Digital Camera Exposure System

Ask a group of photo enthusiasts whether they can guarantee to obtain the ‘correct’ exposure every time they press the shutter release and you will rarely hear more than one or two reply ‘yes!’. I pose this question at the start of my talks on exposure control and it amazes me how few people can honestly say they always know the result will be what they expected. Why is it then in this age of sophisticated digital cameras with multi-mode TTL (through-the-lens) light metering that many people are still unsure about achieving the desired exposure? In this feature we will discuss a simple method of exposure control that can guarantee success every time.

But first let’s clarify what we mean by correct exposure. In fact, I don’t like the term ‘correct exposure’ preferring the more flexible expression desired exposure. The optimum ‘desired exposure’ for a subject is that which results in an image capturing the photographer’s feeling for the subject. However, since we now have the flexibility of digital editing, the final image that captures the photographer’s emotional reaction to a subject may actually require more than one ‘desired exposure’ in the camera. It is now common practice to make two or more camera exposures of the same scene to be combined later in post-production to achieve the photographer’s vision.

A crucial first step to finding your ‘desired exposure’, whether you're at a fashion shoot or doing some photography work for magazines, is how you visualise (interpret) the subject as a picture; the visualisation process will help you to determine the ‘desired exposure’. Individual visualisation is the reason two photographers recording the same subject at the same time may well use different exposures (other things being the same); they wish to create different final images.

Now we know that each of us is likely to want a unique exposure to realise our interpretation of a subject so what do we need to know and how do we go about achieving our desired exposure?

Visualisation

The first important step is to decide how you want to interpret your chosen subject. The idea is to see in your mind how you want the subject to look in your picture. This is called visualisation and it helps you to decide what crucial first steps will be required to achieve the final result. Visualisation determines such factors as: camera viewpoint, lens focal length, depth of field, lighting, ISO setting and decisions regarding how you wish to translate the subject brightness values into image tones.

Subject Brightness Range

Before you can decide how to expose your subject, and achieve your visualised image, you need to determine the existing subject brightness range (SBR) of the scene. Determining the SBR of a scene allows us to know exactly how the scene will record in the camera for a given amount of exposure and whether any modifications are required. Modifications might include making several different exposures, use of a polarising filter to control reflections and/or modify subject colour tones. Knowing the SBR allows you to make informed decisions and control the camera exposure(s) for optimum results rather than simply bracketing and hoping.

The actual SBR for a scene or subject is the difference in brightness value, as measured with a light meter using f/stops, between the darkest and lightest important areas of the subject. Please notice the use of the word ‘important’ in the last sentence. The important areas of the subject for determining the SBR are those in which you want to retain some detail or texture. Be careful not to bother with visually unimportant parts of the subject or areas that are not significant to your visualisation. The amount of detail you can record will be discussed later.

As mentioned, SBR is determined using a light meter. The light meter is used to measure the brightness of the important areas of the subject. The highest and lowest of the meter readings will tell you which are the lightest and darkest areas of importance. The difference in stops between these two readings gives you the SBR of the scene.

Light Meter Issues

To be able to determine the SBR of a scene accurately requires that we have a reliable reflected light meter (a spot TTL or hand spot meter is best) and use it intelligently! Remember, light meters are not intelligent and you need to understand their limitations.

The crucial thing to note is that the factory calibration of the meter may not produce accurate information in the real world. For practical use your light meter should be calibrated to a Kodak 18% Grey Card (I will ignore the various debates that have taken place over the years over this point). Once this is done your meter will attempt to make whatever it measures record as an 18% grey tone. Remember: a light meter does not know what you are pointing it at! To the meter, a white wall is the same as a black cat. Whatever the meter measures, it will indicate exposure settings that will attempt to make that area a mid-grey tone.

Testing a camera TTL meter

To calibrate the camera’s meter use a Kodak Gray Card (or equivalent grey test card) and photograph it in soft overcast daylight. Open the resulting image file in an editing program and use the eyedropper tool and the Info palette to measure the ‘K’ value. If the image tone of the card is lighter than middle grey (value 128 or 50%) the image is over-exposed indicating your camera ISO was set too low. If the image tone is darker than middle grey the image is under-exposed and the camera ISO was set too high. To adjust the calibration of the camera increase or decrease the ISO as necessary until you can reproduce the Kodak Gray card as a middle tone value.

It is essential for each photographer to calibrate their meter to take account of personal working methods and equipment. Usually, the only item of information that we can change on a light meter that will affect the calibration is the ISO setting.  By adjusting the ISO setting on the meter you can fine-tune the accuracy of the meter for your personal methods. This will give you a personal exposure index (EI) for the camera TTL or hand-meter used.

