# The Mechanics Of The Decisive Moment

The Mechanics Of The Decisive Moment

Thursday 15th August 2013 1:00am

In his original book (Images a la sauvette), Henri Cartier-Bresson (HCB) referenced Cardinal de Retz's statement that "There is nothing in this world that does not have a decisive moment". To my mind there is no photograph of worth that does not capture a decisive moment. It is this that gives a still image narrative, and which transforms an image from being a document to being a work of art: for such a moment cannot be captured by instrumentation alone. It requires attention, thought and possibly 'intuition' on the part of the photographer. By demonstrating the impossibility (or extreme unlikelihood) of capturing that moment by instrumentation I intend to add to the argument for photography as art.

HCB himself defined the 'decisive moment' as:

"...the simultaneous recognition, in a fraction of a second, of the significance of an event as well as the precise organization of forms which gives that event its proper expression."

Here he is emphasising the instantaneous coalescence of the event, or if you will, the intuition of the observer. 'Intuition' is a disturbing term. It conjures concepts of divine intervention, something beyond the consciousness of the practitioner. Well, he came from a time and society that still hung on to mystique and mysticism (he suffered a strong catholic schooling). However, if we can accept intuition as the expression of the subconscious mind then we can still find some value in his idea. The crucial thing is to separate the instantaneous moment of capture from the moment of recognition - our hand need not be guided by god at the moment of taking a photograph (or by the mechanics of a camera's software algorithms). The decisive moment can in fact be foreseen.

So, for me, the decisive moment is nothing more nor less than the precise time at which a composition's narrative intent is best portrayed.

Reading around the subject on internet forums will dredge up the usual cacophony of dogma. There's a lot of argument about being in the right place at the right time and having an element of surprise. Arguments that imbue the process with mysticism, that rob us of the right to break down the process and formulate working practices to improve the chance of success. It is not a matter of luck, or the genius of those with great 'intuition'. It is a matter of understanding, recognition, and preparedness. In photography we make our own good luck.

There are also schools of thought that make the decisive moment a genre, a classification of photography. There are arguments about whether a picture is a 'decisive moment picture' or not. This seems simply futile. All photographs of worth contain within them the four perceivable dimensions of our world: the horizontal and vertical as clearly expressed in the flat image; the depth as represented by the depth of field, the plane of focus and the camera-to-subject geometric effect; and the timing as decided by the instant of exposure. All photographs freeze a moment in time. If that specific moment adequately expresses a narrative then the image is an artistic endeavour. If it does not then the image is either a snapshot or else a documentary record. In the first case the image contains a decisive moment. In the latter it does not.

A good test for a photograph's successful capture of a decisive moment is to ask "why was the image taken at just that time?" If there is a reasonably apparent answer then the image likely contains a sense of narrative.

So what does the decisive moment look like in practice? Here are some examples progressing from longer lasting, to shorter lasting moments:

## Examples of the Decisive Moment

### Example 1: Moments That Last - Cocoon

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### Example 2: Moments That Drift - Cranes

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### Example 3: Moments That Pass - Trafalgar Square

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### Example 4: Moments That Flee - Dan Hunt

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### Example 5: Moments That Blink - Woody Bop Muddy

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## Duration Of The Decisive Moment

With regards to capturing them within a two dimensional bounded frame (a picture of finite size), decisive moments have a finite duration. When dealing with things that move longitudinally or latitudinally (or both) through the frame, the duration of the moment is correlated to the size of the moving object (in relation to the frame size) and its speed, within some placement tolerance.*Decisive Moment Duration = Extension * Tolerance / Velocity*

When the moment moves towards the photographer (orthogonal to the plane of the frame) it will have an apparent zero velocity but will exhibit a delta in extension (a change in size). The duration of the decisive moment is correlated to the tolerance we have on that change in size.

*Decisive Moment Duration = Delta Extension / Orthogonal Velocity*

When the moment doesn't move in relation to the frame (e.g. is an expression on a stationary person's face) then the duration of the moment is arbitrary, depending on the nature of the subject.

*Decisive Moment Duration = Indeterminate*

So, apart from the latter (special) case, the duration of a decisive moment is a factor of the size of the thing moving in relation to the speed of its movement, in the frame - within some acceptable tolerance.

