I started this hobby with fixed-focal lens
Amcrest cameras, 2MP then 3MP then 4MP. Then I found out about Dahua and got a Starlight 1080P ptz 25x zoom, then got some fixed lens dahua 4K 8MPs with the big 1/1.8" sensor.
(i'll skip the starlight ptz camera for now since that's a different beast)
I can unequivocally say, without a doubt.... just by going from 2 to 3 or 3 to 4 megapixels, there is noticeable difference in day time visual quality (Amcrest camera). Then, when compared to Dahua's 1831E 1/1.8" sensor that's 4K/8MP, there is HUGE difference in daytime quality. In fact, I also notice that during night time, Dahua's 8MP HFW-1831E bullet outperforms my Amcrest 3 and 4MP bullets. Why is that you ask? When everybody preaches that higher megapixels means less light for each picture resulting in more noisy and dark images. There is some truth to that but there is also another mitigating factor.
First, my Amcrest (dahua rebranded) 1025E spec is a 1/3" 4MP CMOS sensor @ 2688x1520 resolution. It has a minimum illumation of 0.01Lux/F2.0. This ain't that great at night. My Dahua 1831E 8MP/4K has a 1/1.8" sensor @ 3840x2160 but check this out, it has a 0.003Lux/F1.6 and 0.08Lux/F1.6. By a huge margin, the sensor is bigger and the lux rating is much, much better than the Amcrest.
That is why, I could not believe that the 8MP Dahua's night performance was better than my 4MP Amcrest's. Now, let's talk about my Starlight PTZ 49225T-HN. It is a 1/2.8" Starvis sensor with 0.005 Lux. It can zoom 4.8mm to 120mm. That means optical zoom without losing detail. This is not a fair comparison because this is a variable focal camera. If you're looking to capture specific details like license plates, true, you don't need 4K 8MP. Just get the biggest Starvis/Starlight zoom camera there is that is 1080P.
So what is the moral of the story that I learned from my experience and the preaching of others?
1) Find the biggest sensor you can find for the megapixel you want. Don't be fixated on megapixels. If you're increasing megapixels because you want to capture very specific detail likes faces and license plates, you're better off using a varifocal or zoomed lens and allocate more pixels to a very narrow region of view.
2) Understand the tradeoff that generally speaking, yes going higher MP means reduced night time quality. But you should always see better daytime quality versus the lower MP camera. That is the assuming the manufacturer did not put crap 4K sensors and I suspect many american rebranded cameras do that.
3) There is certainly no replacement for higher pixels during daytime for fixed cameras. It's sharper and cleaner, again when you use the right sensors.
4) Why most people see terrible night performance on 4K camera is because the quality of the sensor is not that great, or they have truly, terrible illumination at night. If you have terrible lighting, you need to blast infrared or truly go back to a starvis 1080p. I mean, we're talking pitch blackness.
So how do you solve this problem? Do what I did. I have the Dahua 8MP 4K's complimenting the Starlight PTZ. Each camera around my home serves different purposes. There is no one-size fits all solution. Choose what is more important to you, and if you can't decide, then you'll have to buy two or more cameras like I did.
Where I live, I have plenty of lighting so YMMV.
