How-To: Perfect Exposure
Last update: 30 April, 2013
Setting exposure optimally is quite straightforward in the digital era, and this article will help you nail it every time.
Part 1: The Histogram
A finished image (displayed on a computer screen or printed on paper) has just one purpose — to appeal visually to the viewer. It can appear quite contrasty or flat and dreamy; look very light or rather dark; have a rich tonality or a limited palette with large areas of pure black or paper-white. To each of these looks correspond totally different histograms, and since there is no right or wrong look, there is no right or wrong histogram for a finished image.
There is however a perfect histogram for a RAW image straight out of the camera:
How do you make a histogram perfect?
- If the histogram is cut off at the left, some areas of your image contain completely black pixels. Since black pixels contain no texture or details, you have a black void. To solve the problem increase your exposure.
- If the histogram is cut off at the right, some texture and details are gone and instead you have areas of complete whiteness. Combat this by reducing exposure.
- If the histogram is cut off at both extremes, the scene contains more contrast than the camera’s sensor can capture, so you have to compromise. Say you are shooting a dark mountain in front of bright clouds. The dark mountain will contain areas of complete blackness and the clouds will have some areas comprised of pure whiteness. You now have several possibilities, none of which are optimal:
- If you want to stress the texture of the clouds, decrease your exposure until the histogram is no longer cut off at the right. This will render the mountain even darker.
- If you want to show the pattern of the dark mountain, increase the exposure until the histogram is no longer cut off at the left. Unfortunately even larger areas of the clouds will turn completely white.
- If you are more interested in a compromise, leave the exposure as it is.
- As a last resort shoot multiple images with different exposures and combine them later in an HDR composite.
- Followers of the ETTR (expose-to-the-right) idea tell us that if there is some “space” to the right of the histogram, we should increase exposure until the histogram just starts touching the right extreme. This is motivated by the fact that RAW files carry more information in the brighter tones, so you are capturing the scene within the most sensitive range of your sensor’s pixels. Note that the image will appear overly bright and will need to be darkened in the post-processing phase. ETTR is only relevant for those shooting RAW, so JPG shooters should not ETTR!
Here is an example histogram: a pretty good one, as good as was possible in the given situation, but because it’s cut off on the left, it’s not perfect.
However notice how the histogram extends nicely to the right and fills up the entire space without being cut off. Like I wrote above, the scene had too much contrast and my camera was not able to capture it all. So I deliberately chose to capture all the highlights and since it was not otherwise possible, had to accept some small areas of the image that were completely black.
Here is what the RAW image looked like straight out of the camera. Not that pretty, is it?
But with such a good histogram this image offers maximum freedom in the processing phase, so with a few tweaks in Lightroom I was able to get this:
Part 2: Controlling Exposure
There are the three controls on your camera that affect the brightness of an image — lens aperture, shutter speed, and ISO value — and “Exposure” refers to the values chosen for these variables (for example “f/2.8 at 1/250″ and ISO 400”). Which of the three variables you set yourself or let the camera set depends on the shooting mode (P, A, S, M).
Several observations can be made at this point:
- Changing any of the exposure parameters affects the brightness of everything in the frame. This is the reason why it was impossible to directly shoot a frame that looks like the post-processed image above. If I’d given more exposure, the mountain would have looked lighter, but I’d have large areas of the sky that are pure white, so I would not be able to recover any of the cloud texture. There is simply no dial on the camera which can make the mountain lighter (or darker) but not the clouds.
- Making the aperture one stop “brighter” can be counteracted by making the shutter speed or the ISO value one stop “darker.” Or by making both one half stop “darker.” Thus there are many different exposure settings that result in identical image brightness and therefore in identical-looking histograms.
The three exposure controls affect not only the brightness of your image but also its visual content. Aperture affects how much of your image is in acceptable focus, shutter speed intensifies or freezes motion and the ISO value affects the noise or smoothness of the image.
This leads us to one more observation:
- Identical brightness does not mean identical look. Shooting a scene at f/2.8, 1/250″ and ISO 400 and f/5.6, 1/30″ and ISO 200 will result in identical brightness and identical histograms, but the second image will have more depth-of-field (DOF), moving objects will be more blurred and there will be slightly less image noise.
