How to freeze subject movement

Photos can sometimes get blurred because the subject moves whilst the shutter is open. To avoid this you need to set a fast enough shutter speed so that any such movement is negligible. This page discusses how to do this and provides a Javascript calculator for estimating the required shutter speed for different kinds of subjects.

When I first bought cameras, in the 1960s, a common top shutter speed was 1/500th of a second, and 1/1000th would only be found on top of the range professional cameras. Today, 1/8000th is commonly available.

Somehow, over the years I got into the habit of thinking that 1/125th would be quite fast enough to freeze movement in, say, a group of people, and that 1/1000th would be enough to freeze anything, even a horse race. But my recent experience has made me realise that these lazy assumptions simply aren't true.

Some examples

In this picture of children hugging a hound, which is a crop from a larger photo, everything is pin sharp except the girl's hand. Yet it is taken at 1/200th.

The truth is that most of the time people stay fairly still, but occasionally they are capable of making really quite rapid movements, and a faster shutter speed is needed if you want to avoid such blur.

Try sitting down and moving your hand from your knee to your chin and back ten times. I did this easily in 8 seconds and my hand was moving over a distance of 0.5 metre. That equates to a speed of 1.25 metres/second. As we shall see, a surprisingly fast shutter speed is needed to freeze this.

Canon 5D 200mm f8 1/200

In this horse race, which is also a crop from a larger photo, I prefocussed on the middle of the hedge and took the photo at 1/800th. The hedge is sharp but the horse and rider are not. Of course I could, and probably should, have used a slower shutter speed and panned the camera with the rider, thus keeping the horse sharp (except probably its hooves) and blurring the background. However, I am interested here to know what shutter speed I should have used to freeze this horse without panning.

Canon 5D 90mm f6.3 1/800

The theory of freezing subject motion

Suppose our subject - the horse, the girl's fingers, a basket-ball player or whatever - is moving across the camera's field of view at a known speed V. Whilst the shutter is open for 1/N th of a second, the subject will move a distance V/N.

This subject is imaged by our lens onto the film or sensor of our camera. To do this it applies a large optical reduction, because the subject is quite large and our sensor is pretty small. Assuming we are not in the field of macro photography, the factor by which the lens reduces the size of the subject when imaging it on the sensor is given by R = F/D where F is the focal length of our lens and D is the distance between camera and subject.

So the distance that the subject's image moves across the sensor whilst the shutter is open is R*V/N.

To avoid this movement causing the image to be blurred, we need to ensure that this distance is less than some acceptable limit. So we now have to decide what would be an acceptable limit.

One approach would be to take a very strict view and say "I don't want the image on the sensor moving by more than half the distance between pixels". That would give a figure of about 3-4 microns and would certainly ensure that the image remained sharp even if examined by pixel peepers at 100% in Photoshop. However, that would be a very demanding criterion that would result in impossibly high shutter speeds.

Alternatively, one can consider how big a print one is going to make from the image, what distance one is going to view that print at, and how discriminating the average human eye is. The experienced reader will feel the phrase "circle of confusion" bubbling up at this point. I don't intend to get into that because it is explained much better by Sean McHugh here and by Bob Atkins here (albeit in a different context, that of depth of field). Suffice to say that circle of confusion considerations lead one to choose a figure of about 33 microns for average eyesight, or about 11 microns for people with 20:20 vision.

I simply decided to pick a figure of 20 microns as being a reasonable compromise between these various alternatives. So I am saying that I don't want subject motion to move the image across the sensor by more than C=20 microns whilst the shutter is open.

Finally, we need to note that all the above figures for C apply only to a 35mm film camera or a DSLR with a full-frame sensor. For digital cameras with smaller sensors, C needs to be divided by the camera's crop factor.

Putting this together, we want R*V/N = (F*V)/(D*N) ≤ C. Rearranging, we want N ≥ (F*V)/(D*C).

Obviously enough, the faster the subject is moving (the bigger V is) then the bigger the value of N and hence the faster the shutter speed we require. (Recall that N is the denominator of the shutter speed in this formula.)