Underexposure of photos will prolong the development time.

With the scheme of 1:49, the development will be more accurate. Therefore, the shorter the development time, the more likely it is to make mistakes. It was supposed to take 4.5 minutes, but if you use it for 4 minutes, there will be serious underexposure, exposure value can't set it accurately, and the film development time can't be completely done according to the instructions of the film factory. If the shutter speed of the camera is faster than the calibration and the aperture ratio is smaller than the calibration, the film will be underexposed. If the full amount is owed, it can be considered that the extension time is not linear. I suggest checking it. After the dark exposure of black and white film is completed on the negative, the post-processing cannot affect it. The position of the high part can be changed by increasing or decreasing the expansion time of n+ 1, n+2, n- 1, n-2. Therefore, there is a saying in black and white film photography that the dark part is exposed and the bright part is developed. Find yourself a book. Black and white photography needs to calm down to study and practice.