Motion blur is created when the shutter is open long enough for the subject to move – not very complicated.
Unsurprisingly you control motion blur by changing the shutter speed, first a little experiment to show the effect of shutter speed on motion blur – and then we’ll add some flash magic to make the photos more interesting.
To demonstrate this I needed something that moves in a constant speed so I’ve used a small spring-loaded toy, the camera is on a tripod and pre-focused, the train moves at exactly the same speed in all pictures and I’ve pressed the shutter when the train reached the same location (more or less).
Because I didn’t want to change depth of field the aperture is the same in all pictures (f/5) , I’ve shoot those pictures in Av mode, I’ve manually set the aperture and ISO and let the camera calculate the shutter speed, all pictures are strait out of camera with no processing (except for reducing the size so they fits here).
The settings described below are for example only, the required shutter speed depends on the speed of the subject – and unless you are photographing the same spring loaded toy train I did you will need to adjust.
We start at f/5 1/100sec ISO6400 – the picture is very noisy due to low light/high ISO but there is very little motion blur.
At f/5 1/50sec ISO3200 (look at how halving the ISO exactly doubles the shutter speed) there’s enough motion blur to be really noticeable but not enough to look like we did it on purpose.
At f/5 1/25sec ISO1600 there’s significant motion blur.
At f/5 1/13sec ISO800 the train is so blurry it’s becoming unrecognizable.
Everything continues at 1/6sec ISO400 and 0.3sec ISO200
And finally at f/5 0.6sec ISO100 the train disappeared and all we are left with is some blur
The kind of blur we’ve seen so far actually doesn’t feel like motion, it’s just too blurred – but there’s a very nice technique that turns the picture above into this:
This picture was taken with the same camera settings as the previous one (Av mode, f/5, ISO100, same location and light) but with an added flash, I’ve used the camera’s pop up flash, with flash exposure compensation dialed down all the way to –2 stops (because a direct on-camera flash would slightly over expose the train and background resulting in washed up colors and the unnatural hard shadows pop up flashes are famous for).
What happens is that we get the same blur we got before – and then the flash fires and we get a clear sharp image of the subject at the time the flash fired.
It’s important to note the flash is set to rear curtain sync (or 2nd curtain sync) – in this mode the flash fires at the end of the process right before the shutter closes instead of at the beginning right after the shutter opens, if we didn’t set the flash to rear curtain sync the blur would have been in front of the train instead of behind it.