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| The exposure was reduced in this image during processing to add drama |
Photography, whether analogue or digital, is all about exposure. To record a successful image you need to get a precisely controlled amount of light to the film or sensor inside your camera. How much light depends on the subject, the situation and whether or not you’re trying to create a specific effect. A ‘correct’ exposure is subjective, as intentionally giving an image too much or too little exposure may be necessary in one situation but not in another.
It is important to understand how exposure works so you can control it rather than letting it control you. This applies as much today as it ever did, although in this digital age many photographers see exposure as something that can always be fixed later if they get it wrong. To some extent it can, just as a decent black & white print can usually beteased from a negative that has been under- or overexposed. However, getting it right in-camera is always preferable if optimum image quality is our goal, so you should never leave such a crucial factor as exposure to chance.
The traditional approach to exposing black & white negative film was to expose for the shadows – give the image enough exposure so that detail was recorded in the shadow areas. Some photographers took things a step further and followed the Zone System, made famous by legendary photographer Ansel Adams. This involved choosing a key element of the image, deciding which ‘zone’ you wanted it to fall into (in a scale of 0 to 10, with 0 being pure black and 10 pure white), then setting the exposure accordingly after taking a refl ected meter reading from the relevant part of the scene or subject.
Today, armed with a DSLR, getting the exposure ‘correct’ is far quicker and easier. This is because as well as having a sophisticated integral metering system that’s able to provide accurate readings in almost any situation, you also have a preview screen on the back of the camera that shows you exactly what type of image you’ll end up with if you use that exposure. In addition to the preview image, you have the histogram, which is something that’s even more useful when it comes to assessing exposure.
UNDERSTANDING HISTOGRAMS
The histogram is a graph that shows you the distribution of tones in a digital image, from the darkest shadows (on the far left of the histogram) to the brightest highlights (on the far right). You can think of it as a digital Zone System if you like – with extras!
By assessing the shape and distribution of the histogram, you can quickly gauge the exposure. The images on the preview screen of your camera, on the other hand, are affected by the brightness of your surroundings so they may look lighter and darker than they really are, which makes it difficult to assess whether you have got the exposure right and is more likely to result in exposure error.
I have had photographers on workshops ask why their images are still coming out too dark even when they’ve dialled in +2 stops of exposure compensation. Usually the answer is that the camera’s screen brightness is set to ‘auto’ so it adjusts to suit ambient light levels, often making the preview image much lighter or darker than the actual image recorded on the camera’s sensor. Had they checked the histogram instead, they would have been able to see that the exposure was acceptable even if the preview image wasn’t.
So what is an acceptable exposure? The basic rule with digital photography is to make sure the tones in an image fall within the extremes of the histogram. If they’re pushed over to the left side, this usually indicates underexposure (or an image that comprises mainly dark tones), and if the tone graph actually collides with the far left of the histogram that tells you the shadows have been ‘clipped’. In real terms, this means the darker shadow areas will come out black with no details to recover.
At the other extreme, if the tone graph is pushed over to the right side of the histogram, this is usually a sign of overexposure (or an image that comprises mainly light tones, such as a snow scene). If the tone graph collides with the far right of the histogram, that tells you the highlights have been ‘clipped’ and the brighter highlights will record as white with no detail or tone to recover.
If you make sure that neither the shadows nor the highlights are clipped, you will have an acceptable image to work with. However, in order to achieve optimum image quality you can use the histogram in a more controlled way.
EXPOSURE USING THE HISTOGRAM
‘Exposing to the right’ is the name of the technique and it involves giving as much exposure as you can to an image without ‘clipping’ the highlights. This is similar to using the Zone System to determine the correct exposure for a black & white negative. To use ‘exposing to the right’, you must shoot in raw mode not JPEG, so the images are 12 or 14-bit rather than 8-bit, and are uncompressed so all the data recorded by the sensor is present in the raw files.
The technique is based on the fact that the tonal values recorded by your camera’s sensor aren’t evenly distributed from the shadows through to the highlights, but are heavily biased towards the right side of the histogram – towards the highlights.
Let’s use the following as an example. The sensor in a digital SLR can record a set number of stops in brightness. We’ll assume it’s 5, for the sake of simplicity. If you look at the histogram for an image on your camera’s preview screen, you may find that it’s divided into five sections of equal width from left to right. If not, imagine it is. Each of those sections represents 1 stop of brightness. However, instead of the tonal values that your camera’s sensor can record being divided equally among those 5 stops, 50% of them are recorded in the brightest stop, half as many in the next stop, half as many again in the next stop, and so on.
Most DSLRs record raw images in 12-bit, and a 12-bit image is capable of recording 4,096 tonal values. These are distributed across the histogram as listed below:
First stop . . . . . . . . . . . . . . . . . . . . . . . . . .2,048 tonal values
Second stop . . . . . . . . . . . . . . . . . . . . . . . .1,024 tonal values
Third stop . . . . . . . . . . . . . . . . . . . . . . . . . . 512 tonal values
Fourth stop . . . . . . . . . . . . . . . . . . . . . . . . . 256 tonal values
Fifth stop . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 tonal values
By ‘exposing to the right’ so the histogram extends into the brightest 20% (the first ‘stop’) without clipping the highlights, you’re maximising the number of tonal values in the image so noise and posterisation in the shadow areas are reduced or eliminated. When you look at your camera’s preview screen and when you open the raw file on your computer, the image will usually look overexposed and washed out, but you can adjust the exposure and contrast using the relevant sliders in your raw converter to fix this.
Underexposing is the worst thing for a digital image, as the histogram will be biased to the left side where there are far fewer tonal values. The result will be increased noise when you then lighten the image to correct the exposure.
USING EXPOSURE COMPENSATION
Implementing this technique needn’t involve a drastic change in the way you shoot. Just compose the shot as normal, fit any filters to your lens that you intend to use, then take a shot and check the histogram. If the tonal graph is centred, as it would be for a ‘normal’ image, or biased to the left, dial in +1/3 stop using your camera’s exposure compensation facility, take another shot and check the histogram again. You’ll see that the histogram has shifted to the right. If necessary, dial in +2/3 of exposure compensation and shoot again. Repeat until the highlight warning starts to flash on the preview image (which you can enable/disable in your camera’s menu), telling you the highlights have been clipped. Once that happens, you’ve taken the exposure a little too far.
This may seem complicated, but just like exposing for the shadows with black & white film, or using the Zone System, it’s necessary if you want to achieve optimum image quality – and once you get used to ‘exposing to the right’ you’ll be surprised how quick and easy it is.
KEEP EXPOSURE SIMPLE
Before switching to digital capture, I used a handheld spotmeter and would give myself a headache at times by metering from the highlights, metering from the shadows and trying to work out the optimum exposure. Then the light would change and I’d have to do it all over again!
Thanks to the instant feedback you get from a digital camera – primarily in the form of the histogram
– there’s no longer a need to employ complicated metering techniques to establish ‘correct’ exposure
because you know when you’ve got it right or wrong and you can make changes on the spot.
These days, I rely entirely on the integral metering system of my Canon EOS-1Ds Mark III, which is almost always set to evaluative metering and aperture priority. If I need to adjust the exposure, I just use exposure compensation. It’s a combination that never fails.
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