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Basics - Focus and Exposure

by Peter Rowlands

Reproduced from in focus 57 (April, 1996)


Most lenses can be adjusted to alter the point of focus. This is usually done by rotating the barrel of the lens to move the internal elements in relation to the film. The further away the lens is from the film, the closer the point of focus will be.

A good example of this is with extension tubes on a Nikonos where the 35 mm lens is placed further away from the film by putting an extension tube between the camera body and 35 mm lens.

With reflex cameras you can see the changing effect in the view-finder but with non?reflex cameras you have to estimate how far away you want the point of focus to be and set the lens accordingly.

Both underwater and on land this can take a bit of trial and error to get right and it's the usual case of 'practice makes perfect'.

Underwater, both your eye and the lens are reacting to 'apparent' distances, taking into account the magnifying effect of water so what you see is also what the lens sees. It is only when you start to physically measure the distance that the difference between aparrent and actual feet becomes important so my advice is not to measure and concentrate on getting your visual estimation of distance as accurate as possible.

As we will see later, there are additional criteria which make the exactness of the point of focus much less critical.


There is little point in having a sharp subject if the film is incorrectly exposed. The correct exposure is achieved by choosing the right combination of Shutter speed and aperture.

Shutter speed

The camera body has a shutter which can be controlled to give varying amounts of time which it is open.

A basic shutter like the one in a Nikonos III camera consists of two blinds A and B. In Position 1, the film is covered by Blind A. When the shutter release button is pressed, Blind A moves up and exposes the film to light (Position 2). The second blind (B) follows after a set time depending on the Shutter speed selected and covers up the film again (Position 3).

When the shutter is cocked, both blinds move down without exposing the film and are held in position ready for the next exposure.

For simplistic purposes it is assumed that the time taken for each blind to move is insignificant.

The amount of the delay between Blind B following Blind A determines the 'SHUTTER SPEED'.

The faster the Shutter speed, the less light arrives on the film and vice versa.

Common Shutter speeds are expressed in fractions of a second and range as follows:

1/30th of a second
1/60th of a second
1/125th of a second
1/250th of a second
1/500th of a second

There is an important relationship between each Shutter speed in that, for example, 1/125th is twice as fast as 1/60th so half as much light fails onto the film and vice versa.


The is a mechanical device built into the lens and is basically a hole, the size of which can be controlled. The larger the hole, the more light gets onto the film during the exposure.

For the sake of simplicity, the sizes are designated in 'F STOPS' - the numbers actually relate to the diameter of the divided into the focal length of the lens and normally range as follows: f2.8, 4, 5.6, 8, 11, 16 and 22.

The smaller the number, the larger the and vice versa.

The relationship between each is known as 'STOPS'. Changing the from f5.6 to f4 opens it up 1 stop, doubles the size of the and so doubles the amount of light getting onto the film. This is referred to as 'OPENING UP A STOP'. Going the other way from f4 to f5.6 halves the siie and also the amount of light and is referred to a 'STOPPING DOWN A STOP'.

Some lenses have intermediate settings to allow control of the in half stops for greater exposure accuracy.

Combining Shutter speed and aperture

As said before, the correct exposure is achieved by choosing the right combination of Shutter speed and aperture.

This is done by establishing the light level with either a hand held light meter or one built into the camera. These devices measure the amount of available light and give a reading which is a combination of Shutter speed and aperture. On a hand held meter a dial shows what to use if you want to use a different Shutter speed than that indicated.

Let's assume the given combination of Shutter speed and to achieve correct exposure is:

1/125th Shutter speed and Aperture f8

The same amount of light will fall on the film if the Shutter speed were halved to 1/60th and the aperture were stopped down one stop to f1l to compensate for the extra amount of light failing on the film. The following list will all give exactly the same amount of light on the film:

1/30th f16

1/60th f11

1/125th f8

1/250th f5.6

1/500th f4

Mixing any of these combinations will result in incorrect exposure.

