Why do I have a clear sharp picture during the day and it is out of focus at night?

During daytime , lens will get proper light and if we are using cameras without IR camera will out of focus in night time

• During daytime, the image quality is perfect, however while the old lens obviously adjusted to the changing illumination during dust and dawn, the new lens seems to have problems. The images get quite blurry in the nite. 
• Your pictures will never be as sharp in night mode however, due to there always being a mixture of visible and infra red lighting. The lens can not simultaneously focus both. 

Due to change in the colour, lens of cctv camera auto adjusts in night mode with IR Leds which results out of focus at night.

This is due to the depth of field changing as the light conditions change and can be easily overcome by following set procedures.