Canon Camera Hackers Manual

Contents

38 C H A P T E R 4 Teach Your Camera New Tricksdiaphragm usually offer a shutter priority automatic mode (also called Tvmode). On cameras without a Tv mode, you must set aperture and/or ISOmanually, or use the script presented in section 5.7.4 (see also section4.3.1).4.3.6 Night photographyIn the past, night photography has not been easy with digital cameras. Athigh ISO values, the noise in the image goes up. Shooting at low ISO valuesis not always possible—lack of a tripod may be just one reason. And at longexposure times, the camera’s sensor tends to produce artifacts: hot pixelsthat 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 asingle shot of your subject. Instead, shoot a whole series of images, eachexposed in the same way. Afterwards, these images can be overlaid andaveraged. This will cancel out some of the noise (see below), and the imagewill appear as if shot at a lower ISO speed. It is even possible to shoot sucha series hand-held. PC-based programs like PhotoAcute can automaticallyregister the images with each other and construct an almost noise-freecomposite image.But what if you want to visualize motion (cars on the highway, startracks, 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 nightphotography are extended in several ways:f Shooting RAW. RAW images can be treated with powerful PC-basednoise reduction programs that outperform the in-camera noise reduc-tion applied to JPEG files. These programs usually perform better withRAW sensor data. Your best option is the DNG format: here, bad pixels(as predetermined by the manufacturer) can be subtracted from theimage (section 4.5.2). In addition, RAW images can capture the highcontrast 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. Figure4-24 shows that it becomes possible to obtain low-grain images ofnight scenes even without a tripod when using this technique.f Merging several RAW images (section 4.5.6). This can be an option whenyou take a series of photos with your camera mounted on a tripod. Thisfeature cannot register the images with each other, but it saves post-processing on the PC.