Note: The ‘exposure compensation’ feature built in to most DSLR cameras is the same as changing the actual ISO setting; so your camera’s exposure compensation feature can be used for meter calibration too!

Dynamic range and contrast control

For digital camera users, the problem of the SBR exceeding the tone range of the camera sensor is the same as for film users. The DSLR sensor can only capture, in one exposure, a certain range of subject brightness values. This is the tone range or dynamic range of the sensor.

An essential step is to do a calibration test to find out the normal dynamic range of your camera system. See the following procedure to determine sensor dynamic range.

DSLR Calibration

A Practical Exposure Method

So how do we actually get the result we want? As stated above, an area measured with a calibrated light meter will reproduce as a mid-grey tone in the image if you use the settings indicated by the meter.

So, if you meter a dark area of the subject and you would like it to be dark in the image the exposure indicated by the meter will over-expose that area. Using the meter’s settings will reproduce the dark subject area as a mid-grey in the image. This would be too light. Similarly, if the area of the subject is light, and that is how you want it in the final image, using the indicated meter reading will result in under-exposure of that light area. It too will be made mid-grey in the image. So how do we accurately determine the desired exposure?

You will often read that when the subject is light open up a couple of stops from the meter reading or when the subject is dark you are told to close down a couple of stops. This is hardly being in control and actually ignores how you want the subject to look in the image (your visualisation). What you need is a reliable, repeatable and accurate method so read on.

Based on your visualisation, the first thing to do is decide how you want the subject to look in your final image. Which parts of the subject do you want detail in and how much detail do you want? Using your light meter take readings from the various areas and build up a mental picture of the brightness values of the subject. For practice, it is useful to make a simple sketch and write the meter readings on this.

Once you have this information, determine the lightest area in which you want good detail. In other words, the brightest part of the scene in which you want to see all the texture. For example a white brick wall in sunshine. Now compare this reading to the reading from the darkest area in which you want detail. If the difference between these two readings is the same as your camera dynamic range (e.g. 5 stops) then you have a normal SBR subject and it should record well with your digital camera when given the optimum exposure. Now, to actually determine the exposure to use take the reading from the lightest area with detail and increase it by the number of stops determined from your calibration test for the lightest tone that retains detail, i.e. say 2 stops. The earlier dynamic range test has given you the maximum stops from the metered reading that you can increase the metered exposure of a textured subject and still retain the important detail! Any further increase and you do not record enough information to show any texture; it becomes first featureless white and then you get ‘white out’ in of the image tone. By following the correct procedure detailed here, you will have changed the exposure settings (and hence the subject’s image tonal value) to move the tone up the tone scale. You have ‘placed’ your light tone on the tone scale at the correct position for your visualisation and at the same time obtained the ‘desired exposure’ settings.

Working with the other end of the tone scale, a similar method can be used for ‘placing’ dark areas of the subject. When you meter a dark textured area, such as a detailed shadow, the meter reading makes it mid-grey so it is necessary to adjust this reading to give less exposure. But by how much do you reduce it? Just as with light tones, the answer depends on how much detail you want. The maximum reduction in exposure you can give a dark subject and still retain the slightest hint of detail is the lower limit from your calibration test, e.g. 4 stops. However, this may be too much reduction if you want good detail. Your calibration may indicate that the most you can reduce the exposure and retain good detail for a dark subject is actually 3 stops less than the meter reading. So in the case of our dark, detailed shadow, a reading from the area needs to be reduced by no more than three stops to retain the good detail in the subject A two stop decrease would make the shadow a lighter grey with detail but this may then look slightly too light. It depends what you want from the image based on your visualisation.

With this knowledge of texture limits you can determine whether the areas you want detail in will actually retain that detail. If the difference between the lightest and darkest readings of the important subject areas is equal to or less than the dynamic range of your camera you can expose as above by placing the light tone and taking the shot (the dark tone will naturally ‘fall’ on the correct lower tone in this example).

If the SBR required for your visualisation exceeds the dynamic range of your camera you will need to consider using multiple exposures and combining them in post-production. That is the subject of another feature.

If you have followed the above method you now have enough understanding of how to achieve the desired exposure for any subject. You also have a method of evaluating a subject in terms of brightness values to help you decide on your personal interpretation of the scene.

If you followed this discussion you now also understand the Ansel Adams zone system! Now all you need is lots of practice.