Returning to our earlier examples I will look in more detail at the duration of the moments. In these examples where movement is involved I segment the image so a sense of distance can be determined. I also highlight the acceptable area for placement of the key compositional items.

**Example 1:**

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**900s**.

**Example 2:**

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**60s**. The pigeon (strip 9) is at full wingspan (0.6m) and travelling at perhaps 20kmph (or 5.5m/s, it was a leisurely old bird. Racing pigeons fly on average at 92.5kmph).

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**0.6s**.

**Example 3:**

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The man was walking briskly, about 4mph or 1.8m/s. Average stride for a man is about 1m and we can see his stride covers 1.5 segments. To move completely across the frame, he has to cover 7 segments. So for his distance from the camera he has to take 5 x 1m strides at 1.8m/s, which is 2.8s to completely cross the frame.

Each strip is 2/3rds of a meter and takes 0.37s to cross (2/3rds divided by 1.8m/s). The man is placed in the centre of the third strip. I could have placed him anywhere from the very start to the very end of the third strip, so I had one strip of tolerance in my composition - but, because the woman at the back is also moving I cannot afford to exercise that tolerance. So that makes the decisive moment the time to cross that one strip. The window for this decisive moment is

**0.4s**.

**Example 4:**

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**Example 5:**

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**0.13s**.

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**0.07s**.

The following table expresses the earlier examples in these terms. Note, for 'action' images the decisive moment duration

**is**given by Size * Tol * Velocity:

Example | Motion Type | Size % of frame | Tol. | Velocity seconds to cross frame | Decisive Moment Duration |

1 - Cocoon | None | 90% | n/a | 0s | 900s |

2 - Cranes, cloud only | Longitudinal | 10% | 2 | 300s | 60s |

2 - Cranes, with pigeon | Longitudinal | 3% | 6 | 3.3s | 0.6s |

3 - Trafalgar Square | Longitudinal | 14% | 1 | 2.8s | 0.4s |

4 - Dan Hunt | None | 25% | n/a | 0s | 0.3s |

5 - Woody Bop Muddy, hand | Latitudinal | 25% | 2 | 0.25s | 0.13s |

5 - Woody Bop Muddy, rice | Orthogonal | 36% | 1 | 0.2s | 0.07s |

So in these examples, with the exception of stationary or slow moving subjects that do not have fleeting changes of expression, the decisive moment has a duration in the range of around 0.4 to 0.1s. With an average human reaction time when concentrating of around 0.15s, the effective decisive moment duration is somewhere between 0 and 0.2 seconds. Let us say that:

**on average the decisive moment has a duration of 0.1s.**

Of course there are many forms of subject I have not considered, most especially high-speed action, such as: a kingfisher in flight (20mph); an F1 car (average speed around 100mph); the world's fastest recorded tennis serve (163mph). In these situations the photographer will generally pan which has the effect of prolonging the decisive moment.

## Mechanics of Capturing The Decisive Moment

The main reason that I'm discussing the duration of the decisive moment is to consider how best to capture it. Options for triggering the camera's shutter release are:- Manually, once with anticipation of the moment
- Manually with a burst of several high speed shots around the moment
- Automatically by employing some form of external sensor trigger
- Automatically on a time lapse

For less specialised applications the photographer has to choose to manually trip the shutter and can apply one of the two manual options in the hope of doing so at just the right moment (within the 1/10th of a second that the moment lasts on average).

So, can firing a burst of shots at a high rate of frames-per-second (fps) capture the decisive moment?

Nikon's D300 delivers 6fps. At a shutter speed of 1/30th of a second the probability of capturing the moment is 1 in 3. To demonstrate this, consider the following diagram:

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The circles show the one second timeframe split into thirty intervals of 1/30th of a second each. The points when the shutter is open are shown in yellow. The interval between shots is shown in blue. This diagram shows four possible points when the 1/10th of a second (=3/30ths of a second) decisive moment may happen:

- Point A (in red) will be a missed shot as it falls in the interval when the shutter is closed.
- Point B (in amber) will probably be an unacceptable shot since the decisive moment is forming whilst a shot is being taken.
- Point C (also in amber) will probably be an unacceptable shot since the decisive moment finishes in the same interval that the shutter fires, and so has passed too far.
- Point D (in green) will be a good shot since the decisive moment has become established before the shutter opens and remains stable until after the shutter closes.

