Test recording using the blue channel in 4:2:0 subsampling—reveals chroma artifacts before you roll main shots. Standard prep on DCP productions.
Before the main shoot, you do a blue record — a test recording that exclusively uses the blue channel (chroma information) and is recorded in 4:2:0 format. This sounds technically dry, but it's one of the most important diagnostic methods to find out how your camera handles color information and where subsampling artifacts occur before the first real scene is shot.
The practical sense: 4:2:0 chroma subsampling means that color information is not captured on every image line — but only on every second line, vertically and horizontally. This saves bandwidth but creates problems with certain image subjects: sharp color edges bleed, thin colored lines disappear, and especially with greenscreen keying or in color grading, you'll later notice that the chroma resolution is insufficient. The blue record precisely reveals this while the scene is still in the can.
On set, you test with your planned camera, the same codecs and bitrates as for the actual shoot: a few minutes of footage with critical subjects — sharp lines, hair edges against complex backgrounds, greenscreen scenes, or the planned studio lighting. Afterwards, you watch the recording on the monitor or later at the editing suite: How does the blue channel behave under these conditions? Where do banding artifacts occur? How stable is the chroma stability during fast movements?
The result then determines your decision: Do we need higher subsampling (4:2:2 or 4:4:4) to guarantee clean keying results? Can we work with this camera and this format, or does the budget need to be reallocated? Some DoPs do this test recording as a routine, others only when the production design or keying becomes particularly demanding. With modern digital cameras with variable compression, this is not a mere formality — it saves you correction work in the grade later and prevents unpleasant surprises during final delivery.