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Sporting Chance

Page 6

as your speed, your baseline exposure speed is 1/500ths (or 1/2000ths at 400ISO). Using a 2x converter will cost you two stops and so you drop to an almost unmanageable 1/125th if you stay on ISO 100. Using a converter is a double whammy, you need faster shutter speeds to accommodate the higher magnification, but you are losing aperture at the same time. As with all photography, the settings are a compromise between camera shake, subject speed, digital noise and sometimes the simple need to just get a shot at all costs!




Shutter Speed Selection

Assuming that you need sharp images, it is possible to compute the required shutter speed to stop action under various circumstances. Typical subject speeds are shown in the table (they vary – international class athletes go faster, hit faster, bowl faster, etc). For a 300mm lens the shutter speeds at various distances and speeds are shown. By way of example if we take a hockey player moving at 20mph (equivalent to12s for 100 metres) and a distance of 20m, we require 1/6000th to stop the action. This would need an aperture of f2.8 and an ISO of 200 in bright sunshine, close to the limit of available technology.

If the subject is now a bowler, he will be at 60m minimum but you have to add the arm action to his basic speed of 20mph, which brings the ball speed up to close to 90mph for a test bowler. Again this demands a shutter speed of 1/4000th. With an 800mm lens you would have to go to 400ISO, probably faster.

These calculations assume that stopping action is vital. In reality this is not so. Aesthetics demands a bit of blur to create a sense of movement. The trick is to make it look like you meant it, not that you were incompetent! Note two other things. An image stabilising lens is of no help in stopping subject movement, all it will do is prevent you from adding camera shake to an already slightly fuzzy subject. The calculations assume that the subject is moving across the field of view. If the action is coming diagonally towards you, you can grab about a half stop to your benefit; if the action is directly towards you, you can grab just over a stop.

This assumes that the subject is moving at a single speed. However, a bowler running up to deliver is going at 20mph and if you are panning at that speed he is stationary on the film. However his arm is moving at 70mph at the ball but is effectively at zero mph at the shoulder. The ground is moving at an effective -20mph (backwards across the frame) which is why it is possible to have a near sharp bowler and a nice blur on the crowd in the background.

The speed differential is even greater if you are panning a racing car. Here the axle of the wheel has zero translational speed, the bottom of the wheel is stationary (if you are panning exactly) but the top of the wheel is moving forward at twice the speed of the vehicle. While all this is going on the rotational speed of the wheel is whatever it is. The net effect is almost sharp writing on Jensen Button’s helmet, a blurred background and rotationally blurred wheels. If, in addition, the car is pitching under braking or due to a bumpy surface then only the logos in the centre of that pitching motion will be sharp. This is an effect you can sometimes see on motorbikes as they dig their noses in under heavy braking into a bend.



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Last Modified: Friday, 16 August 2013