Canon Camera, Руководство хакера

Страница: 47 / 257

38
C H A P T E R 4 Teach Your Camera New Tricks
diaphragm usually offer a shutter priority automatic mode (also called Tv
mode). On cameras without a Tv mode, you must set aperture and/or ISO
manually, or use the script presented in section 5.7.4 (see also section
4.3.1).
4.3.6 Night photography
In the past, night photography has not been easy with digital cameras. At
high ISO values, the noise in the image goes up. Shooting at low ISO values
is not always possible—lack of a tripod may be just one reason. And at long
exposure times, the camera’s sensor tends to produce artifacts: hot pixels
that show up as tiny bright dots. Hot pixels are the main reason why man-
ufacturers limit the maximum exposure time.
However, inventive photographers have found a way around this limita-
tion. The basic idea is: shoot at a higher ISO speed, but don’t just take a
single shot of your subject. Instead, shoot a whole series of images, each
exposed in the same way. Afterwards, these images can be overlaid and
averaged. This will cancel out some of the noise (see below), and the image
will appear as if shot at a lower ISO speed. It is even possible to shoot such
a series hand-held. PC-based programs like
PhotoAcute can automatically
register the images with each other and construct an almost noise-free
composite image.
But what if you want to visualize motion (cars on the highway, star
tracks, etc.)? In these cases you need long to very long exposure times.
Without the CHDK, you are lost. But with the CHDK, your options for night
photography are extended in several ways:
f
Shooting RAW . RAW images can be treated with powerful PC-based
noise reduction programs that outperform the in-camera noise reduc-
tion applied to JPEG files. These programs usually perform better with
RAW sensor data. Your best option is the
DNG format: here, bad pixels
(as predetermined by the manufacturer) can be subtracted from the
image (section 4.5.2). In addition, RAW images can capture the high
contrast typical of night exposures better than JPEG images.
f Higher ISO values (section 4.3.1). This results in more noise, of course;
but you can take several hand-held shots and average them later. Figure
4-24 shows that it becomes possible to obtain low-grain images of
night scenes even without a tripod when using this technique.
f
Merging several RAW images (section 4.5.6). This can be an option when
you take a series of photos with your camera mounted on a tripod. This
feature cannot register the images with each other, but it saves post-
processing on the PC.