Recently I went to film a Lancaster Bomber on a Taxi run and I wanted a short DoF, setting my lens to f2.8 or sometimes f4. It was daylight, so light levels were high and due to the wide aperture setting of the lens, the only way to obtain the correct exposure was to increase the shutter speed. Everything looked fine apart from the fact that the propeller blades of the plane appeared to be almost stationary; the high shutter speed of (probably) 1/1000s saw to that!
The problem with shooting at a high shutter speed is that the it sort of takes away the "film" look of the footage, where the shutter speed is always set to TWICE that of the current frame rate being used (as each frame is only exposed to light for half of the time). Therefore, at 25p, a shutter of 1/50s should be utilised. Similarly, when shooting at say 50p, a shutter of 1/100s should be used. So therefore, if you want to use a wide aperture setting for shallow DoF, gain the correct exposure and only use the optimum shutter speed, how you can achieve this; there are no more variables to play with!
The answer (after some research!): some kind of Vari-ND filter attached to the lens which enables the user to control the amount of light actually entering the lens to begin with. Philip Bloom, and others, recommend the Singh-Ray filters (http://www.singh-ray.com/varind.html), but these are very expensive. Therefore, I have found a DIY alternative whereby two polarising filters can be bought and one reversed: http://www.petapixel.com/2010/08/27/how-to-build-a-cheap-and-simple-variable-neutral-density-filter/
Not sure how well it works, but worth a try :)
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