming years and keep your same set of lenses, so lenses are possibly the best and longest lasting investment in photography and video. They keep their value and quality glass never goes out of style.
One of the defining looks of cinema is the blurred background, or bokeh. This effect results from a narrow depth of field (DOF), or the distance between the nearest and farthest points that is in focus. For example, a shallow (or narrow) depth of field could render a dog's nose and ears unfocused while his eyes remain in focus.
Larger camera sensors enable shallower DOF but the lens is also plays an important role. A lens' aperture is the hole that lets light into the camera. A wider aperture results in a narrower the depth of field and a more blurred background. All other settings equal, it will also let in more light and your image will have a greater exposure (will be brighter). In photography, the shutter speed can be adjusted to compensate for a wider aperture; in this way, you can blur the background beautifully while keeping the image properly exposed. In video, this is avoided (to learn why, read How to Choose Shutter Speed For Video On Your DSLR). Instead, the ISO can be adjusted – learn more about ISO and digital sensors here.
To see how your choice of aperture affects depth of field – and blurs the background – check out the video at B&H Photo.
Apertures are expressed in terms of f-numbers. The lower the f-number, the wider the aperture. So an f-number of 4 (f/4) is narrower than an f-number of 2.8 (f/2.8). Each lens has a maximum and minimum aperture and lenses with wider maximum aperture (lower f-number) are generally more expensive. However, they are also more desirable, because they allow the photographer or filmmaker to blur the background and draw attention to the subject (these are often called “fast” lenses).
Tip: Most zoom lenses have different maximum apertures at different focal lengths. So if your lens has a maximum aperture of f/3.5 at 35mm, it may have a maximum aperture of f/5.6 at 55mm. For this reason, you should avoid changing focal lengths (zooming) while filming because you will notice distracting exposure variations through the shot. Or get lenses with a constant aperture; i.e, a lens with only one aperture value listed. They're more expensive, but necessary for zooming in video.
A key feature for video work is image stabilization (IS on Canon lenses, VR or vibration reduction on Nikon). It can help correct rolling shutter and, though it shouldn't replace other stabilization devices such as shoulder mounts, can smooth out camera shake in handheld work. See an the effects of image stabilization, even on a tripod, on Philip Bloom's video on Vimeo.
Prime Vs. Zoom
Prime lenses are often preferred to zooms because they are lighter, faster, more inexpensive and have great build quality. They will also improve your skill as a filmmaker; the hallmark of an amateur video is excessive zoom or relying on the zoom to frame the shot instead of carefully thinking about composition for every shot. That said, a zoom lens can provide more opportunity for capturing a moment because you don't need to physically move to recompose a shot.
When choosing a focal length, you first must consider the sensor size of your camera. If you have a Canon 5D Mark ii, you have a full sensor camera and the focal length written on the lens is the focal length you can expect to see in your pictures. However, if you have the Canon 7D or 60D, your sensor has a crop factor of 1.6. Nikons generally have a crop factor of 1.5. Crop factors change the apparent focal length of your lens and make it seem more “zoomed in.” That is, if you use a 30mm lens on a 60D, you'll have the equivalent of a 50mm lens – to get this figure, multiply 30 (the lens focal length) by 1.6 (the crop factor) to get about 50mm.
If you've taken photography classes, you've probably been handed a 50mm lens and told to shoot until you're comfortable with the lens. The 50mm lens is considered “standard” because it is close to how you view the world through your own eyes. This is a great starting point for video. Ideally, you'll have a standard lens, a longer lens (such as 100mm prime or 70-200mm zoom) and a wide angle lens (35mm or 16-35mm). Alter these focal lengths to choose the correct lens for your camera according to your camera's sensor size.
Lens Brands and Lines
The best lines to look at when considering lenses for video are Zeiss compact primes and ZE/ZF series, or the Canon EF series (the L series within the EF series are considered the best for video). However, if you're on a budget, you've probably clicked on the links to lenses I've mentioned and realized that these are not really an option for you at this point. See my article on choosing lenses on a budget for more practical advice if you're in this category. You can make beautiful images with very cheap lenses but even with inexpensive lenses it's important to consider the maximum aperture available, focal length and whether it's a prime or zoom. Also, you can make due with one very nice prime, as illustrated in Philip Bloom's Sofia's People shot entirely on a Zeiss 50mm lens.