The black card is redundant and there is no discernible application. The white and gray cards are also only a temporary solution for the road, in the studio should be at least 12 cm x 12 cm be available. The gray card for exposure metering and white card for exposure metering when shooting at night (2 stops further open). The gray card can check the white balance or perform manually. There are a few examples on the web as it is performed.
