Canon Camera Hackers Manual

Contents

36 C H A P T E R 4 Teach Your Camera New Tricksf The sensitivity of the Zebra feature can be controlled by UnderExposureThreshold and OverExposure Threshold. The range goes from 0 (low sen-sitivity) to 32 (high sensitivity).f The next two options only apply to the Zebra blink modes. When aZebra pattern is drawn, both the Canon indicators and the CHDK indica-tors (for example, the DOF calculator) are erased by the pattern. To re-store these information items during the blink pauses, set the optionsRestore Original Screen (for the Canon items) and Restore OSD (for theCHDK items).f The option Draw Over Zebra applies to all Zebra modes. When this optionis set to value OSD, all OSD information items will be drawn on top ofthe Zebra pattern and thus remain visible. When set to Histo, only thehistogram is drawn on top of the Zebra pattern. When set to Nothing,the Zebra pattern will be drawn on top of all items. With a blinking Zebrapattern, I would rather set this parameter to Nothing and enable theprevious two options (Restore original screen and Restore OSD). This way,both the Zebra pattern and the indicators are completely visible.f Finally, there is a special Zebra mode that can be switched on by RGBzebra. This mode displays overexposed areas in different colors depend-ing on which color channel is overexposed. For example, if the bluechannel is overexposed, the Zebra pattern will be blue; if both the redand green channels are overexposed, the Zebra pattern will be yellow(red+green), and if all channels are overexposed, the Zebra pattern willbe black. The drawback of this mode is that it cannot show under-exposed areas.4.3.5 High-speed photographyOne amazing feature of the CHDK is the provision for ultra-short shutterspeeds. Before we go into the details, let’s take a look at general shuttertechnology.Traditional cameras from the analog era use mainly two types of shut-ters. One is the focal plane shutter, which is also used in DSLRs. Here, twocurtains move across the focal plane, and the light passes through a slitbetween the two curtains. The width of the slit determines the exposuretime. By choosing a very narrow slit, a focal plane shutter can offer fastshutter speeds down to 1/8000 sec. The downside of this technique is thatthe slit needs time to travel across the focal plane. Fast-moving objects oftenappear stretched or contracted depending on the direction of the object’smovement in relation to the slit’s movement. Another problem with thistype of shutter is flash photography. The entire sensor area must be uncov-ered by the curtains while the flash is emitting light in order to avoid unevenillumination. So shutter speeds cannot be very high when flash is used.