This article was originally written about five years ago but was never published on my web site, so I have slightly updated it to publish now. It is not a complete guide to exposure, which is a big subject, so I have included some links to simple and clear articles on various aspects of exposure and controlling depth of field on the excellent Cambridge in Colour, Luminous Landscape and other web sites.
What is exposure?
To beginners, exposure may be just a word, a strange concept that they haven’t come across in the context of their new hobby, so here is a simple definition (based on digital capture):
“Exposure is the amount of light collected by the sensor in your camera during the capture of an image.”
The more exposure, the brighter the image, less exposure, the darker the image. Simples! Once you have exposed your image, you can view it, save it, edit it, project or print it.
The Three Variables
There are essentially three variables which make up the exposure:
- Aperture – given as an f/ number – is the size of the opening in the camera’s shutter which determines how much light can pass through to the sensor. The bigger the f/number, the smaller the aperture and the less light passes through the aperture.
- Shutter Speed – in seconds or fractions of a second – is the time the shutter is open to allow passage of light.
- ISO setting – the sensitivity of the image sensor – the bigger the number the higher the sensitivity setting.
Exposure is controlled by a combination of settings of the three variables. The Cambridge in Colour web site describes exposure as being like capturing rain falling into a bucket and it is worth getting to grips with the basics by looking at their clear and simple tutorial. In my view, achieving the best print or projected image is dependent on achieving a good exposure in the first place.
If you look at a book on traditional film photography, you may see the advice that exposure is controlled by two variables, the shutter speed and the chosen aperture. However, a third variable, sensitivity (ISO film speed) was dialed into hand-held exposure meters to calculate the final exposure. As film cameras became more sophisticated, the film speed was automatically read from the film into the camera and added into the automated exposure calculation along with shutter speed and aperture.
Now that most of us have a digital camera, the ISO, or sensitivity of the sensor, is taken into account whether manually or automatically set, along with shutter speed and aperture, for every shot we take.
Reciprocity – same exposure, different variables
In order to maintain the same exposure with a given sensitivity, as the aperture is opened up to allow more light to pass through, the shutter speed must be reduced to maintain the same exposure. Varying the ISO (sensitivity) setting, will have a knock-on effect on the other two variables. Maintaining the same exposure by adjusting the combination of aperture and shutter speed at a given ISO is called reciprocity. So, 1/125th sec at f/8, will give the same exposure to the film or digital sensor as 1/250th sec at f/5.6, which is the same as 1/500th sec at f/4. Increase or decrease the ISO setting and the shutter/aperture settings will have to be compensated accordingly. The great thing about automatic cameras is that all the calculations are done for you!
The choice of aperture/shutter speed/ISO combination is what allows us to develop different styles of working, from “sharp from here to infinity” shots to arty pictures with movement or shallow depth of field.
For more on this see Depth of field tutorial at Cambridge in Colour
The camera’s metering system
The camera’s automated process is facilitated by the in-camera metering system which measures the light falling on the sensor. The camera’s software will instantly calculate a programmed set of values of all three parameters, if you wish, and beginners’ cameras are peppered with many different “scene modes”. Set a landscape mode and the camera will compute and automatically set the “best” combination of aperture and shutter speed (and sometimes also ISO) that you need for such a scene, based on the algorithms that have been worked out by the manufacturer, in this example to give the best depth of field possible whilst rendering a “correct” exposure.
However, many beginners seem to want to set everything manually and I must admit I wonder why. Modern cameras are so good at getting it right most of the time and you only need a slight change in conditions to occur, e.g. the sun going in or pointing the camera in a different direction and the exposure can change dramatically, so why not let the camera work exposure out for you with a programmed exposure? or, take more control by choosing one of the “priority modes”…
e.g. set the ISO you want (to control sensitivity but be aware the higher the ISO setting the more noisy or granular your picture becomes.) Then set…
Aperture priority – set your aperture to control depth of field and let the camera set the shutter speed
or…Shutter priority – set the shutter speed to allow to freeze movement or show blur for example and the camera will set the aperture
In-camera meters have a fundamental difference from traditional hand-held meters. Hand held meters have the facility to measure both reflected light, i.e. the light bouncing back from the subject and also incident light, which is the amount of light falling on the subject. Because incident light metering will measure the actual amount of light falling on the subject, it will output the same exposure information for any subject, be it white, black or multicoloured or multi-toned. Therefore dark items will be rendered dark and light items will be rendered correspondingly light on the captured image. So, incident light metering has always been a “safer” option for metering difficult subjects like figures against a bright sky, where the in-camera reflected light meter might, for example, choose to render a sky as “mid-grey” tone and a figure against the sky as “shadow”.
The “correct” exposure, the “Grey Card” and the fluff in my tumble dryer
Knowing your camera’s characteristics becomes crucial. I know that I can trust my camera to expose most average scenes acceptably, but I also know that most of these automatic exposures will give about 1/3 stop less than I prefer. (A stop is a halving or doubling of exposure). Experience tells me when I need to over-ride the metered exposure and use manual or adjusted auto settings instead. In fact I rarely use manual and generally choose the exposure compensation setting to achieve my preferred exposure.
