I would like to ask you questions about black cards, gray cards and white cards.

Usually just a gray card, mainly used for metering. The most commonly used gray card is 18%, because the machine automatically processes a photo by photometry according to the gray scale of 18%. In the case that the backlight is not accurate, you can use the gray card to measure the light at the same angle of the subject.

In the past, when film was used, some manufacturers (such as Kodak) put the gray scale of 18% on the film cover, which can be used as a gray card. But now these things are hard to find, and it will be inaccurate to do it yourself. Another method is to put the back of the hand and the object being shot back at the same angle as a gray card for metering. Although it is not very accurate, emergency is possible.

The gray card in photography is mainly used to control the exposure accurately with the exposure meter. Nowadays, all machines have their own photometry, so gray cards are rarely used in general photography, but it is very useful to control the exposure accurately.

Finally, there are two similar props, the reflective whiteboard, which is used to reflect light, and the black one is used to block light, which is much larger than the gray card.