Dash cams explained: resolution and frame rate, field of view, sensors at night, parking mode and the UK rules

The clip you will actually need

A dash cam earns its place once: in the thirty seconds around a collision or a near miss. What you need from that clip is a plate you can read, the time, the place and your speed, and a steady picture of the few seconds before. Every specification on the box matters only as far as it serves those. Resolution is one of five things that decide whether the plate is readable, and not the most important at night.

Resolution and frame rate

Resolution is how many pixels cover the scene: 1080p is about two million, 1440p nearly four, 4K eight. More pixels make a distant plate bigger in the frame, which is what you want for a car two lanes over in daylight. Frame rate is how many pictures a second: 30 is standard, 60 halves motion blur on a plate crossing the frame, 120 lets you step through a fast event.

The two trade against each other on a fixed budget of processing and card space, and the makers choose differently.

Field of view

The lens angle, 130 to 160 degrees on cameras here. Wider sees more of a junction and the pavement, and makes every object smaller, so a plate at the same distance has fewer pixels on it. Narrower reads plates further away and misses a car coming from the side. Around 140 degrees is where the makers have settled for a front camera, which is why five of the six cameras on our shortlist use it; the Viofo’s 160-degree rear lens is wider because a rear camera’s job is to see what is alongside as well as behind.

The sensor at night

Night is where cheap cameras fail. Headlights blow out the plate they illuminate, and everything else is noise. Two things help. A larger, newer sensor with better low-light sensitivity, which is what Sony’s STARVIS 2 line is and why the two Viofos here are built around it; and HDR, which combines a bright and a dark exposure so the lit plate and the dark car both register. Garmin’s Clarity HDR, Viofo’s dual-exposure HDR and 70mai’s HDR night vision are the makers’ names for the second. A fast lens, f/1.3 to f/1.8 on the cameras here, is the third.

GPS, and what the clip carries

GPS stamps every clip with position and speed and lets the app plot the route. For evidence it is the difference between a picture and a record: where, when, how fast. Every camera on our shortlist records it; Garmin’s screenless Mini 3, which we left off the list, does not. Nextbase’s 10 Hz GPS updates ten times a second; Garmin’s X110 adds Galileo.

Power: supercapacitor or battery

A dash cam needs power to finish writing the last clip when the car is switched off. Lithium batteries do it, and die on a hot windscreen. A supercapacitor stores a few seconds of charge, does not mind 60 degrees, and lasts the life of the camera; it cannot run the camera without the car. The cameras here that state a supercapacitor are the two Viofos and the 70mai; Garmin’s X110 keeps a small battery rated for twenty minutes. If the car sits outside in summer, prefer the capacitor.

Parking mode and hardwiring

Every maker sells parking mode: recording when the car is parked, triggered by a bump from the G-sensor or by motion, ideally with a buffer so the seconds before the bump are kept. Every maker’s small print then says the same thing: the camera needs power with the ignition off, which the 12 V socket does not supply in most cars, so you need a hardwire kit to a fused circuit, fitted by you or a garage. Garmin adds a subscription to its Vault service for its Parking Guard alerts. Budget for the kit with the camera, and if the car lives on a street, buy the kit before you buy 4K.

The UK rules

Three apply to a private driver. The camera must not obstruct the driver’s view: mount it behind the mirror, in the wiper-swept area, out of the sightline; a camera low on the driver’s side can be an offence. Footage is admissible, and insurers and police will take it. And recording people: a forward-facing camera in a private car raises no data-protection duty, but a cabin camera, audio recording of passengers, or any use at work does, and a sticker telling people they are recorded is the minimum. Switch the microphone off unless you need it.

How to read a dash cam listing

1. Resolution and frame rate together, including what happens with the rear camera on.
2. Sensor and HDR. A named sensor is a good sign; HDR is the night feature.
3. GPS. Yes or no.
4. Supercapacitor or battery.
5. Parking mode, and the hardwire kit it needs.

Which spec, by situation

• City driving at night: 1440p at 60 fps on a low-light sensor with HDR, GPS.
• Motorway miles in daylight: 4K at 30 fps with a polariser.
• Car parked on a street: any of the above, plus the hardwire kit and buffered parking mode.
• Nothing on show: a screenless 1080p HDR camera behind the mirror, accepting no GPS.

Sources