Another way of adjusting exposures is by changing the ISO sensitivity setting. Sometimes photographers forget about this option, because the common practice is to set the ISO once for a particular shooting session (say, at ISO 100 or 200 for bright sunlight outdoors, or ISO 800 when shooting indoors) and then forget about ISO. The reason for that is that ISOs higher than ISO 100 or 200 are seen as "bad" or "necessary evils." However, changing the ISO is a valid way of adjusting exposure settings, particularly with the Canon EOS 40D, which produces good results at ISO settings that create grainy, unusable pictures with some other camera models.
Indeed, I find myself using ISO adjustment as a convenient alternate way of adding or subtracting EV when shooting in Manual mode, and as a quick way of choosing equivalent exposures when in Automatic or Semiautomatic modes. For example, I've selected a manual exposure with both f/stop and shutter speed suitable for my image using, say, ISO 200. I can change the exposure in 1/3 stop increments by tapping the ISO-Flash Exposure Compensation button and spinning the Main Dial one click at a time. The difference in image quality/noise at the base setting of ISO 200 is negligible if I dial in ISO 160 or 125 to reduce exposure a little, or change to ISO 250 or 320 to increase exposure. I keep my preferred f/stop and shutter speed, but still adjust the exposure.
Or, perhaps, I am using Tv mode and the metered exposure at ISO 200 is 1/500th second at f/11. If I decide on the spur of the moment I'd rather use 1/500th second at f/8, I can tap the ISO-Flash Exposure Compensation button and spin the Main Dial three clicks counterclockwise to switch to ISO 100. Of course, it's a good idea to monitor your ISO changes, so you don't end up at ISO 1,600 (or higher, if ISO Expansion is enabled) accidentally.
ISO settings can, of course, also be used to boost or reduce sensitivity in particular shooting situations. The EOS 40D can use ISO settings from ISO 100 up to 1,600 (or ISO 3,200 if you've set C. Fn. I-03 to 1).
The camera can adjust the ISO automatically as appropriate for various lighting conditions. In Basic Zone modes, ISO is normally set between ISO 100-800. When you choose the Auto ISO setting, the 40D adjusts the sensitivity dynamically to suit the subject matter. In Basic Zone Auto, Landscape, Close-Up, Night
Portrait, and Flash Off modes, the 40D adjusts ISO between ISO 100-800 as required. In Sports mode, ISO is set between ISO 400-800. In Portrait mode, ISO is fixed at ISO 100, because the 40D attempts to use larger f/stops to blur the background, and the lower ISO setting lends itself to those larger stops.
When Auto ISO is chosen when using Creative Zone modes, sensitivity will be generally set to ISO 400-800 in Program, Av, and A-DEP modes, except when ISO 400 would produce overexposure, and a lower speed (down to ISO 100) will be used instead. In Tv mode, Auto ISO normally produces an ISO 400 setting, but for very bright or dark subjects the 40D will change the ISO in the range ISO 100-800. In Manual exposure mode, Auto ISO is fixed at ISO 400.
When using flash, Auto ISO produces a setting of ISO 400 automatically, except when overexposure would occur (as when shooting subjects very close to the camera), in which case a lower setting (down to ISO 100) will be used. Remember that if the Auto ISO ranges aren't suitable for you, individual ISO values can also be selected in any of the Creative Zone modes.
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Although we usually tend to think of the digital camera as the best thing since sliced bread, there are both pros and cons with its use. Nothing is available on the market that does not have both a good and a bad side, but the key is to weigh the good against the bad in order to come up with the best of both worlds.