With the development of mirrorless camera technology, the distinction between photo-taking devices and video cameras has become further blurred. Today, there are plenty of mirrorless stills/video cameras available that can shoot footage comparable to the standard of professional video tools, but at a fraction of the cost.

In this guide we'll be explaining the key technologies and features of today's hybrid stills/video cameras, to help you make the right buying decision. To help you navigate the jargon-heavy world of video, we've created a glossary of terms which you'll find at the end of this article.

Resolution

The most quoted video specification you'll see for a camera is the output resolution, typically 1080p/Full HD, 4K or even 8K on the latest cameras. Most recent TVs can display 1080p/Full HD, and the ability to show 4K video, which has twice the resolution, is becoming increasingly common. Shooting 4K footage gives some flexibility during the editing process, even if your final output will be 1080, but the files tend to be a lot larger and require more storage and a more powerful machine for editing.
The same is true to an even greater degree with 8K capture: it affords you some creative flexibility (in terms of cropping or stabilizing your footage) if you're outputting a 4K video, but the storage and processing requirements are even greater. Most people will find good quality 4K more useful than 8K footage, most of the time.

1080/Full HD, 4K, 8K explained:

These are the most common resolutions used in video. Full HD (High Definition), also called '1080' is 1920 x 1080 pixels. 4K can refer to DCI (4096 x 2160 pixels) or the more common UHD (3840 x 2160 pixels), 8K also has DCI and UHD versions, which double the resolution in each dimension to 8192 x 4320 pixels and 7680 × 4320 pixels respectively.

An important consideration beyond the quoted output resolution is how the footage is captured: the best cameras capture greater-than-4K resolution and downscale to give highly detailed 4K output, but other models have to sub-sample (only capturing some lines of their sensor, or lumping pixels together) which gives a less-detailed result that is more prone to glitches. Finally, some cameras have to crop in and use a small area of their sensor, which lowers quality (especially in low lighting conditions), and means your footage is more 'zoomed-in' than in your camera's photo mode, making it harder to get a wide-angle view. This is a detail most manufacturers don't publish, so you'll need to learn the lingo and read reviews to be certain.

The final factor to consider is rolling shutter: the wobbly, Jello-like distortion of subjects that move quickly past the camera. This is caused because cameras capture their video one line at a time, scanning down the sensor: on a camera where this is slow, there's more of a risk of your subject moving and being in a different position by the time the camera is capturing the bottom of each frame. Generally, cameras with smaller sensors are quicker to read-out, so are less prone to this problem.

Frame Rate

Most video is shot at approximately 24 frames per second or 30 frames per second (with 25 fps being the standard for TV broadcast outside North America). But many cameras offer faster frame rates, which can be used in a number of ways. 60p footage can do a better job of representing motion, so can be a good way of capturing bursts of action. The alternative is to capture 60p or faster and then slow it down to 24 or 30p, to give a slow-motion effect. Most cameras can't offer fast frame rates at their highest resolution, but 1080 capture at 120 fps or faster is not uncommon, which can be great if your project doesn't have to be 4K.

Recording limits

Another detail to check is whether a camera has any recording restrictions. Some models can only record for 29 minutes and 59 seconds (an old restriction that related to import duty), but most end up being limited simply because high-resolution video capture generates a lot of heat.
The processing needed to capture video generates heat and most stills/video cameras aren't very effective at dissipating this heat, eventually requiring them to shut down to cool off. Pro video cameras have cooling fans but most stills/video hybrids simply try to transfer this heat to the camera's body panels, where it can escape into the environment. The best of these designs can continue shooting for extended periods, while other models let you disable their overheat limits (or, at least, make them less stringent). This is rarely a problem if you plan to shoot lots of short clips to edit together but will prevent you leaving the camera running at something like a school recital, especially if you try to shoot in 4K or higher. Fast frame rates can cause similar headaches in terms of heat and storage.

Audio

Once you've found a camera that shoots good footage at the resolution you want, a key thing to consider is audio. Most audiences are more forgiving of poor-looking footage than they are of bad-sounding video, and it's a factor easily overlooked if most of your experience is photographic.
A microphone input socket is a must: the internal microphones in cameras tend to be simple affairs that will pick up ever movement of the operators hands or clothes moving nearby, so you'll want to be able to attach an external microphone. The next most valuable feature is a headphone socket so that you can check the volume level and monitor for distracting background sounds: the human brain is great at filtering-out the sound of a car passing or an airplane flying overhead but you won't be able to remove it from your audio recording, when you watch the footage back.

Autofocus

One of the biggest distinctions in modern cameras is how reliably their autofocus works when capturing video. Unlike stills shooting, video captures all of the camera's attempts to focus, as well as the moments it's in focus, so you'll need a camera that's decisive and dependable if you're hoping to trust it to autofocus while you're recording.
The best performers are able to reliably track subjects you've chosen (especially human subjects), and let you decide whether they should re-focus rapidly (to keep a moving subject in focus), or slowly and smoothly, for when you want to draw attention from one subject to another. Autofocus depends on both the camera and the design of the lens you use, so it's worth doing a degree of research (and, perhaps, testing), before you decide to rely heavily on autofocus.

Focus peaking and Zebras

The alternative to autofocus is, as you might expect, to focus manually. This is the way a lot of professional video is still shot. Most modern cameras let you use autofocus to set your initial focus position, before you start recording, then provide a 'focus peaking' function that highlights the edges of the in-focus points in your scene. When used with an appropriate lens (ideally one with linear focus response, where the focus always changes by the same amount as you turn the focus ring) and a bit of practice, manual focus is pretty workable, but a lot of subjects can be arranged so that you don't need to re-focus very often.As well as focus peaking, most cameras let you 'punch-in' to the video: giving a magnified view of part of the scene to check critical focus. Whereas nearly all cameras will punch-in before you start recording, only some will let you zoom-in to double-check your focus while you're recording, which is a useful option to have.
The other useful video tool worth checking for, when researching a video camera is the option of overlay a Zebra pattern onto the screen, indicating a specified brightness. It's a useful tool for judging exposure, and can be adjusted to check for over-exposed regions or to check you're exposing skin-tones correctly (getting exposure correct in video is much more critical than in stills, where you can shoot Raw to preserve some latitude for adjustment).

Settings carry-over

One detail that won't be mentioned on a camera makers' website is whether exposure and other settings are carried over from stills to video shooting. The ideal photo settings are often drastically different from the ideal video settings, so we prefer when exposure, white balance and focus modes are kept separate.
Even with separate (or separable) settings for stills and video, it's not uncommon to have to add darkening (neutral density) filters to your lens when jumping from stills to video capture, but not having to constantly adjust your settings can help make switching back and forth a lot simpler.