Image acquisition - difference between frame rate and exposure

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Chris Whyte 2019-10-24

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https://ww2.mathworks.cn/matlabcentral/answers/487259-image-acquisition-difference-between-frame-rate-and-exposure

评论： Jayabrata Dhar 2020-1-27

Hello,

I have a fairly generic and ostensibly simple question, but I am struggling to find a satisfactory answer.

Using image acquisition toolbox and a generic USB camera, I have limits of control of the exposure between -8 and -1. I am taken to understand that these are basically 2^n values, so 2^-8 = 1/256th of a second etc.The frame rate of my camera is fixed at 30 frames per second.

I am reasonably well versed in writing programs with iamge acquisition toolbox to work with live feed video, however I now have an application where I need to take a long exposure image, not a video. I have attempted to set the camera at Exposure of -1 (0.5 second exposure time supposedly) and set the exposure mode to manual. As mentioned previously the camera has a fixed frame rate of 30fps for my given camera resolurtion. When i try to capture my photo using the image acquisition command getsnapshot I get a frame from the 30fps video feed the camera is recording. I have played around with the logging and triggering options but my images are always acquired from the 30fps video feed,

My question is therefore what is the point of the exposure control if I cannot acquire images with a set exposure level, and is there another toolbox or set of commands I should be looking at to take still images instead of from a video feed. Everytime I try to look at taking still images with a USB camera I always get directed to taking images from live feed videos.

Also, while I understand I could take multiple images and sum them to simulate a long exposure image, this is not ideal for the situation at hand as I am not satisfied with the resultant signal to noise.

Thank you for your help.

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Jayabrata Dhar 2020-1-27

I have a similar doubt but on a different level. First on your point, if you change the frame rate (say reduce it), more light will hit the sensor and hence it will be brighter. So, it seems exposure is dependent on frame rate; however, if there exists an exposure pin hole whose opening can also be independently controlled, then one can assume they are mutually exclusive.

Now the thing is when I set the exposure to manual and set a certain frame rate, I expect the exposure to still change the image brightness, but in MATLAB it does not happen. In fact, with my imaging source camera, the frame rate does not even change as I want. For example, if I set a frame rate of 1.00, I expect my preview video should be very slow, but it is not slow (and MATLAB also displays the rate at which it shows the preview which also does not change when I change the frame rate). It seems MATLAB cannot control the hardware that well or I am making any mistake.

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