Output framerate

The output framerate of a video camera determines the amount of time that each frame is displayed. 

If the video output is set to 60 frames per second, that means that each frame is 1/60th of one second long, 16.67 milliseconds. 

Shutter Speed or sensor exposure time

If you have a look into the settings of an Avonic camera, you'll find a range of options for the shutterspeed to be set. 

Key point to remember is that some settings have no effect and others have a dramatic effect on the video output characteristics, it depends on the setting of the video output framerate. Ultimately the video output framerate determines how much time there is available per output frame. 

Shutter setting in fraction of a second                        Necessary Exposure time in milliseconds

Amount of frames put out per second                         Time per frame in milliseconds

Now, if you have a closer look at the two tables above, you can see that some combinations are not possible simply, because a specific shutter setting may take more time than there is available in a displayed frame on the output of the camera.

Example 

If the video output is set to 60 frames per second, this means that each frame is 16.67ms long. Any shutter setting that needs more time than that 16.67ms, is useless because there is no more time in the frame. 

Therefor, when 60fps is selected, 1/50th, 1/30th and 1/35th shutter settings have no function because they take longer to be exposed on the sensor than the given time in the output frame. 

This phenomenon also works the other way around and can be used to your advantage: if the camera must be used in challenging light settings, the user can opt to set the output framerate to a lower framerate of let's say 50 frames per second. The length of each frame is now 20% longer when compared to to 60 fps, leaving more time for the sensor to be lit. In the case of an output 50fps, each frame is 20ms leaving room to use the shutter setting of 1/50th of a second. 

Lowering the shutter speed matters only until you reach the frame rate. After that it has no effect (in Avonic cameras)

If more exposure time is necessary, it is possible to lower the framerate further, however lowering the output framerate and lenghtening the exposure time per frame may lead to some (unwanted) motion blur. Theoretically, you can set the output to 25fps and have an exposure time per frame of 40 ms, that is approximately 2.5x more exposure time per frame than is possible on 60 frames per second