Adobe Photoshop CS5 for Photographers - Martin Evening [177]
Therefore, when capturing a high dynamic range image the objective is to make sure you capture the entire contrast range in a scene from dark to light. You can do this by taking spot meter readings and manually working out the best exposure bracketing sequence to use, plus how many brackets are required. An alternative (and simpler) approach is to use a standard method of shooting in which you first measure the best average exposure (as you would for a single exposure) and bracket either side of that using either 3, 5 or 7 bracketed exposures at 2 EV apart. This may not be so precise a method, but a 5 bracketed sequence should at the very least double the dynamic range of your camera.
There are several benefits to capturing a high dynamic range. First of all, you can potentially capture all the light that was in the original scene and edit the recorded information any way you like. Secondly, providing you manage to capture all the individual brackets without any subject movement, a merged HDR image should contain smoother tonal information in the shadow regions. This is because more levels are captured at the bright end of the levels histogram (see ‘Digital exposure’ on page 186). Because of this the overexposed brackets will have more levels with which to record the shadow detail. When you successfully capture and create an HDR image, there should be little or no noise in the shadows and you should have a lot more headroom to edit the shadow tones without the risk of banding or lack of fine detail that is often a problem with normally exposed digital photos.
HDR shooting tips
The first thing you want to do is to set up your camera so that it can shoot auto bracketed exposures. This can usually be done via the camera controls. Some cameras only allow you to shoot just three bracketed exposures, others more. With the Canon EOS range you should find that by tethering your camera to the computer you can use the Canon camera utilities software to set the default to five or more exposure brackets. The bracketing should be done based on varying the exposure time. This is because the aperture must always remain fixed so that you don't vary the focus between captures. Next, you want the camera to be kept still between exposures. It is possible to achieve this by shooting the pictures with a hand-held camera and keep as still as possible, but for best results you should use a sturdy tripod with a cable release. Even then you may have the problem of mirror shake to deal with. This is where the flipping up of the mirror on an SLR camera can set off a tiny vibration which can cause a small amount of image movement during the exposure. However, this is mostly only noticeable if using a long focal length lens. When shooting on a tripod this can be a problem, but if you shoot hand-held, the vibrations are usually dampened by your hands holding the camera. So apart from using a cable release, do enable the mirror up settings on your camera if you can. As a Canon user it has been frustrating going through the custom function menu options to set the camera to mirror up mode, but setting the mirror lock up has been made easier with the latest EOS 1Ds MkIII camera.
The ideal exposure bracket range will vary, but an exposure bracket of five exposures of 2 EV apart should be enough to successfully capture most scenes. You can use just three exposures that are 2 EV apart and get good results, but you won't be recording as wide a dynamic range. As you shoot a bracketed sequence watch out for any movement between exposures such as people moving through the frame, cars or where the wind may be causing movement.