Tips, ideas and advice for budding underwater photographers

by Nick Robertson-Brown FRPS

Part 1: Getting to grips with your camera settings – Aperture

When you look at your subject underwater, without using artificial light, there are essentially three factors that you can change on your camera to create the image you want. These three factors are aperture, shutter speed and ISO and all three have an effect upon how the image will look. Each will have their own individual characteristic upon the outcome of your image, and usually, by changing one of these factors, one of the others will need to be adjusted to compensate for what you have done. In order to maintain the same amount of light in your image, the exposure value (EV), increasing the light from one of these factors will mean you need to decrease the light using one of the other two.

It is a case of balancing the light to get the right value for the image you want to create. How you balance this value will give you, the photographer, control of how you wish the image to look. In this first article, we are going to look at Aperture.

Aperture

The aperture is the space behind the lens through which the light enters the camera, and hence, the sensor. The aperture works very much in the same way as the pupil of the human eye. In bright light, the pupil will close down to a small round opening, hence restricting the amount of light that falls on the retina. As it gets darker, the pupil dilates to allow more light to enter the eye. The aperture of the camera’s lens can be opened and closed, just like the pupil of the eye, and if you have your camera in the auto setting, this is one factor the camera may use to change the exposure value. This is why I think it is important that you should use the manual setting so you have control of how you want to present your image.

Aperture Setting

The aperture setting is referred to as the f-stop or f-number, and the smaller the number, the larger the aperture. Mechanically, the size of the hole is determined by a circle of blades that cause the central aperture to open and close by the overlapping of these blades on each other. Closing the aperture will restrict the amount of light hitting the sensor and the f-number increases. This f-number is not just a random figure, but is in fact a ratio of the focal length of the lens to the physical size of the aperture. In most compact cameras there is no mechanical closing of an aperture but its attributes are produced electronically to give the same effect.

The figure above shows the f-value and all camera lenses are calibrated using the same f-stop scale. The range of aperture numbers does vary from one lens to another, with the greater range generally equating to the greater price. The scale, however, is always constant and each increase in f-stop number equates to half the amount of light that is allowed through to the sensor.

Depth of Field

Opening and closing the aperture, or decreasing and increasing the f-stop, would appear to be a simple way of changing the light level on the sensor. As the aperture is opened however, the depth of field is reduced, and this has a very noticeable effect upon the image. The depth of field is defined as the amount of the image which appears to be acceptably in focus, and changing the depth of field on your subject can create very different images.

As you can see from the above image, the pygmy seahorse is in sharp focus. The rest of the coral behind it is totally out of focus, and this has the effect of making the seahorse, your subject, pop out from the picture.

And again with this ghost pipefish, the subject is in focus, but the background is blurry and there can be no misunderstanding as to what the subject is.

This does not necessarily mean that you should always use a small depth of field to make your subject pop out, and there are times when the environment around the subject is important. This technique is used by many underwater photographers as it allows you to virtually eliminate any messy background by using a low f-stop number to create a small depth of field. This is ideal for when your subject will not come out into the open. The blurry background in an image is called “Bokeh”

Of course, sometimes you want to show the critter in its environment and to do this, you need to increase the f-stop (close down the aperture). As a result of closing down the aperture, the exposure value will go down as you are letting less light onto the sensor. Within the exposure triangle, there are two other factors you can adjust and I will be looking at these controls next time.

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