Low megapixel cameras have less noise in low light because of bigger pixels, right? Camera manufacturers AND smartphone manufacturers use that in their advertising, but is it true?

NO. Tony Northrup puts three sets of cameras to the test, for both still images and video. The cameras tested include the Sony a7S III, Sony a7R IV, Sony a1, Canon R5, Canon R6, Fujifilm XH2, and Fujifilm XT4. In EVERY case, the high megapixel camera outperformed the lower megapixel camera in lowlight, high ISO photography AND video. Every time.

Tony tests the fullframe cameras at ISO 25,600 and the APSC cameras at ISO 12,800. You can download the sample files here: https://sdp.io/lowlightsamples

0:00 Introduction
0:40 Sony a7S III vs Sony a7R IV
2:41 Canon R6 vs Canon R5
3:23 Fujifilm XT4 vs Fujifilm XH2
4:05 Misleading advertising
4:30 Cool rain metaphore
5:21 Sony a7S III vs Sony a7R IV for video
6:15 Sony a7S III vs Sony a1 for video
7:13 Summary
8:31 How to get better lowlight performance