Taking control of Exposure
David Pike

You cannot rely on your light meter - whether it is a TTL meter or a hand-held unit - despite the claims from camera and light meter manufacturers. All light meters suffer from the same basic problem, they assume that you are photographing a medium toned subject and that you want this to be shown as a medium tone in the final image.

There is no doubt that modern light meters are trying to get to grips with this problem by building in better assessment of colour and using micro-chip processors to compare light readings within the frame to a database of sample exposures and then using this 'experience', to set the most appropriate exposure. However, they have not yet cracked it and it is still important for you as the photographer to take control of exposure setting - light meters are very useful tools, but they are not infallible.

The starting point for exposure setting is the exposure suggested by your light meter but there will be many circumstances in which you will have to adjust the exposure suggested. This is often referred to as exposure compensation and there are several ways that it can be done:

a) Manual adjustment
b) using the exposure compensation dial
c) using a mid-toned subject
d) changing the film speed

Grey seals at Donna Nook, Lincolnshire These two photographs show the importance of understanding how parts of an image can affect the overall exposure. The brightness of the seal pup has led to an uner-exposed image in the lower picture. I used the compensation dial to 'add' exposure to achieve a more pleasing result in the upper image.

The Achilles heel of light meters
The problems of exposure setting start with the fact that all light meters, including through the lens (TTL) in-camera meters assume that the tone of the subject that they are measuring is a mid-tone.

As a result, the exposure setting suggested by a meter reading will render the colour of the subject as a mid-tone - tone in this context refers to how light or dark the colour is - regardless of what the actual tone of the subject is.

Understanding tones
Given that light meters are all calibrated to return an exposure as a mid-tone, understanding what is mid-toned and what is not is a useful photographic skill to acquire.

Grey Wolf (captive), The Wild Bunch Ranch, Idaho I had the metering mode set to spot meter and the darkness of the wolf's coat has led to an over-exposed image in the left-hand picture. In the right-hand picture I used the exposure compensation dial to 'subtract' exposure, to achieve a more pleasing result.

Mid-tone is not limited to the medium-tone grey that is often recommended to determine an appropriate exposure, mid-tone can be applied to any colour. The skill that you need to master is to recognise what is a mid-toned yellow, blue or red etc. There is no easy way to do this, but practice is important and can be done anywhere, with or without a camera.

The next step, having learnt to recognise what mid-tone colours are, is to then learn how to judge the difference between the colour-tone of a subject and the mid-tone. This is not too difficult if you think about a colour-tone scale. At one end of the scale the colour tone fades to white, while at the other end it gets increasingly dark, becoming black - again this is true for any colour. The mid-tone of the colour is half way between the two.

The ability of film to capture tones is limited, and on slide film, the difference between burnt-out white and detail-less shadows is about five stops. This then is the full range of our colour-tone scale. Bright white therefore lies 2½ stops from mid-tone in one direction and dark black lies 2½ stops from mid-tone in the other direction, looking something like this:

Japanese Crane, Hokkaido, Japan Knowing how your camera will react to different conditions is important. In this image the white snow is almost full balanced by the very dark background. I found through experience that the matrix meter on my F5 gave a good exposure setting - on brighter days I found that 'opening up' from the suggested exposure was necessary.

white
	Very light	+2.0
	Quite light	+1.5
	Lighter	+1.0
	A bit lighter	+0.5
	Mid-tone	0
	A bit darker	-0.5
	Darker	-1.0
	Quite dark	-1.5
	Very dark	-2.0
Black

The terms used in the table may seem a little trite, but these are the phrases that I use in my head when I assess the tone of a colour.

So how do we use this scale in exposure compensation. We know that the exposure set by the camera's TTL meter will render the colour of the subject as a medium-tone. If this is what we want, then we take the picture. However, if we want to show the subject as a different tone (either to reflect reality or for creative effect) then we will have to adjust the exposure to achieve the desired result. If the subject's tone is between mid-toned and white then we need to increase the amount of light striking the film (open up) by perhaps one to one and a half stops. This will mean increasing the aperture and/or slowing the shutter speed by the required amount. The opposite would apply if we want to achieve a result darker than mid-tone (stopping down).

