I have a question about shutter priority mode?

Nope, that's the widest aperture the lens can accomodate - it can probably close the aperture to f/22-f/32 depending on focal length. Yet: Do consider to change this lens to something better, f/6.3 is outside the working range of your camera's autofocus module - thus the lens needs to trick it into trying, which results in all sorts of problems like unreliable and often hunting focus tries...

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regards
Karl Günter Wünsch