Focus Shift Issue. Clear in B&W, Blurry in Colour

I have recently set up a Dahua IPC-H-3500 with a Fujinon DV4x12.5SR4A-SA1L 12.5 - 50mm 5mp Day Night Auto Iris lens. I have it set to 12.5mm. It is set up for numberplate capture and is illuminated by a 96LED illuminator at night.

The exposure is set to Manual with a Maximum exposure of 1/500 sec. I set up the exposure at night under IR and get excellent clear number plate capture by night. Slower exposures wash the plate out.

By day I get similar quality if I leave the camera in black and white but everything is blurry in colour.

By day in B&W

By Day in Colour

It almost appears as if I have lower depth of field by day in colour than I do at night under IR which seems to run contrary to everything I've read. The lens is IR corrected so theoretically visible and IR should focus on the same plane.

Anyone have any ideas as to why this might be? Should I refocus at night with the IR off? Refocus in colour at night?

The Fujinon DV4x12.5SR4A-SA1 is a manual iris lens.

Use auto iris lens to correct focus shift...

The Fujinon DV4x12.5SR4A-SA1 is a manual iris lens.

 

Use auto iris lens to correct focus shift...

It appears I linked to the manual iris version of the same lens. My lens is definitely a DC Auto Iris lens. Fixed the link now.

Check your Exposure Mode, Auto Iris, and Day/night Mode Settings.

Does focus shift problem occur when in auto exposure mode as well?

Also, manual exposure mode might disable day/night settings.

Check your Exposure Mode, Auto Iris, and Day/night Mode Settings.

Exposure is set to manual range with a maximum of 2ms (1/500 sec). Auto iris is on and day/night is currently set to black and white. It was set to auto but would drop into colour by day with the resulting blur so I have fixed black and white

Does focus shift problem occur when in auto exposure mode as well?

Auto is not really an option for me because I need high shutter speed so the IR does not wash out the plates by night. I might try doubling the shutter speed by day and see if that helps.

Also, manual exposure mode might disable day/night settings.

The camera was still switching fine.