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10/08/2006 11:52:02 AM · #1
Blink-free photos, guaranteed
from Velocity/Science In Motion: //velocity.ansto.gov.au/velocity/ans0011/article_06.asp

CSIRO physicist Dr Piers Barnes explains to writer and occasional photographer Nic Svenson how many shots she should take to get one where no-one's blinking. This is Nic's story...

"Anyone who's played photographer at family functions knows that, even if everyone stays perfectly still, there's always someone who blinks.

I often have to take group photos and I wondered just how many shots I'd have to take to get one where no-one's blinking. I started counting: people, photos, photos spoilt due to blinks... It was taking forever! I couldn't make up a rule after ten counts. To be what's known as statistically significant, I'd need around two hundred.

I whinged to my colleague, Dr Piers Barnes, and he said: 'You don't need data, we can model it.' Trying not to feel like an idiot for thinking science is based on hard numbers, I set about finding some figures to plug into the formula Piers was working on.

It turns out that the average number of blinks made by someone getting their photo taken is ten per minute. The average blink lasts about 250 milliseconds and, in good indoor light, a camera shutter stays open for about eight milliseconds.

Figuring out the number of photos to take so I can expect to get one where no-one's blinking relies on probabilities (I'd like to guarantee a good shot, but apparently this is impossible as there's always a chance someone will blink). When sorting out probabilities, you have to consider what might influence them.


The number of photos required to be 99% confident of getting a good one

For our purposes, it's fair to say that blinks are independent. If a group of people are looking at a camera, one person's blinks won't influence another's and, unless you've got something in your eye, your blinks don't influence each other either. It's also safe to say that blinks are random; they don't happen every six seconds.

This means we're looking for the probability of a random event - a blink - occurring during a window of time - how long the shutter's open - that's much shorter than the event itself.

Piers says the probability of one person spoiling a photo by blinking equals their expected number of blinks (x), multiplied by the time during which the photo could be spoilt (t) - if the expected time between blinks is longer than the time in which a photo can be spoilt, which it is.

This makes the probability of one person not blinking 1 - xt. For two people it's (1 - xt).(1 - xt) and for a group of people it's (1 - xt)n, n being the number of people.
This means (1 - xt)n is also the probability of a good photo. Therefore, the number of photos should be 1/(1 - xt)n.

Let's test this: each shutter opening results in either a good photo or a spoilt one. If you make a graph of a lot of these successes and you'll find it follows what statisticians call the normal distribution. Even if you know nothing about stats, you've probably heard of the bell curve - well, that's what the normal distribution looks like.

At one end of the curve the trials are 100% successful: the photographer got all good shots. In the middle, the number of good and bad photos is split 50:50. And, at the other end, are all dud trials: the photographer got no good shots.

Piers then figured out how many shots I'd need to be 99% certain of getting a good one. He found that photographing thirty people in bad light would need about thirty shots. Once there's around fifty people, even in good light, you can kiss your hopes of an unspoilt photo goodbye.

Piers also came up with a rule of thumb for calculating the number of photos to take for groups of less than 20: divide the number of people by three if there's good light and two if the light's bad."

Nic's story originally aired on Radio National's The Science Show with Robyn Williams. The show's unique content has given Aussies the inside goss into scientific issues, personalities and topics ranging from the physics of cricket to prime ministerial biorhythms. To catch the show, tune into Radio National on Saturdays at 12.10 pm (repeated Mondays at 7.10 pm), or visit www.abc.net.au/rn/science/ss/
10/08/2006 12:17:26 PM · #2
Originally posted by lynnesite:

...It's also safe to say that blinks are random; they don't happen every six seconds...


Not sure I buy completely random. To keep the eye ball moist a human would have to blink within some time period - no doubt dependant on the person and humidity and other stuff (I have no idea how long but probably a few seconds).