In-camera meters measure only the light that is reflected from the surfaces of the subject(s), so the in-camera computer can then only “guess” at how much light is actually falling on the subject, because it doesn’t know what the subject is. Well, it’s more than just a wild guess, it’s a calculation based on the calibration of the meter, which is set to assume a scene is “mid-grey”.
In an “average” scene, there will be a variety of different colours and tones, from light to dark, and if you were to mix them all together, you would end up with roughly mid-grey. Don’t believe me? Then look at the colour of the fluff coming out of the filter in your tumble dryer. If like me, your fluff is only collected and disposed of “every now and then”, by the time that you do empty the filter, you will have tumble dried a variety of dark and light clothes, sheets, towels etc, whites and blacks and coloured items. Believe me, the fluff is always grey! You have tumble dried an average scene! (hint to myself to clean the filter more often).
The camera meter is calibrated to assume, in the same way, that the reflected light it is measuring comes from an average scene measuring (integrating or mixing down to) mid-grey. Mid-grey is a neutral grey, approximately half way between the brightest white paper and the darkest black that can be printed on paper. It was the basis of the Kodak Grey Card that can be purchased as an exposure and white balance aid. Kodak developed the grey card in co-operation with Ansel Adams. Adams chose 18% grey reflectance as being, he believed, equivalent to Zone 5 in his Zone System for exposure. Kodak, it is said, would have preferred 14% reflectance for their grey card, but eventually they agreed to 18%. However, when Kodak first issued the grey card, it was issued with instructions to add 1/3 stop of exposure more than the grey card measurement indicated! Confusing, or what?
These days, each camera manufacturer chooses it’s own value for mid-grey. Various sources on the internet indicate that the variation is from 12-15% reflectance but generally not the 18% traditionally cited. So if you meter from a Kodak 18% grey card, your average scene will tend to be a little underexposed if your camera manufacturer has calibrated to 14% for example. You may think exposure doesn’t matter and can be fixed afterwards, but we all want to achieve the very best results from our efforts, so why not exposure correctly in the first place?
Our problem, as photographers, is that not all scenes are average. If you point your camera at a subject that does not have a mix that integrates to mid grey tones, your meter will still assume a scene that is “integrated to grey” and so your camera will overexpose dark toned subjects or underexpose light toned subjects.
Most cameras offer a number of different metering options to help us to choose the best option for the subject in hand and to achieve the best possible exposure. They may be called such as “centre spot”, “centre-weighted average”, “evaluative” or “partial” for example. Your camera instruction book will explain what these are and how much importance is given to the various zones or areas within the frame. Choose the best for the job. Centre spot metering will give better exposure of a small subject against a bright background; a landscape with bright sky might be better with an evaluative metering mode. The best way to learn is to try them and see which give you the best results in a given situation. Learning by experience is invaluable.
The histogram and exposing right
Histograms are not right or wrong, good or bad. They are just information. The histogram shows how the tones of your image are distributed from the deepest shadows to the brightest highlights. It can tell you if your image has lost shadow and highlight detail. IMO, it is just as important to look at the histogram as it is to look at the preview! Now is the time to take another shot if you think you haven’t got it right rather than trying to fix it later. If you are unsure what the best exposure will be, take a variety of shots at different exposures (bracket them) and learn that way. Look at the histograms for each shot and see which gives you the best final result.
In real-life as opposed to theoretical photography, there is not an “absolute” correct exposure, but there is a range of exposures that will achieve an acceptable end result in terms of sharpness, dynamic range and noise (or lack of). Everything we do in making an exposure is a compromise, a trade-off between sharpness and smoothness and depth of field, being the best that we can do in a given set of conditions, which are rarely ideal!
But the histogram is still one of the most useful (vital) tools in checking your exposure at the time of taking the shot. In most cameras it can be programmed to overlay the image preview on the LCD display. You’ll also see the histogram in your photo-editing programme where it can help you to get the best out of your editing and warn where you are losing detail.
The camera histogram may be divided into zones, 4 or 5 typically, dividing off your shadows, dark, medium and light tones. Cameras often capture images in 12 or 14 bit. A 12 bit image can record 4,096 different tones. Your histogram may show 5 zones, approximately equivalent to the 5 stop dynamic range of the sensor. The left hand section, the shadows, contain only 128 levels out of 4,096. The rightmost section, the highlights contain 2,048 tones out of your 4,096. In other words, half of the information that you capture is in the rightmost section of your histogram. So don’t waste it. Bias your exposures to include the topmost values in the histogram. Go as far as you can “to the right” by increasing exposure as much as you can without losing highlight detail. The image may look too bright when you import it into the computer but it’s a simple matter to adjust the tones in the RAW converter. Work in 16 bits in the computer and you will keep more of the captured information in the image and will end up with a higher quality image to project or make your print.