Aspect ratios, simply put (for those who are unaware), are the ratios between the width and height of a single frame of video. Television has had a 4:3 ratio (4 units of width to 3 units of height), until the popularization of “HD” television, using a “widescreen” ratio of 16:9. I’m not going to go through the entire history of aspect ratios in cinema, as there is a great retrospective available on vimeo.
Most DSLR cinematography is done, by default, with a 16:9 ratio, as the maximum capture size for their video is generally 1920x1080, abbreviated as 1080p (to additionally indicate progressive, rather than interlaced, scan). It is relatively easy to enforce a “wider” aspect ratio by dropping lines at the top and bottom of the frame, which most watch-at-home film viewers will identify as “black bars” at the top and bottom of the frame. This does, however, introduce an interesting issue – that of composing inside that frame. That ends up looking something like this:
Standard compositional rules generally make use of the two frame diagonals (top left to bottom right, and bottom left to top right) and both horizontal and vertical thirds. Most compositional choices for “balanced” frames have tended to rely on these invisible divisions to create a pleasing image, even though the rules are really guidelines, and are not absolute. If you, as a cinematographer, decide that you’re going to use an alternate aspect ratio, you cannot properly compose by using the thirds or diagonals indicated by any common guide or viewfinder in 16:9 ratio, as they will result in a vastly skewed product, compared to the original on-camera image. I’ve seen a number of forums on DSLR cinematography which had suggested everything from using a black marker to denote the frame to using black tape to cover the segments outside of the final crop. A more “modern” solution for Canon DSLR owners is using Magic Lantern cropmarks. If you’re shooting with another DSLR brand, you may be at the mercy of the firmware, or any of the aforementioned hacks.
NOTE: I’d like to mention, at this point in this article, that there are other compositional rules, including golden ratio/mean/spiral/rectangle, so please don’t take the rule of thirds and diagonals to be the only important and pleasing ways to frame an image….
Besides attempting to find a way to follow common compositional rules, there is another side to aspect ratio: selecting the appropriate one for the story you are attempting to tell. Apart from the 16:9 ratio being the “standard format” for HDTV broadcasts and web-based series, it is much taller (i.e., it offers more headroom for shots) than Cinemascope (2.35:1) or some of the other aspect ratios. Depending on the types of shots which you are using to compose your film, it may be advantageous to have a wider, yet thinner, view of the world which comes with “shorter” aspect ratios. Again there is no “one size fits all” for aspect ratios, and it should be a conscious decision for the material which you are filming.
An added advantage to using a shorter aspect ratio is that, due to the camera bodies recording the entire frame, regardless of target aspect ratio, additional editing and recomposition “wiggle room” is available in the editing and post-production process. If something isn’t framed in the exact way it should be, this can be corrected later by adjusting the vertical positioning of the frame within the crop. Most (if not all) NLE software has vertical positioning, including Adobe Premiere, Sony Vegas, and Final Cut Pro, so you shouldn’t have any issues doing this. I’m aware that the ideal cinematographer does everything in-camera (as I have vociferously advocated in past), and does not rely on post-production to fix mistakes, this does allow for that possibility.
Aspect ratios additionally carry some subconscious baggage with them, due to the associations we have made with previous material filmed using them, much in the same way we associate film grain with high-end Hollywood pictures, even when, in many cases, that film grain was added back to the digital footage in post-production.
The quick takeaway is that you shouldn’t embark on a project without having made a conscious decision regarding the aspect ratio which you are using for that project, considered the ramifications and technical concerns associated with that decision (including making sure that you can properly frame and compose shots in that aspect ratio), and storyboarded and/or prepared your material considering your chosen aspect ratio. It is yet another variable which can be tweaked to help make an already good project that much better, and like all power, it comes with responsibility. So, choose – but choose wisely, and as always, good luck!