Can someone explain why sensor size isn't reflected in image quality?

[loofer]

I'm fairly new to dashcams but I'm looking for a good quality camera with a screen and a moveable base for under £140, primarily interested in image quality with little need for additional features.

I'm looking at Mio Mivue, Garmin, Vicovation and Nextbase cameras and noticed that most use 2-4MP sensors but the Nextbase 512GW uses a 6.8MP one. I know there are other factors involved but that's such a huge step up I would expect next generation clarity and yet the footage doesn't really look much different from it's near rivals. Can anyone explain this, maybe it's just Youtube compressing the footage so the advantage can't be seen but I don't really buy into that.

 

[Nigel]

Are you looking at the video on a Full HD display, or a 4K display?

A Full HD display only has 2MP resolution so can't display all the detail from a 6.8MP sensor.
Most people view YouTube at 1080 or lower resolution even if they have a 4K resolution display.
Most people record at FHD anyway, in which case there is no point having a sensor with more than 2MP resolution.

Image quality is more related to the size of the pixels on the sensor, the bigger the pixels the better the image quality, the lower the noise and the more sensitivity in low light and thus less motion blur. Given that the sensors in most dashcams are very similar sizes, the more mega pixels on the sensor the smaller the pixels have to be to fit on. Thus high megapixel sensors give noisier and worse quality images, especially in low light conditions. The only advantage of a high megapixel sensor is better resolution if there is enough light to avoid motion blur, and if your display is high enough resolution to display it.

Image quality also depends on bitrate, to get advantage from a high resolution sensor you need a camera with high bitrate to store all the detail, and a memory card that can keep up. Some of the 4K cameras are still using similar bitrates to the 1080 cameras, sometimes even lower bitrates, and so most of the extra resolution gets lost in compression!

Sensors and processors are getting better, at some point we will get some good 4K dashcams, but at present the best compromise comes from the 2MP sensors. Any less than 2MP and there is not enough resolution to read plates so 2MP is currently the most popular. For people who drive mainly in good daylight, it is worth looking at the higher resolution cameras, but maybe not for UK in November!

Take a look at the best-dash-cams-of-2018 page, there are other options to those that you listed.

 

[loofer]

I'm watching it on a 1440 monitor but most of the footage is only 1080, having said that even the few 4k uploads I've watched aren't substantially clearer. I guess I should have just worked the numbers to see that 1920 x 1080 = 2MP and 2560 x 1440 = 3.7MP so anything more than that is just going to bring noise rather than clarity

I was also guessing that the smaller FOV cams might give better clarity as you're getting more pixels per degree of view even though you sacrifice coverage, the Garmin 55 looks good on paper but the footage again doesn't blow me away even though it's on the best buy list.

The more I look the more confused I am... thanks for the feedback though, lots of intersting stuff to add to the equation!

 

[Nigel]

The higher resolution cameras do tend to be wider field of view so the pixels per degree does not go up as much as you would expect.