Research: High Speed Image Capture    

Posted on By Tony

Introduction

I researched this topic to be able to capture my response to Exercise 3.1 – capture a frozen moment in time.  I had decided to capture a bursting water balloon.

There are several challenges to be faced to capture a perfectly sharp image.

  1. I use a Nikon D800 and its fastest shutter speed is 1/8000th of a second. Whilst fast, this is not fast enough to freeze the image.
  1. I wanted (needed) to use flash to illuminate the water droplets coming from the balloon but the fastest shutter speed that can be used with a flash is 1/250th of a second. There is an excellent explanation for why this is the case by Young (Young, 2011)
  1. Establishing a technique to fire the shutter at the same time as the balloon bursts.

I started researching youtube as this is the kind of popular image that people like to provide instruction for.

Dealing with exposure length (points 1 and 2)

I found an excellent video presented by Evan Sharboneau (photoextremist.com, 2011) which was created for a different subject matter (dropping vegetables into water) but covered the technical details of how to capture a high-speed image.

As noted above, the shutter speed of the camera is not sufficiently short to freeze an image but it is not possible to go faster in any case as to sync with the flash the maximum is 1/250th.  The solution to these challenges is to make use of the flash duration to control the length of the exposure.  Sharboneau describes that the duration of the flash is significantly faster than the shutter speed.  He also makes the point that the power setting of the flash controls how long the flash duration is, rather than affecting the brightness of the flash (which is what I thought changed).

I use a Nikon SB900 and researching its specifications at the Nikon website, the duration of its flash is as follows:

1/880 sec. at M 1/1 (full) output
1/1100 sec. at M 1/2 output
1/2550 sec. at M 1/4 output
1/5000 sec. at M 1/8 output
1/10000 sec. at M 1/16 output
1/20000 sec. at M 1/32 output
1/35700 sec. at M 1/64 output
1/38500 sec. at M 1/128 output

As the details show, any power output less than 1/8 power will result in a flash duration that is shorter than the shortest shutter speed of the camera.

Next comes the technique to make use of this feature.

Two things are needed.

  1. Shoot in relative darkness so that the ambient light is not registered in the image. It is only what is lit by the flash, for the duration of the flash, that will form the image.  Thus the flash alone is controlling the exposure.
  2. Set the camera so that the shutter will be fully open for at least the duration of the flash. Sharboneau’s technique is to set the camera to 1/250th of a second.  On a Nikon D800, at this speed, it means that the entire focal plane will be open at the same time.  Whilst it is open, the flash will execute and provide an exposure across the whole frame.

Having adopted this technique, and provided the flash is set to 1/16th power or below, then it will be the duration of the flash that is controlling the duration of the exposure and it will be a faster(shorter) exposure than could be delivered by the shutter alone.

To control the brightness of the image, one can make use of aperture setting and/or iso setting.

Capturing a Balloon (point 3)

I found an additional video on youtube by Merino (Merino, 2016) who explained the setup required to capture the exact moment the ballon bursts.

Merino makes use of the technical technique described above and simply times one hand to stick a pin in his balloon whilst at the same time firing the shutter with his other.  I was surprised at this given that this technique requires coordination between the two hands to match each other exactly to capture a moment that lasts approximately 1/20,000th of a second!

I checked a number of other tutorials and found that this is the technique a large number of people recommend. So I accepted this and moved on to the exercise itself.

Main Learning Points

The main things I have learned in this research are:

  • To reach very high exposure speeds, it is necessary to use flash to control the length of the exposure as the speeds are too high for the mechanical shutter of a standard DSLR.
  • Flash durations vary according to their power output settings (rather than the brightness of the flash).
  • Using a specific combination of shutter speed, flash speed, and low ambient light, effective capture speeds of up to 1/38500 can be achieved.

 

Bibliography

Young, D. (2011). Understanding Nikon’s Auto FP High-Speed Flash Sync Mode. [online] Darrellyoung.blogspot.co.uk. Available at: http://darrellyoung.blogspot.co.uk/2011/10/understanding-your-nikons-flash-sync.html [Accessed 15 Apr. 2017].

photoextremist.com, (2011). High-Speed Photography Tutorial with Splashes and Flashes. [online] Available at: https://www.youtube.com/watch?v=JUtrNJN_4zY [Accessed 15 Apr. 2017].

Merino, M. (2016). High Speed Photography with water balloons. [online] Available at: https://www.youtube.com/watch?v=sn01RmVjNOE [Accessed 15 Apr. 2017].