The Human Eye Vs a Camera

In this post I would like to discuss the concept of how cameras view the world differently to the human eye.

Generally video cameras open a shutter 25 times a second to scan an image projected onto a sensor or a film to produce 25 pictures. Then when these are shown back at the same rate they were taken (25fps) they produce the illusion of a moving image. 

The human eye, however, views the world differently. Instead of seeing through multiple still frames over time, it tracks on to objects and views them as a constant beam of photons focused on to the retina by the lens and the cornea. 

Video cameras are generally designed to emulate the human eye so that the videos they produce play out like something we would see with or eyes. This is why they generally film at 24-30 frames per second as it gives the effect of slight motion blur on fast moving objects, the same as we see when viewing fast objects. We see motion blur on fast moving objects because our eye's can not keep track of objects moving at speeds that are too high so our brains fill in the missing bits of information with motion blur. (Alais, Apthorp, Karmann & Cass, 2011)  [1]

Although video cameras do a good job of emulating our vision there are some movements that it struggles to view as our eyes do. Objects that oscillate at a frame rate that is the same, close to or a multiple of the camera's frame rate produce an effect in the video that we can't see through our vision due to the fact that we don't see in frames per second. When an object is oscillating continuously at the same exact frequency of the camera's frame rate the object will look perfectly still despite the fact it's moving. This is because the camera is taking each frame at the exact same point in the object's oscillation cycle. So when it is played back the object appears to be perfectly stationary. When the object oscillates at a frequency that is just outside of the camera's frame rate, for example the camera is recording at 25fps and the object is oscillating at 24Hz, the object appears to be moving extremely slowly as if it's in slow-motion. This is because the camera is now taking each frame at an ever so slightly different point in the object's oscillation cycle. 

For this effect to be fully visible the camera needs to have a fast enough exposure to capture the high speed movement of the object so as to avoid motion blur. I demonstrate this effect in the video embedded below. I used a speaker to provide oscillations of a sine wave at various frequencies around the camera's frame rate of 50fps.

To me, this concept reveals to me how much we take the way we see for granted. Our vision is limited by the biology that allows us to see the way we do and has evolved to work in a very specific way. We can take concepts like this to develop our technology to see things that we could not ordinarily see with our eyes.

References

1.  Alais, D., Apthorp, D., Karmann, A., & Cass, J. (2011). Temporal Integration of Movement: The Time-Course of Motion Streaks Revealed by Masking. Plos ONE, 6(12), e28675.