Tips, ideas and advice for budding underwater photographers
by Nick Robertson-Brown FRPS
Part 2: Getting to grips with your camera settings – Shutter Speed
On a digital SLR, the shutter is a mechanical device that blocks the light to the sensor of the camera, but can be opened to allow light onto the sensor when the release button is pressed. Whilst Micro 4/3 and compact cameras may not have a mechanical device for controlling the amount of light that hits the sensor, the action of the shutter is mimicked electronically. Whether it be mechanical or electronic, the speed of the shutter is the second item of the light triangle and the higher the shutter speed, the less time there is for light to get through to the sensor. As stated in part one, the aperture restricts the amount of light entering the camera too, but as discussed, the more light we allow through the aperture, then the smaller the depth of field will be. This leaves us with a trade-off in how we allow the light onto the sensor. If you are photographing moving objects, such as fish, the shutter speed becomes the dominant factor in deciding the level of light. If your shutter speed is not fast enough moving objects may look blurred and out of focus. This is called motion blur and you can use it to great effect if you get it right, as it will give the impression of speed and motion as it moves through the water.
Working within a range of shutter speeds, with and without strobes, in order to freeze fast the moving action and make it pin sharp, use settings of 1/125 to 1/250 (or higher if your camera will synch with the strobes). But remember, as the shutter will not be open for long, not much light will make it onto the sensor. For slow moving subjects, or to achieve some motion blur, work with 1/40 to 1/125 and you will also get more light on your image.
For the cameras that do not have a mechanical shutter, the light is allowed through to the sensor all the time it is switched on. It is, however, only when the release button is pressed that the pixels in the sensor, which are already charged with light, start reading it and then pass the digital image through to the sensor. It is this moment that is recorded by the camera and the balance between the shutter speed and the f-stop will determine how your image looks.
The shutter speed is also important for defining the background colour of the water, particularly if you are using flash or strobe. In this instance, the flash will freeze the action close to the camera, but as light will only travel a few metres underwater, the light from the background will be presented at the sensor according to the aperture or the shutter speed. This will be covered in greater depth when we get to the part of this series on artificial light, but put simply, the aperture should be set to expose for the subject when the flash fires and the shutter speed can be adjusted, within a small range, so that the background will lighten as you increase the time the shutter is open. Conversely, as you close the shutter, the background colour will become darker. This interplay of aperture and shutter speed will give a light value that will vary in a similar way to how it does in air. It is when you introduce artificial light that the options become greater.
For a black background shot, start out with settings similar to f8 (or above), 1/200, ISO 100 and a low strobe power setting and then make changes in the shutter speed and watch the background colour change.
The shutter speed settings on the camera are not merely random numbers that the manufacturer has come up with. When changing the aperture, the f-stop scale is a mathematical constant. Each stop doubles or halves the amount of light getting through to the sensor. The shutter speed is also scaled in this way; each stop (or setting) will double or halve the amount of light getting through to the sensor. This means that if you wish to maintain the same exposure value on your image, if you open the aperture by one stop, you must double the speed of the shutter by one stop.
Do you want to learn more? You can pick up a copy of Nick’s book “Underwater Photography Art & Techniques” by clicking here.