Lenses: Focal Lengths, Depth-of-Field

While older, manual SLR lenses are very inexpensive, there are disadvantages to using them for cinematography; these shortcomings are chiefly noticeable while pulling focus. For one, most still lenses breathe, meaning, they actually change focal lengths slightly while racking focus, which can be visually distracting for those used to cine lenses (which do not). As a related matter this breathing is accompanied by a physical lengthening (or shortening) of the lens while pulling focus, which can wreak havoc with the ability to use a matte box (see the matte box section). However, SLR lenses are exponentially cheaper than cine lenses — not to mention your DSLR probably doesn’t have an Arri PL mount or any way of easily affixing cine lenses — so go with used SLR glass and be happy that you’ve got a video camera with interchangeable lenses for several times less $$$ than any of us would’ve imagined a few years ago.

When it comes to choosing focal lengths, the first lens you should purchase is a 50mm prime, which is wide enough to get room coverage, yet not so wide that it introduces much geometric distortion. Hitchcock used it for the majority of his shots, and I shot 90% of The West Side with a 50 (pictured); they’re workhorse lenses and are generally inexpensive. When buying lenses you will generally want faster glass for the best low-light performance and flexibility (f/1.4 is basically as fast as it gets). To the layman, f/stop numbers seem undifferentiated, but the drop-off in light transmission from stop to stop is significant; exposure is logarithmic, so f/1.4 transmits twice as much light as f2, f/2 twice as much as f/2.8, f/2.8 twice as much as f/4, etc. After the 50mm one should expand in both directions, adding a 28mm or 35mm lens on the wider side as well as an 85mm and 135mm on the telephoto side (these numbers are approximate, given the varying lens lineup of different manufacturers). It’s also a good idea to have a general zoom lens to hit particular focal lengths, especially on the longer side, so something like a 70-200mm would serve you well.

If you’re shooting on a full-frame camera like the 5D, the focal length listed on the barrel of a SLR lens will be accurate. However, if you’re shooting on a DSLR with an APS-C or similar-sized sensor, you’ll need to calculate the corresponding crop factor of your camera to get the accurate focal length. For example, if you’re shooting with a 7D or T2i you’ll need to multiply by 1.6, so a 28mm lens becomes 45mm, a 50mm becomes 80mm, and so on and so forth. This makes crop-factor cameras better for telephoto work (they’re a popular choice of wildlife and sports photographers) because your lenses automatically gain more “reach” — but it can often be harder to find good wide-angle lenses. The 1D Mark IV has a crop factor of 1.3, which effectively splits the difference between full-frame and APS-C.

If you’re just getting started and don’t have a whole lot of money for lenses, ask friends and family if they have an old SLR they’re not using, and if you can borrow/have their old lenses. Get an adapter for that brand and try them out (note that some manufacturers designed their lenses with rear tabs or flaps that may interfere with your camera’s mirror, so you may have to file down such protrusions). If they perform well, then you’ve just saved yourself some money with the added bonus of clearing out your aunt’s closet space. You’ll probably want to upgrade eventually, but this can be a good way of shooting ASAP and learning on the fly.

Once you’ve got your lenses and start shooting, you’ll notice that fast primes have a very narrow depth-of-field when wide open. This is mostly a blessing: for years, guerilla filmmakers have been after a narrow depth of field (previous video cameras in this price range had tiny imaging sensors, which yielded images with a very deep DOF), as most audiences associate shallow DOF with narrative filmmaking. A shallow DOF also allows you to defocus backgrounds that might not be production-designable and direct the viewer’s attention to what you deem important in a scene. However, when working with full-frame sensors like that of the 5D, your DOF can become too narrow, so that it’s nigh impossible to achieve sharp focus on a moving object (such as a person’s face). Also from Shane (he’s talking about a full-frame sensor): “With a Canon 85mm lens, at a 1.4 T-stop you have a 1/32” of focus.” This means: your focus on a subject’s face might be limited to the tip of their nose and not their eyes, or vice versa; this can be distracting to say the least, especially over the course of a shot. Even if your subject is stationary (say, in an interview), when they lean forward they might drop out of focus if you’re using a wide-open aperture, and so you’ll have to choose your f/stop wisely. Note that, at the same f/stop, perceived DOF is deeper on wider lenses and shallower on longer lenses (I say perceived because technically this is not true; see this article on DOF for why). To further understand the relationship between sensor size, lens length, and f/stop, you can use a calculator to figure out the depth of field for a given lens (this is calculated based on “acceptable sharpness” a.k.a. the circle of confusion).