How to calculate the difference of a few seconds between the brightest and darkest exposure value in spot metering test?

Using aperture priority mode to measure facula, the camera will give the shutter speed, which is assumed to be 1 sec. Then measure the light in the dark, and the camera also gives the shutter speed, assuming it is 5 seconds, write it down. In this mode, the same parameters are used, such as aperture.

Turn to manual M and select the spot light value in the dark. At this time, the shutter speed is 5 seconds. Cover the highlights with a black card and shake it quickly and slightly. Press the shutter to start shooting and start timing. Remove the black card in 4 seconds, and finally expose it in the dark and light at the same time 1 second.

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It takes skill to shake the black card, and it depends on the situation when using it. If the shutter speed required for exposure in a bright place is very fast, reaching a few tenths of a second, then you have no time to calculate the time to remove the black card. At this time, it is best to use a gradient filter to solve it.