Why does the Xiaomi 15 Ultra flash black when using Romantic Star Trails to take photos?

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The preview in the long exposure algorithm and the actual film are two signals. When you press the shutter, the preview interface switches back to the film interface, so there will be a sudden darkening action. The current test environment is relatively bright, so it will be more obvious. When you use this function in a starry sky environment with very weak ambient light, this phenomenon will weaken or disappear.