树莓派3B +相机的快门速度卡在0或12时如何解决？

Question

我正在尝试使用picamera库更改相机的快门速度（而不是帧频）。当我这样做时，结果证明值是0或12。在线示例表明情况不一定如此。

这或多或少非常简单-尝试设置cam.shutter_speed，无论如何，该值始终强制为0或12。

import pygame
from pygame.locals import *
import car
from picamera import PiCamera
import datetime



c = car.Car()
cam = PiCamera()
cam.vflip = True
cam.hflip = True
cam.start_preview()

speed = 0

pygame.init()
width, height = 1280, 1000
screen=pygame.display.set_mode((width, height))
motion = 0 # -1 for backward, 1 for forward
direction = 0 # -50 for left, 50 for right, in between for lesser angles
playerpos=[100,100]



while 1:
    screen.fill(0)

    for event in pygame.event.get():
        # test events, set key states
        if event.type == pygame.KEYDOWN:
            elif event.key == K_UP:
                speed = speed + 1
                cam.shutter_speed = speed
                print("Shutter speed set to " + str(cam.shutter_speed))
            elif event.key == K_DOWN:
                speed = max(0, speed - 1)
                cam.shutter_speed = speed
                print("Shutter speed set to " + str(cam.shutter_speed))

通常，我希望快门速度将保持指定的值。我知道它受fps的限制，但不应限制为0或12。