5.7 Example scripts 159
5.7.2 Bracketing
We already discussed bracketing in section 4.6. While early CHDK versions
relied mainly on scripts for bracketing, the newer builds of CHDK provide
enough built-in support for most bracketing applications. We can therefore
keep this section short.
Where scripts can optimize the bracketing process is in the area of Focus
Stacking. The standard CHDK function for bracketing (in Extra Photo
Operations>Bracketing in Continuous Mode>Subj.Dist.Bracket Value (MF))
uses an increment (in mm) between the different shots of a series. This in-
crement increases in proportion to the current focusing distance. If you
specify 100 mm as a bracket value and start at a distance of 100 cm, the
camera will shoot a series of photos with 100cm, 110cm, 121cm, 133.1cm,
… This is not bad, but it would be better to increment the focusing distance
based on the actual Depth of Field ( DOF). This range increases with the focal
distance and not at all in linear proportions. In addition, the DOF depends
on the aperture and the focal length. A script can utilize the DOF and
produce an optimized series of photos.
The CHDK is able to compute the total DOF, the near limit, and the far
limit. These values can be shown on the display (section 4.2.8), but they are
also accessible in scripts. Unfortunately, we cannot use them in this script.
When the camera is in normal focusing mode, values for the near and far
limits that would fall into the macro range are returned as –1 (infinity). And
vice versa—when the camera is in macro mode, values outside the macro
range are returned as –1. However, we want a script that covers the full
range of distances from close-up to infinity. Fortunately, it is quite easy to
compute the far limit for each subject distance. Given the subject distance
f and the hyperfocal distance y, the far limit g is computed as:
g = y*f / (y-f)
provided that f is smaller than y. The hyperfocal distance is easily obtained
from the CHDK with:
y = get_hyp_dist
When starting at close range, we can therefore compute the far limit g for
each shot and use this value as the new subject distance for the next shot.
We stop as soon as we reach a specified end value or infinity.
Working this way, we get a series of shots with nicely overlapping
sharpness ranges while keeping the number of images at a minimum.
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