Spring Beech leaves, Lincolnshire These two images of freshly opened leaves of a beech tree demonstrate that it is not just bright whites and dark browns and blacks that can throw a TTL meter off balance. I shot the upper image using a D1 camera set to spot meter with no compensation - the bright greens of the leaves led to an under-exposed image. The lower shot was taken by adding 1 stop through the exposure compensation dial.

Manual Compensation
Lets imagine we have to photograph a bride in a white dress on a sunny day. The camera's TTL meter assumes that everything is mid-toned and therefore sets an exposure that renders the dress a dismal grey in the final picture, since we want a white dress we have to make some adjustment.

We know we want to add light to the image to brighten it, but how much? Using the scale above we also know that if we go to extreme, 'opening up' by 2½ stops, then we will get a featureless white dress, with no detail. To retain some detail therefore we want to increase exposure - by 1½ or perhaps even 2 stops. Lets imagine that we are shooting with Fuji Provia 100F (ISO 100) and the camera has set an exposure of 1/250 at f22. To adjust the exposure by two stops we would either 'open up' the aperture to f11, or slow the shutter speed by two stops (to 1/60), or by opening up the aperture and slowing the shutter speed by one stop each.

The opposite adjustments would have to be made if we took a meter reading from the groom's black morning suit - the camera would render this as a dull grey and the amount of exposure adjustment would depend upon how dark we wanted to make the suit.

The important point in these examples is that it is the photographer, not the camera that is in charge. The photographer is setting the exposure to achieve the desired result in the final image. The camera's meter provides a starting point for setting exposure - but not the final exposure for taking the picture.

Using the exposure compensation dial
Many modern SLR cameras now have an exposure compensation dial. This significantly simplifies the whole issue of adjusting exposure and allows the camera to remain in auto-exposure mode.

The exposure compensation dial is marked off in half or sometimes thirds of a stop, usually up to +3 and -3 stops. To use this method of exposure adjustment all that you have to do is determine what level of adjustment you believe is appropriate and then to 'dial in' that amount of compensation. To increase exposure by one stop you turn the dial to +1. It cannot get any simpler than that.

	Kamikochi, Nagano Prefecture, Japan I use the exposure compensation dial most of the time to adjust exposures. This shot on a clear winter's day was taken with an addition of one third of a stop dialled in using the exposure compensation dial.

Using mid-toned subjects to set the compensation
Another method of setting an appropriate exposure is to take advantage of the camera's assumption that everything is mid-toned by taking a reading from a mid-toned subject. The critical point with this approach is that the mid-toned subject has to be in the same light as the subject that you want to photograph. The exposure suggested by the camera's TTL meter is then either locked in by pressing the exposure lock button, or noted and reset manually after re-composing on the subject that you want to photograph.

If we return to our wedding group - the bride's mother is wearing a mid-toned blue dress. It is a simple matter, therefore, to put the cameras meter on to spot mode, take a meter reading from this dress, press the exposure lock button, recompose on the bride and shoot. Remember that if the light changes (for example due to clouds in the sky or changes in the position of the subject) a fresh reading from your mid-toned subject will have to be taken and used to re-set exposure.

Changing film speed
The final method of adjusting exposures is to take advantage of the fact that all exposures are set in relation to the film speed. Lets assume that the appropriate exposure determined by metering off the bride's mother's dress is 1/250 at f11 with Fuji Provia 100F (ISO 100).

The meter reading from the bride, however, is 1/250 at f22 - a 2 stop difference from the appropriate exposure. To adjust the exposure we need to 'open up' by 2 stops. This can be achieved by changing the ISO setting in the camera, to make the camera believe that there is a slower film loaded. In this example, the ISO setting would have to be 25 (a 2 stop change from ISO 100). In response to the slower film setting the camera will either widen the aperture or slow the shutter speed by 2 stops, thereby achieving the desired exposure.

Conclusion
The camera does not know what you want to achieve - an auto-exposure camera may set the exposure for you, but it is not necessarily going to set the appropriate exposure. The camera's auto-exposure setting is only a starting point. In order to achieve an appropriate exposure, an exposure that renders an image as you want it, then you have to take control of the exposure and compensate for the camera's inadequacies.

Posted in Posted by cheewooi at 5:50 PM