Shutter vs Frame Rate

By: John Honovich, Published on Feb 27, 2013

Shutter speed and frame rate are frequently confused. They are certainly related and here's the key point - a camera's shutter speed can never be slower than its frame rate but it can be much faster. In this tutorial, we explain how, why and the practical implications of this.

Shutter Fundamentals

Shutter speed refers to how long the camera's sensor is exposed to light. It can be exposed for 1/10,000s or 1/100s or 1s, even more or less sometimes. If the shutter speed is 1/10,000s than it could open many times per second (30, 60, etc.). However, if the shutter speed is 1/2s, it can only do it twice at most. When you have very little light, you want to open the shutter for a long time (like 1/30s, 1/15s or even less). However, when it is very bright out, you want to open the shutter very briefly (like 1/1,000s or greater). See our shutter speed tutorial for more.

Frame Rate Impact

Each frame needs its own complete shutter. If you created 2 frames from the same shutter / exposure, the two would have the exact same input and would product the same picture, everything else being equal.

Imagine you had the camera set a 1s shutter. Could you do 30 frames per second?

No, unless you cheated. You could simply generate 30 copies of the same image from that single exposure. However, if you wanted to have 30 unique frames where each image differed from the previous, showing any motion / change, you would need 30 exposures in a second. To get those 30 exposures, the shutter speed has to be 1/30s.

Key Takeaways

  • If your shutter speed is slower than the frame rate, the 'true' frame rate must drop or the camera must insert copies of the same frame over and again. If you care about maintaining the frame rate, be careful about slow shutter. Not only may it cause blur but it will drop the effective frame rate. For example, many Axis cameras out of the box, drop to 6fps in low light because they are programmed to slow down to a 1/6s shutter speed.
  • When it is bright out, a camera's shutter speed will be far faster than the frame rate and the user does not need to do anything. The camera will make sure it works without intervention.
  • If you use a 60fps camera (becoming more widely available with new chip generations) and want to maintain that 24/7, the slowest exposure will now only be 1/60s, meaning that the amount of light captured in low light will drop by half compared to 30fps cameras.

 

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