And now the final conclusion:
Part 3: Set up your Camera Properly
You might have to dig into the menus in order to achieve the following:
- For an SLR, make sure that when you shoot an image, the image gets displayed immediately afterwards along with a histogram and blinking highlights. For a mirrorless camera make the histogram and/or blinking highlights visible in the viewfinder (before taking the image).
- Make exposure compensation as easy as possible. On some cameras you have to press a button then turn a dial, on others you simply turn one of the two main dials.
- Turn Auto ISO off since it does not work as well as it should on most cameras. This is a shame, because the ideal auto ISO behavior is easy to describe and program, yet only Pentax and some of the newest Nikons do it well. Once you’ve mastered the manual ISO process below, turn Auto ISO on and see if it behaves like you want it to.
Part 4: Aperture Priority and Exposure Compensation
Most images do not benefit from motion blur, so the only creative choice is depth of field. In order to control this optimally switch the camera to aperture priority (denoted by A or Av on most cameras). On higher-end cameras you have a dial dedicated to changing the aperture. On budget models you have to press some button then turn a dial (check your manual, the button is usually labelled “Av +/-” or something similar). Note that on an SLR type camera you have to switch to live view or push the DOF preview button to see the effect of changing the aperture, while on a mirrorless body this happens fully automatically.
In aperture priority you set the aperture manually and the camera automatically chooses a shutter speed that results in a properly exposed image. This works well most of the time and when it doesn’t, you can use the exposure compensation function. When you move the exposure towards +, the shutter speed gets longer, image gets lighter and the histogram moves towards the right. Setting a negative exposure makes the shutter speed shorter, the image darker and the histogram moves to the left.
Part 5: Shutter Speed and Image Sharpness
In the film days there was a rule of thumb that in order to get sharp images your shutter speed should not be longer than the reciprocal of the focal length. For example, if you are using a 50 mm lens, your shutter speeds should be 1/50″ or shorter (remember: shorter means 1/60″, 1/125″, 1/250″ and so on). Let’s call this magical shutter speed Tmax.
In the digital age, due to the fineness of the digital pixels, Tmax changed to “double the focal length.” In other words for a 50 mm lens use shutter speeds of 1/100″ or shorter.
But then image stabilization came along and allowed us to shorten Tmax by 2-5 stops. With my Olympus E-M5 I can shoot a 50 mm lens at 1/20″ and get consistently sharp images, so my personal rule of thumb is “the reciprocal of half of the focal length”. (Of course IS only counteracts the movement of the photographer and the camera, not the subject, so on the rare occasions when you have a quickly moving subject, you will need to set a shutter speed shorter than Tmax.)
By the way, the reason why most implementations of Auto ISO fail is because they do not take the focal length into account when calculating Tmax. Very annoyingly, the E-M5’s Auto ISO does that, but it “forgets” about IS and defaults to “double the focal length” shutter speeds instead of “half the focal length”, so it sets the ISO value two stops higher than necessary, which makes it unusable for me.
Part 6: The Process for Setting Optimal Exposure
For the overwhelming majority of images we want no motion blur and as little noise as possible, while the amount of DOF depends heavily on the subject. So we need a process which sets our desired aperture, a shutter speed which is short enough to eliminate motion blur and the smallest possible ISO value for a minimum of digital noise. And at the same time produces a perfect histogram…
- The camera is in aperture priority, so set an aperture that gives you the desired depth of field (DOF). A small number (like f/2.8) gives you a shallow DOF, a larger number (like f/11) leads to pretty much anything being in acceptable focus.
- Look at the histogram. If it’s not optimal, use the exposure compensation dial to make it optimal. You are aiming for a histogram which is not cut off or squeezed towards one end or the other. If the scene contains too much contrast and the histogram does not fit entirely, make sure that it’s cut off at the end which visually less important.
- Look at the shutter speed chosen by the camera. If it’s shorter than Tmax, lower the ISO value for less image noise. If it’s longer than Tmax, increase the ISO value.
You can skip step 3 if your Auto ISO takes the lens focal length and whether IS is on or off into account. Also, if the light is very low, you’ll have to set a very high ISO value, which will result in a noisy image. Try this instead:
- Choose a wider aperture and live with the reduced DOF, or
- Use a tripod, reduce the ISO value to the minimum and don’t worry about the shutter speed exceeding Tmax.