The chance of capturing a good shot of a moment using a burst of exposures is given by the following formula (many thanks to Holly Hayes for help in deriving this):

For the D300 example, this gives:Given:De - Duration of eventTs - shutter speedNs - number of shots takenfps - frames per secondProbability of capturing good shot is(De - Ts)*(Ns-1) / ( (Ns/fps) - De )

Which is just about 1 in 3.De = 0.1sTs = 0.033sNs = 6fps = 6(0.1 - 0.033) * (6-1) / ( (6/6) - 0. 1 ) = 0.37

The formula assumes that we will start shooting before the moment happens, which in all likelihood we will: therefore the first shot is bad and we subtract one from the number of shots.

Notice that as the shutter speed decreases the chance of getting the shot increases. This is because a shorter shutter speed is less likely to fall at the start/end of the moment; e.g. for a shutter speed of 1/120th of a second (0.008s):

Also notice that the number of shots taken has little effect on the probability of capturing the moment, since number of shots appears on the top and the bottom of the division; e.g. if we shoot twice as many:(0.1 - 0.008) * (6-1) / ( (6/6) - 0. 1 ) = 0.51

The frame rate has a significant effect. Consider if we invested the £5,000 or so needed to upgrade from a D300 to a D4 so that we could shoot at 10fps, keeping all other parameters the same:(0.1 - 0.033) * (12-1) / ( (12/6) - 0. 1 ) = 0.39

Here the probability of getting a good shot is 2 out of 3, as opposed to 1 out of 3 with the D300. Pushing the equation to the limit, shooting 10 shot at 1/8000s (0.000125s):(0.1 - 0.033) * (6-1) / ( (6/10) - 0. 1 ) = 0.67

It is clear that a higher frame rate is highly desirable.At 6fps: (0.1 - 0.000125) * (10-1) / ( (10/6) - 0. 1 ) = 0.574At 10fps: (0.1 - 0.000125) * (10-1) / ( (10/10) - 0. 1 ) = 0.999

Charting shutter speed against probability for 6fps and 10fps gives:

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In conclusion

**can**firing a burst of shots at a high rate of frames-per-second (fps) capture the decisive moment?

The answer seems to be 'yes, sometimes'. Using a high-end camera and restricting shutter speeds to 1/125th of a second or higher for moments slower than 0.1 seconds with placement tolerances of 10% or more then we can rely on the camera to implement the photographer's artistic judgement.

But there are a lot of constraints there. In practice the photographer will achieve greatest success if they use continuous shooting with caution. If the photographer takes the time to learn the behaviours of their subject, and to anticipate the decisive moment, the chance of success will be greater. The formula becomes:

And the chance of success shooting with a 1/30th of a second shutter speed at 6fps rises from 37% to 45%, almost 1 in 2 as opposed to 1 in 3.(De - Ts)*Ns / ( (Ns/fps) - De )

The success will be all the more fulfilling since it relies less upon good fortune. The capabilities of the technology can certainly play a supporting role, especially with high-speed or uncertain action, but with today's technology intuition remains firmly the province of the seeing eye.

## Summary

Contrary to dogma I consider all photographs of worth to contain a 'decisive moment', otherwise they are snapshots or documents.Depending on the subject the moment can have a wide range of durations. When dealing with action, the moment's duration is a function of the size of the moving object and its speed. If the action is orthogonal to the plane of the image the moment's duration is a function of the change in apparent size of the moving object. If the action doesn't move (e.g. is a change in expression) then knowledge of the subject is required to understand the duration of the decisive moment. For example, expression related to speech may be a function of the speed of the speech.

The decisive moment in action shots may well be around 0.1 seconds on average, although panning may be required. Moments of this duration cannot be reliably captured by a camera's high-speed continuous shooting mode in a wide range of situations. High-speed shooting can be a help, but only if the photographer approaches the shot with the anticipation needed to 'intuit' the moment.

This reliance on human skill transfigures the mechanical craft of photography to an artistic endeavour. Despite advances in technology, photography remains an art.