[Les Meehan]

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 cameras and light meters that many people are still unsure about achieving the desired exposure? Let’s 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 desired exposure for any subject is that which produces an image which captures the photographer’s feeling for the subject. Therefore, how you visualise (interpret) the subject will determine the exposure required. Two photographers recording the same subject at the same time may well use two different exposures (other things being the same) because 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 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 steps will be required to achieve the final result. Visualisation determines camera viewpoint, lens focal length, depth of field, lighting, film choice (only film speed for digital users) 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 we need to determine the existing subject brightness range (SBR). Determining the SBR of a scene allows us to know exactly how the scene will record on the film and whether any modifications are required. These might include using a filter to change local tonal values, altering development time to retain detail in the lightest areas or using pre-exposure to boost shadow detail. For digital users replace development suggestions with digital contrast control methods, i.e. two exposures, RAW conversion, curves adjustments.
The SBR is the difference in brightness 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 SBR are those in which you want to retain some detail or texture. Be careful not to bother with visually unimportant areas of the subject. How much detail you can record will be discussed later.

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 these readings 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.

It is essential for each photographer to calibrate their meter to take account of personal working methods (non-digital, for each film in use). 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 (non-digital, for a particular film) 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/meter/film tested.

Non-digital users, don’t always assume that because the film speed on the box is ISO 125 that this is the best speed to set on your meter! Also, for cameras that use DX coding the exposure compensation dial can be used to effectively change the film speed.

Contrast Control

Non-digital users - Film Development
Film development and exposure are intrinsically linked. Once you have visualised your subject and made the necessary exposure to obtain the image detail you need, it is essential that the film be given the necessary development. If the film receives too little development ALL the tones of the subject will be darker than you expected. Conversely, too much development will result in ALL the tones being lighter than you desired. However, since development affects heavily exposed parts of the film more than less exposed parts incorrect development also distorts the original SBR of the subject. In other words, too little development will make the image of the subject have less contrast than originally planned and too much development will make the image have more contrast. By the way, if you have your film developed at a lab do not assume they are doing it correctly, especially with monochrome film!

Digital users
For digital camera users, the problem of the SBR exceeding the tone range of the camera are just as significant as for film users. This is why it is often recommended that digital users think in terms of the way colour transparency film works. An essential step is to do a calibration test on your camera to find out the normal tone range of the camera. Do this as follows:

To test the contrast range of your camera make a series of exposures of a neutral (not coloured) textured surface in even light (an overcast day and a grey towel are a good combination, avoid brightly coloured things for this as your light meter is colour sensitive). Make the first exposure at the camera suggested exposure. Close down one stop (reduce exposure by one stop) and make the second exposure. Make four more exposures each time reducing the exposure by one stop. If your camera does not allow manual control use the ISO to obtain the changes (alternatively chuck it away and buy a proper camera). This sequence should produce a series of progressively darker images of the towel going into pure black.

Next, return to the camera suggested exposure (recheck this in case the light levels on the target have changed) and make a second series of exposures but this time increasing the exposure (making the towel lighter each time) by one stop for each shot. Make about five exposures, this sequence will make the towel lighter until it goes into pure white.

Put the images into your editing software WITHOUT ANY MESSING ABOUT! If shooting in RAW, don't do anything during the conversion as you want basic info. You want to see the 'pure' images. Zoom each to 100% and carefully examine the TEXTURE of the towel (or whatever textured subject you used). Due to uneveness of lighting/camera things just examine the centre portion of the images for consistency.

Find the darkest image that just shows the minimum texture. This is your lower limit of exposure for detail. Note down how many stops less this is from the original meter reading, i.e. 3 stops less, or whatever.

I have posted an example of the low and high tones test for my Fuji S2 Pro in the Gallery section. Also, a film test.

Examine the lighter images and again find the one that just retains a little detail. This is your upper limit for detail. Note the difference in stops from the original reading.

At the end of this simple test you will KNOW exactly how dark a subject can be allowed to go before it loses all detail and goes completely black. Also, you will know how light a subject can go before it turns completely white. Note down and remember how many stops there are from the darkest to the lightest textured image: this is your camera’s maximum tone range that it can record texture in.

OK, lets say your result show that the last detail in the dark images is the image that received three stops LESS exposure than the camera recommended. Also, the lightest image to retain minimal detail was the one that received two stops more exposure. You can safely say that the camera has a texture range of 5 stops (which is pretty usual for a DSLR), this is the difference in stops not the number of stops of exposure which would be 6.

So, you now have enough info to accurately expose any subject or scene by asking yourself two question: How dark or light do I want this subject and how much detail do I want to have in it? When I say 'subject' I am refering to a selected area of a scene from which you take a meter reading e.g. a shadow or a light area.

A Practical Method

So how do we actually get the result we want? As stated above, a calibrated light meter will try to produce a mid-grey tone in the image. 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. 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. What you need is a reliable, repeatable and accurate method so read on.

The first thing to do is decide how the subject will look in your final image (visualisation). 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 around 5 stops then you have a normal SBR subject and it should record well with your digital camera. 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 above test has given you the maximum 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 textureless white and then you get ‘white out’in the image. This has ‘placed’ your light tone on the tone scale at the correct position for your visualisation.

Working with the other end of the tone scale, a similar method is 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, i.e. 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 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.

With this knowledge of texture limits you can determine whether the areas you want detail in will actually retain that detail. If the difference in the lightest and darkest readings is 5 stops 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).

Non-digital users: For negative film it is usual to base the exposure on the reading from the darkest area (although with a 5 stop SBR you could use the lightest reading). For transparency film base the exposure on the reading from the lightest area because ‘washed out’ high values are less acceptable than black shadows on slides. If the SBR is less than 5 stops you may wish to consider increasing the film development to increase contrast (if this is what you want). Increasing the development of a film makes the lighter areas of the subject even lighter. Conversely, if the SBR is more than 5 stops you may need to consider reducing development. These development changes work for both monochrome and colour film.

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. If you wish, you can read more at my site index.