**How to guarantee a group photo without capturing anyone blinking**

It is a truth universally acknowledged that whenever a group photograph is taken there is always at least someone who manages to be snapped with their eyes shut. Blinking is a natural eye function that spreads tears across and removes irritants from the surface of the cornea and conjunctiva. For the perfectionists amongst us and for those with an enquiring mind the obvious questions are is it possible to get a group photo without someone blinking in it and how many shots will you need to take to be sure you have one picture with everyone wide-eyed?

The obvious answer is just one, provided you give each of the subjects a pair of matchsticks to prop their peepers open. However, if you want a natural photo or at least as natural as a group photo is ever likely to be, then you need to resort to some mathematics and probability theory. Fortunately someone cleverer than I has cracked their grey cells to shed some light on this first world problem. Step forward, Dr Piers Barnes, a physicist from the Commonwealth Scientific and Industrial Research Organisation.

The starting point is the blink. The average number of times a person blinks when they are having their photo taken is ten and an average blink lasts 250 milliseconds. Unlike yawning where one person can trigger off a spate of copy-cat yawning amongst bystanders, there is no evidence that one person blinking influences another. Each blink is an independent event and when we have a group of people each of their blinks will be independent of each other’s. The only occasions when this might not be the case is if the group are standing in something like a sandstorm but let’s ignore this unnecessary complication. Each blink will also be random – they won’t all occur uniformly every six seconds.

In good indoor light the shutter of a camera stays open for eight milliseconds, a period of time considerably shorter than the duration of a blink. So from a probability theory perspective the chance of someone blinking while a photo is being taken is the expected number of blinks which we will call x multiplied by the period of time (t) during which the photo could be spoilt. The reciprocal, 1 – xt, is the probability of one person not blinking while a photo is being taken.

Following this logic through, if you have a group of people posing for a photograph – we will denote the number by the symbol n – then the probability of a good group photo with no one blinking would be 1 minus xt to the power of n and the number of photos required to get the perfect shot will be 1 over 1 minus xt to the power of n. With me so far?

Plotting the results of the formula on to a graph you will find you have a normal distribution which will enable you to calculate the number of shots you would need to guarantee, at least statistically speaking, a perfect photo for any size of group. What it does mean is that if there is a group of fifty or more, there is virtually no chance of an unspoilt photo. Remember that when you are planning your wedding photo list.

Of course, in the heat of the moment even the brainiest of photographers might not be able to make the necessary calculations. Helpfully, Barnes has developed a rule of thumb for calculating the number of shots for groups of under twenty people. In good light divide the number of people by three and in poor light use two as the denominator.

So now we know. Happy snapping!

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