Blink-Free Group Photo Calculator (how many shots so nobody blinks)

Understanding the blink-free group photo calculator — concept and principle

In a big group photo, someone almost always blinks. This calculator estimates the chance that one photo catches everyone with eyes open and how many photos you should take to be confident of at least one good one, from the number of people, their blink rate, blink duration and shutter speed.

Each extra person multiplies in another 92.6 %, so 10 people are all open 46 % of the time.

How it is calculated

Window per person w = blink duration + exposure time (1 ÷ shutter value)

P(one person not blinking) = e^(−r × w), with r = blinks per minute ÷ 60

P(everyone's eyes open) = e^(−n × r × w)

Average photos per good shot = 1 ÷ P

Photos for confidence C: N = ⌈ln(1 − C) ÷ ln(1 − P)⌉

The principle

Blinks happen at random moments at a fairly steady average rate, which is what a Poisson process describes. A person spoils the shot if a blink is under way at any moment during the exposure, which means a blink must have started within a window as long as the blink plus the exposure. For a Poisson process, the chance of no event in a window is e^(−rate × window). With several independent people, those chances multiply, so the chance of a clean shot falls exponentially with group size. Repeated photos are independent tries, so the chance that all N fail is (1 − P)^N.

Worked example

Default inputs: 10 people, 15 blinks per minute, 300 ms closed per blink, 1/125 s shutter, 90% confidence.

r = 15 ÷ 60 = 0.25 blinks per second; w = 0.300 + 0.008 = 0.308 s.
One person: e^(−0.25 × 0.308) = 0.926, so the chance one person blinks is 7.4%.
Everyone open: e^(−10 × 0.25 × 0.308) = e^(−0.77) = 46.3%.
Average photos per good shot = 1 ÷ 0.463 = 2.2.
For 90% confidence: N = ⌈ln(0.1) ÷ ln(0.537)⌉ = ⌈3.70⌉ = 4 photos.

Each point is the calculator re-run with only “Blink rate (blinks per minute)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Blink rate (blinks per minute)	Photos to take
3.75	2
7.5	3
11.2	3
15	4
18.8	5
22.5	7
30	10

How to read the result

Small groups need only a couple of shots; large groups need many more because the chance of a clean frame drops exponentially. Research has measured average blink rates in the teens per minute for adults, but nerves, bright light and a countdown change real behaviour. Taking a burst, asking people to close their eyes and open them on "three", and using faster shutter speeds all help.

Common mistakes

Assuming the number of photos grows in proportion to group size; it grows faster.
Ignoring that blinks last a few hundred milliseconds, much longer than the exposure.
Expecting the result to be exact for any specific group.

Keep in mind: The model assumes independent, random blinking at a steady rate. Real groups behave differently, so treat the number as a guide and take a few extra shots.

Frequently asked questions

How long does a blink last?

Studies with high-speed cameras report around a third of a second, though it varies.

How often do people blink?

Adults often blink roughly 12–20 times per minute at rest, varying with activity and mood.

Does a faster shutter help much?

A little; the blink itself is much longer than typical exposures.

Sources: J. Kaminer et al., "Characterization of the spontaneous blink in humans", Journal of Neuroscience 31(31): 11256–11267, 2011 · K.-A. Kwon et al., "High-speed camera characterization of voluntary eye blinking kinematics", Journal of the Royal Society Interface 10(85): 20130227, 2013

Checked 9 Nov 2026. Poisson model of random events (probability of no event in a window = e^(−rate × time)); blink rate 17.6 ± 2.4 blinks/min in 10 adults: J. Kaminer et al., J. Neurosci. 31(31):11256–11267, 2011; blink lasting about 1/3 s and adults blinking about 12 times per minute: K.-A. Kwon et al., “High-speed camera characterization of voluntary eye blinking kinematics”, J. R. Soc. Interface 10(85):20130227, 2013.

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