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The Canon 40D introduces a new feature to improve highlight detail. This has always been a weak point of digital versus film cameras: digital cameras do ugly things the instant highlights get even a little out of control. There is no shoulder as we have in film.
Canon cryptically calls this "Highlight Tone Priority," which makes no sense in English. Setting this mode extends the highlight dynamic range. The 40D only operates from ISO 200 to ISO 1,600 in this mode.
There is only one setting. I'll refer to it in different ways.
The 40D is brilliant in how it tells you if you've set this mode. The zeros in the ISO indications, both in-finder and everywhere else they appear in the 40D, change from zeros to lower case o's if you're in the extended highlights mode.
For example ISO 200 will read as ISO 200 in the regular mode, and reads as ISO 2oo in the extended highlights mode.
How to set this mode
Highlight tone priority is set in MENU > (JUMP to orange custom functions) > Spin wheel down to C.Fn II: Image > SET > C.Fn 3: Highlight Tone Priority > SET > Enable.
It looks exactly like the regular mode, unless you go out of your way to shoot subjects with wild highlights.
Most normal subjects will look identical with this mode on or off.
I had to make a lot of shots to find something which would make the difference this obvious. When trying it yourself, be careful not to confuse variations in exposure from shot-to-shot with the effects of this setting. Most cameras shooting subjects like this will give different exposures each time.
California Fall Sunset. Mouse over to see the effects of highlight extension.
As expected, this feature does help preserve brilliant highlights, but it's not a big deal. I really did have to shoot a lot and pick out these two shots which made it this obvious.
Normal shots show no difference. This feature won't help you if you overexpose a shot.
I leave this mode activated all the time. You can't get to ISO 100 while you're in it.
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