Exercise 3.3 – Timeframes

What do the timeframes of the camera actually look like? If you have a manual film camera, open the camera back (make sure there’s no film in the camera first!) and look through the shutter as you press the shutter release. What is the shortest duration in which your eyes can perceive a recognisable image in bright daylight? Describe the experiment in your learning log.

Because the lens elements can distort the view, I have done this exercise using ‘bare’ shutters. I was intrigued as to whether the result is the same with leaf shutters and focal-plane shutters.

Incidentally (a minor niggling point) I regard ‘shutter’ as a misleading name for a device that is really an ‘opener’.

For the leaf shutter, I took a large-format lensboard and removed the lens elements leaving a bare Synchro-Compur with speeds between 1 sec and 1/400s. I was able to perceive a sunlit scene at all available shutter speeds.

For the focal-plane shutter, I removed the lens of a Pentax Spotmatic SLR. This has a horizontally-travelling focal plane shutter with speeds between 1sec and 1/1000 sec and a flash-synchronisation speed of 1/60s. I was interested in whether I got different results below the synch speed (where the shutter is fully-open for a time) and above (where the second-curtain is released before the first is fully open). Again, I was able to perceive a sunlit scene at all shutter speeds.

I wonder if this counts as a ‘failure’ in the experiment, as I found no cut-off speed at which the image was lost. Perhaps removing the lens made it too easy. (Alternatively, perhaps attempting the exercise with a lens in place, as anticipated by the course notes, distorts the results)

However, there were two interesting observations to be made:

First, the focal-plane experiment is a demonstration of persistence of vision. I saw the whole scene and the whole of the circular lens-throat outline even at the higher speeds where the shutter is never fully open. Effectively, a continuous set of moving slit-images appeared as a whole.

Second, the eye takes time to focus. If I concentrated on the shutter, I could feel my eye muscles adjusting focus to the scene. At higher speeds, it is necessary to ‘pre-focus’ on the distant scene in order to see it properly through the shutter.


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