Figure 4-38 The Bracketing submenu is reached via ALT >MENU >Extra Photo Operations > Bracketing in Continuous Mode. Here we see a typical configuration for shooting HDR images.
Setting up the camera for an unlimited series of bracketing shots is very easy under the CHDK. Just go to the Bracketing submenu and set the values for the entries found there:
- TV Bracketing Value. This entry is used for modifying the exposure time during a series. A value of 2 results in an exposure time multiplication factor of 22 = 4. So, if you start with 1/100 sec, you will get a shutter speed series of 1/100, 1/25, 1/400, 1/6, 1/1600, and so on. This type of bracketing is typically used for HDR work (section 4.6.2). It can also be used for getting the perfect exposure when AV bracketing is not available.
- AV Bracketing Value. This entry allows you to modify the aperture for the individual shots in a series. For example, if you set the value to one f-stop and the aperture to 5.6, you will end up with a series shot at apertures 5.6, 4.0, 8.0, 2.8, 11, etc. AV bracketing is not suited for HDR work because the depth of field changes with each aperture change; but it’s a good option for getting a perfectly exposed single image. This menu entry is only available on cameras with a diaphragm.
- Subj.Dist.Bracket Value. This entry is used for focus stacking, which is discussed in the next section.
- ISO Bracketing Value. This entry allows you to modify the ISO value. The subentry Value Factor specifies a factor that is multiplied with the ISO Bracketing Value—just to save you a few hundred keystrokes. The result is added and subtracted from the original ISO value. For example, if you start with an ISO value of 100, an ISO Bracketing Value of 2, and a Value Factor of 10, you will end up with a series of 100, 120, 80, 140, 60, etc. ISO bracketing is well suited for classical exposure bracketing when you need the perfectly exposed single image, especially if AV bracketing is not available. It is not as well suited for HDR work (see above) because the signal-to-noise ratio changes with each ISO setting.
- Bracketing Type. There are three bracketing types. Choosing “+/-” will result in a series with alternating over- and underexposed images, typically used for finding the perfect exposure and for HDR work. The option “-” will result in a series of increasingly underexposed images. Finally, the option “+” will result in a series of increasingly overexposed images. The last two options are often used for focus stacking.
- Clear Bracket Value on Start. If this option is enabled, the bracketing values will be reset to default when the camera is powered up.
- Add RAW Suffix. When this option is enabled and you shoot RAW images, a suffix is added to the names of the resulting RAW files, for example, IMG_2041_01.DNG, IMG_2041_02.DNG, IMG_2041_03.DNG. This allows you to easily identify the bracketing series later. However, in the CHDK file browser of DryOS cameras (section 2.2), the files will show up as IMG_20~1.DNG, IMG_20~2.DNG, IMG_20~3.DNG, because the file browser is limited to 8.3 (At most, eight characters in front of the dot, three behind it) filenames on this platform.
To enable bracketing, you must also set Disable Overrides in the submenu Extra Photo Operations to Off (section 4.3.1) if you enabled the option Include AutolSO & Bracketing in the same submenu. Then switch the camera to Continuous mode. On cameras that support multiple shots taken with the Custom Timer, Custom Timer is another possible option. This would allow you to predefine the number of shots to be taken.
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