Canon Camera Hackers Manual

Contents

28 C H A P T E R 4 Teach Your Camera New Tricks4.3 ExposureMost compact cameras come with automatic exposure only. Usually theysupport a variety of automatic program modes such as portrait, landscape,sports, kids&pets, night shot, sunset, snow, etc. However, the smaller andlower-priced models usually do not feature aperture priority, exposuretime priority, or a fully manual mode. There are simply no provisions to dialin an aperture value or a shutter speed. If they offer a manual mode, it onlyallows dialing in an exposure correction value, and that’s it.The CHDK has set out to remedy this situation. It allows overriding thevalues determined by the camera’s exposure system with values dialed inby the user. This is possible for aperture (or neutral density filter), exposuretime, sensor speed, and flash power. In addition, the CHDK offers a highlycustomizable Auto ISO system and advanced features for exposure control,such as a live histogram and zebra areas. We will discuss those features inthe following sections.4.3.1 OverridesAs already mentioned, the CHDK allows overriding aperture, exposuretime, ISO speed, and flash power. This works in all camera modes, auto-matic and manual. So, for example, if the camera is in automatic mode andyou override the exposure time only, the camera will automatically choosethe right aperture; the camera will be in Shutter priority (Tv) exposuremode.But not all cameras can do this. Cameras equipped with an ND filterinstead of a diaphragm cannot adjust the aperture and completely revertto manual mode. In this case, you must adjust the ND filter state and theISO speed yourself to obtain a properly exposed image. The density of theND filter depends on the maximum aperture—the ND filter simulates asetting of f/8 at the focal length at maximum aperture. So, if the maximumaperture is f/2.8, the density of the ND filter will be three f-stops. With atelephoto setting where the aperture is perhaps only f/4.9, the aperturesimulated by the ND filter would be around f/14. With the CHDK in place,you can normally keep the ND filter out and use very short shutter speedswhen conditions become too bright. An ND filter makes sense if you needrather long shutter speeds for compositional reasons (waterfalls, architec-tural images, etc.).So, with diaphragm-less cameras your options for exposure control aresomewhat limited. You can’t just open or close the aperture to compensatefor variations in shutter-speed. Instead, you must adapt the sensor speed.For example, if the camera computes a shutter speed of 1/100 sec and youoverride it with 1/1000 sec, you have to make up for the 3 1/3 stops less