Blink-Free Photo Calculator
Enter the size of your group and the lighting conditions, and this calculator tells you exactly how many frames to shoot so you have at least a 99% chance of getting one photo where nobody has their eyes closed. The result comes from the Ig Nobel Prize-winning 2006 research by Nic Svenson and Piers Barnes, which modelled human blink rates, blink duration, and camera exposure time to derive a precise shot count.
Formula
Worked example
For 5 people in good light: blink probability per person = (10/60) x (0.01 + 0.25) = 4.33%. Probability all 5 have open eyes = (1 - 0.0433)^5 = 80.0%. Shots for 99% confidence = ceil(ln(0.01) / ln(1 - 0.800)) = ceil(4.605 / 1.609) = 3 shots.
The science of blinking and group photos
Humans blink about 15 to 20 times per minute under normal conditions, but the rate drops to around 10 blinks per minute when someone is concentrating - like looking at a camera. Each blink lasts approximately 250 milliseconds. A typical outdoor camera exposure is about 1/100 second, while a low-light or indoor shot might use 1/30 second. During that window, there is a small but real chance that any given person will be mid-blink. With a single subject the odds are in your favor, but multiply that risk across a group of 10, 20 or 30 people and a single perfect frame becomes surprisingly rare. This is the problem Nic Svenson and Piers Barnes quantified in their 2006 paper "Blink-Free Photos, Guaranteed," which won the Ig Nobel Prize for Mathematics.
How the formula works
The core calculation has three steps. First, compute the probability that one person blinks during a single exposure: blink rate (blinks per second) multiplied by the sum of exposure time and blink duration. Second, raise (1 minus that probability) to the power of the number of people to get the chance that everyone has open eyes in one frame. Third, use the geometric distribution to find how many frames you need so that the probability of at least one fully blink-free frame exceeds your chosen confidence level. In practice: T = ceil( ln(1 - confidence) / ln(1 - p_all_open) ). The classic "divide by 3 in good light, divide by 2 in bad light" rule of thumb is a practical approximation of this exact formula.
Tips for photographers
Shoot outdoors or in bright, even light whenever you can - it is the single biggest lever you have, cutting the required shot count almost in half. Use your camera's burst or continuous mode to fire the recommended number of frames quickly, before people move or lose their pose. Count down aloud ("3, 2, 1, shoot!") and ask everyone to take a slow, deliberate blink just before the countdown, then keep their eyes wide on zero. This "blink reset" approach clears the muscle fatigue that causes involuntary blinks. For very large groups (30 or more people), consider splitting into two or three sub-groups, photographing each separately and compositing the images together afterward.
When group size becomes unmanageable
Beyond about 50 people in low-light conditions, even shooting 20 or more frames still leaves a meaningful chance that at least one person will be blinking in every frame. At that scale, professional photographers use image-editing software to composite the best eyes from multiple frames onto a single base image. Alternatively, video capture followed by frame extraction gives you hundreds of frames to choose from. Modern cameras and smartphones also offer "best shot" modes that automatically select the frame with the most open eyes from a short burst.
Shots needed by group size
| Group size | Good light (shots) | Low light (shots) | Rule of thumb (good) | Rule of thumb (low) |
|---|---|---|---|---|
| 1 | 1 | 1 | 1 | 1 |
| 2 | 1 | 1 | 1 | 1 |
| 3 | 1 | 2 | 1 | 2 |
| 4 | 2 | 2 | 2 | 2 |
| 5 | 2 | 2 | 2 | 3 |
| 6 | 2 | 3 | 2 | 3 |
| 8 | 3 | 3 | 3 | 4 |
| 10 | 3 | 4 | 4 | 5 |
| 15 | 5 | 6 | 5 | 8 |
| 20 | 6 | 8 | 7 | 10 |
| 25 | 8 | 10 | 9 | 13 |
| 30 | 9 | 12 | 10 | 15 |
| 40 | 12 | 15 | 14 | 20 |
| 50 | 15 | 19 | 17 | 25 |
Number of photos required for a 99% chance of at least one blink-free frame, based on Svenson & Barnes (2006). Good light assumes a 1/100 s shutter; low light assumes 1/30 s.
Frequently asked questions
How many photos should I take for a group of 10?
In good light you need about 3 to 4 photos for a 99% chance of one blink-free frame. In low light that rises to 4 to 5. The quick rule of thumb is 10 / 3 = 3 in good light and 10 / 2 = 5 in low light. This calculator gives you the exact figure based on the Svenson & Barnes probability formula.
Why does lighting make such a big difference?
Better light lets you use a faster shutter speed, typically 1/100 second outdoors versus 1/30 second indoors. Since a blink lasts about 250 milliseconds, a faster shutter captures a shorter slice of time and therefore has a smaller window during which a blink can be caught. This cuts the per-person blink probability from about 4.7% in low light down to about 4.3% in good light - a small difference per person that compounds significantly across a large group.
What is the Svenson and Barnes rule of thumb?
Nic Svenson and Piers Barnes of the Australian CSIRO published a formula for blink-free photos in 2006. Their practical rule of thumb: divide the number of people by 3 if the light is good, or by 2 if the light is poor. So for 12 people in good light, take at least 4 shots. This is an approximation of the full probability formula, and the exact figure may be a shot or two higher for very high confidence levels.
What confidence level should I choose?
99% is the standard used in the original research and works well for most situations. If you are a professional photographer who needs a guaranteed result, use 99.9% - it adds one or two extra shots but nearly eliminates the risk. For casual snapshots where a near-miss is acceptable, 95% is fine and requires slightly fewer frames.
Does this apply to video or only to photos?
The formula applies to any single still frame. Video captures 24 to 60 frames per second, so the effective exposure per frame is very short, making individual frames more likely to be blink-free. With video you can simply extract the best frame after the fact rather than calculating how many clips to record.
Is there any way to reduce blinking on command?
Yes. Ask your subjects to take a deliberate slow blink a second or two before you shoot, then keep their eyes wide open on the countdown. This "resets" the blink reflex and reduces the chance of an involuntary blink during the actual exposure. Keeping the session short also helps - fatigue and boredom both increase blink rate.
Why do people blink more in bad light or with flash?
Flash causes a startle reflex that can trigger a blink. Dim light also causes the eye muscles to work harder as pupils dilate and refocus, which can increase involuntary blinking. Consistent, diffused light from multiple sources is ideal because it avoids the harsh on-axis flash that triggers the most blinks.