The effect of changing Shutter speed

The faster the Shutter speed, the more you will be able to 'FREEZE' the action and get sharp results of fast moving subjects. With slower Shutter speeds you will have to hold the camera steadier to get sharp results.

As a rough guide to the slowest speed you can use when hand holding a camera, the speed will be the reciprocal of the focal length of the lens. For example with a 35 mm lens the slowest sensible speed would be 1/35th sec i.e. 1/30th and for a 135 mm telephoto it would be 1/135th sec i.e. 1/125th.

The effect of changing aperture

Changing aperture produces a much more important effect and is often fundamental to achieving sharper results overall.

For simplicity's sake we'll assume that, at Aperture f2.8, only the subject focused on will be sharp. Stopping the aperture 'DOWN' will bring more into sharp focus either side of the centre point of focus and will give the advantage of not having to be too critical about the exact point of focus. This increased focus area is known as 'DEPTH OF FIELD'.

Here is a typical depth of field chart for a 35 mm lens focused at 2 metres:


































Note how the focus range increases as the is stopped down.

The range of the depth of field varies with the focal length of lens being used - the wider the angle of the lens, the more the depth of field so focusing is much less critical.

The effect of using different film

In order to achieve correct exposure you have to take the speed of the film into account. The faster the film, the quicker it reacts to light and vice versa. Common film speeds are:

50 ASA, 100 ASA, 200 ASA, 400 ASA

Just as with aperture and Shutter speed, there is a relationship between the film speeds in that 100 ASA is twice a fast as 50 ASA so 100 ASA is ONE STOP faster than 50 ASA and vice versa. For example, if the correct exposure for 100 ASA is 1/125th © f8, it must be altered to 1/125th © f5.6 for 50 ASA (or any combination which achieves the same amount of light).


If you want to change any of the exposure settings, you must counteract the change in order to retain correct exposure.

If you want increased depth of field you 'STOP DOWN' the aperture but must compensate by slowing down the Shutter speed to let more light in but this might result in subject blur.

If you want to freeze the movement of a fish you will need to use a fast Shutter speed but will have to 'OPEN UP' the aperture to compensate. This in turn will limit the depth of field so you will have to be extra careful with the focus.

Exposure meters and light readings

The precise level of available light is best measured by using a light meter. This can be either a hand held meter or one built into the camera.

Fortunately, light levels underwater are fairly consistent in that the light is diffused but care should be taken when taking a light reading to make sure the correct exposure is achieved.

There are three main types of light metering systems:

Centre weighted



Centre weighted

This is the most common and like that in the Nikonos IVa and V cameras. As the name implies, the meter takes more into account from the centre of the frame than from elsewhere and this does suit the majority of subjects underwater. The usual weighting is 75% from the centre circle of the frame and 25% from the rest of the area.

With this sort of system, care should be taken when the sun or any brighter area is in the frame as it may fool the system into thinking there is more light available than there actually is and this will result in under exposure. In this case tilt the camera slightly down or away from the bright area but still include some of it and this should achieve a correct exposure. With an automatic camera this is not practical as you will be pointing incorrectly so you will have to take a reading with the camera pointed down, note the reading and set the camera controls manually.

Matrix metering

Matrix metering, and derivations of it, works by splitting the frame into sections and takes each of these into account. This is more accurate than centre weighted.

In the case of Nikon F801s and Nikonos RS matrix metering the frame is divided into 5 sections and each section is taken into account. The meter then averages out all of the 5 seperate readings to arrive at a correct exposure.

With this form of metering, bright areas will be taken into account and should not fool the system as much as with centre weighting.

Spot metering

As the name implies, the meter cell reads only the light from a very small part of the frame.

This is ideal for critical exposures of certain areas but is not really practical to use with automatic cameras as the readings could well be inaccurate for the rest of the frame.

This system is best used with the camera on manual.


Most modern camera systems offer exposure automation for both available light and flash which greatly simplifies the photographic process and increases the success rate. However there will be times when even the most sophisticated metering system can be fooled and it always pays to understand how to achieve a correct exposure manually.

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