Maybe it would work better if you made everyone blink at the same time and then hit the shutter a little after they all open their eyes? Interesting concept...
10/08/2006 12:21:48 PM · #3
Just take two or three shots, pick the best of the bunch, and clone in faces or eyes of the ones that blinked or moved. It's a lot better than taking 30 shots. :-)

10/08/2006 12:53:38 PM · #4
just tell everyone to close their eyes and right before youre about to take the picture.. tell them to open them..
Blink Free..
10/08/2006 02:44:26 PM · #5
Originally posted by behindthescenes:

just tell everyone to close their eyes and right before youre about to take the picture.. tell them to open them..
Blink Free..


This also gives you larger pupils which normally makes the eyes more attractive.
10/08/2006 07:14:32 PM · #6
Originally posted by behindthescenes:

just tell everyone to close their eyes and right before youre about to take the picture.. tell them to open them..
Blink Free..

Just be ready to wait until they finish blinking if they kept them closed for very long or the lighting is very bright.

---

Concentrating, while not moving also increases the blinking. Try it. Look at something in the room, concentrating on just looking at it without moving (blinking is moving). Typically the eyes start getting dry and burning almost immediately. Telling the group to 'look at the camera -- hold it..." is basicly the same as telling most of them to start blinking.

The best I can suggest is to have the group move their eyes as a group. 'With just your eyes, look at (something to theleft) -- good; look at (something to the right) -- good; look at the camera -- click -- good."

David
10/08/2006 09:17:09 PM · #7
Now, how about squinting. It's virtually impossible for me to have my photograph taken outside in the sunshine without me squinting (I usually wear sunglasses and a hat when I'm outside.)
10/08/2006 09:21:24 PM · #8
i picked up a good tip from a tv show (cheesy drama thing, nothing to do with photography at all...). the photographer in it had his models look down, he'd count to 5, then tell them to look up and smile. i've used it, it seems to work. they smile, their eyes are open, and it's relatively candid, unposed looking.
10/08/2006 09:27:19 PM · #9
Originally posted by xianart:

i picked up a good tip from a tv show (cheesy drama thing, nothing to do with photography at all...). the photographer in it had his models look down, he'd count to 5, then tell them to look up and smile. i've used it, it seems to work. they smile, their eyes are open, and it's relatively candid, unposed looking.


that sounds like a fantastic idea...I am going to take that on!
10/08/2006 10:52:36 PM · #10
Actually some blinks are a product of the flash. Certain people have a faster blink reflex than others. If I remember right, most E-TTL flashes prefire to get a meter reading. In most people their reflex is not fast enough to be caught, but in some they are halfway through the blink when the shutter triggers. My wife and mother-in-law are famous at this. They always gets shots where they look "dopey" because their eyes are half closed...

Message edited by author 2006-10-08 22:53:19.
10/08/2006 11:11:59 PM · #11
Originally posted by xianart:

i picked up a good tip from a tv show (cheesy drama thing, nothing to do with photography at all...). the photographer in it had his models look down, he'd count to 5, then tell them to look up and smile. i've used it, it seems to work. they smile, their eyes are open, and it's relatively candid, unposed looking.


I like that idea, but kinda throws out the focus on the eyes idea :-)
10/09/2006 04:35:47 AM · #12
This is very funny - they were awarded the Ig Nobel award(for the worst science)


10/09/2006 05:36:18 AM · #13
I've just been at a photography studio, having the privilege of seeing some friends choose some photos to print of their eldest daughter's special event for turning seven. Of the seven photos of the family group (the parents and the two girls; the younger one just turned 4), the father has his eyes shut in two of them.

Oh, and the photos were viewed on a TV screen using a light box and camera (which converts the large format negatives to positive images).

10/09/2006 12:52:26 PM · #14
Originally posted by fotomann_forever:

I like that idea, but kinda throws out the focus on the eyes idea :-)


yeah, this is best for groups, or those who are really uncomfortable with being photographed. not for portrait sessions or head shots. unless you're shooting with a small aperture
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