3. Compression & Bit Rate
Compression refers to a method for reducing the amount of data a DSLR produces; in the case of video-shooting DSLRs, all cameras currently employ some method of compression. If you’re used to shooting photos in JPEG format, you’re used to capturing compressed images; while RAW can also employ compression, it is generally thought of as “uncompressed.” This is because, as far as shooters are concerned, when we’re talking about compression we’re talking about lossy compression — meaning, a codec (compression algorithm) that throws out data in order to reduce file size. As you can imagine, tossing portions of an image has negative side effects, and while many codecs deal with images perceptually in order to minimize their perceived impact, the difference is there. For example, if you upload a video to YouTube, the service recompresses your video in order to optimize it for internet delivery; you might not notice this compression, but check out this video that’s been recompressed a thousand times (original here) and you can see that every compression step throws out data along the way. On the positive side, however, lossy codecs are also the reason we can record hours of footage to inexpensive flash memory devices like CF and SD cards.
The most common compression formats in DSLRs are h.264 and MJPEG, and while both are lossy, h.264 is generally much more efficient (it introduces less artifacts at the same bit rate as MPJEG). Bit rate is the amount of data per time that a given codec adheres to; higher bit rates are almost always better because they use less compression. At press time there are no DSLRs that shoot uncompressed video.
4. Depth of Field
The amount to which objects in the foreground, mid-ground and background are all in focus at once is a function of depth of field. A shallow depth of field would mean that only one plane was in focus; a wide (or deep) depth of field would mean that all planes are in focus at once. Depth of field is determined by the focal distance and aperture size (see below for more on Aperture). DSLRs exploded in popularity almost singlehandedly because of their ability to render images with a shallow depth of field. This is chiefly due to their massive sensor sizes (see the next chapter, “Choosing a DSLR,” for an examination of sensor sizes), which are exponentially larger than previous video cameras. On a basic level, shallow depth of field (DOF) allows filmmakers to blur out areas of the image they deem to be unimportant or undesired.
5. Exposure & Aperture
Exposure refers to the amount of light allowed to enter the DSLR sensor (or any imaging surface). When shooting stills, DSLRs use a mechanical shutter to regulate exposure by opening for the desired amount of time (1/60th or 1/1000th of a second, for example) and then closing. DSLRs are generally rated to last for hundreds of thousands of shutter cycles, but at 24 frames per second, couldn’t your DSLR reach that limit very quickly? No, because in video mode, DSLRs use an electronic shutter — the sensor basically turns on and off to regulate exposure, instead of relying on a physical barrier (i.e., the mechanical shutter) to regulate light. Aperture refers to the adjustable opening near the rear of the lens that lets light through — the amount of light it transmits is generally referred to as the F-stop (T-stop is very similar, except it’s measured instead of calculated). We’ll go more into depth on aperture in the “Lenses” section of the guide, but keep in mind that the size of the aperture does not only affect the amount of light, but also the angle of light rays hitting the sensor — a narrow aperture creates an image with a wide depth of field, whereas a large aperture creates an image with a shallower depth of field.
6. Focal Length
Technically, focal length refers to the distance over which collimated rays are brought into focus. An easier way to think of it: focal length refers to image magnification. A longer focal length, e.g. 100mm, makes distant objects appear larger, whereas those same objects will appear smaller with a shorter focal length, e.g. 35mm. Focal length also refers to angle of view; longer focal lengths have a narrower angle of view, whereas shorter focal lengths have a broader angle of view. When it comes to focal length, a picture is worth a thousand words, so here are images taken with the camera in the same place, but with lenses of different focal lengths attached: