@Blund here are the C225 2K vs C260 4K images stacked for others as a reference. The detail is better on the newer 4K but the contrast and saturation is worse. It has a washed out blueish grey noise over it which makes the older but brighter 2K more usable.

  @Solla-topee It has.This is the factory settings. Original as choise and 50% for the light setting. 
The "Warm" setting is too warm. The "Cold" setting is way too cold, and the "Soft" setting I do not know what to think of, so I stick to the factory setting called "Original"


The problem seems to be that both the optics and the sensor are of a quality that do not match the requirement for a 8MP camera.

  @Solla-topee 
It is under warranty. It's only a few weeks old. 

But it is not broken. It just does not have the expected quality.
I paid the same for the C260 and the C225.
I should have known better. If it cost half the price of other 4K cameras, there might be a reason....
 

  @Solla-topee 
 I have tested another C260. Same result, but now I got the explanation why.

The C260 have an image sensor just marginally larger than the C225. 1/2.7" vs 1/3 For the 225v1. The resolution is 2560 × 1440 px vs 3840 × 2160 px for the C260, which is a huge difference. The lens is F1.6 for both.

The C225 will always be better in low light and give more color and less noise, given the design choices for the things considered most important. Result: Much more light per pixel and a much better signal to noise ratio. And much better images for moving objects for comparable frame rate. All at a cost of a lower resolution.


So the C260 is as good as it can be, given the compromises taken when designing this camera with a 4K 8MP resolution. 

Wish I had learned these tings before buying. Resolution is not everything, and high resolution can actually be a bad thing if the rest of the hw/fw do not match.


Perhaps a future C270 with an upgraded 1/2" sensor will do the trick....

  @Blund C260 has a smaller sensor 1/2.7 vs the C225 with a 1/2.9 sensor, both have F1.6 Aperture and 4.3mm Focal Lenght. The C260 should perform better in general and offer a greater depth of field with more of the scene in focus from front to back. But it is just a bad sensor that gives below average performance especially in medium and low light conditions. You can ask TP-Link to try and increase shutter speed which can maybe improve the performance a bit.

  @Martin-TP 1/2.7" is not smaller but bigger. Bigger is better.

And the C225v1 has a  1/3” Progressive Scan CMOS Starlight Sensor. The v2 has 1/2.9".