blown highlights

Started Jun 30, 2013 | Discussions thread
Sailor Blue
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Re: blown highlights
In reply to 24Peter, Jul 2, 2013

24Peter wrote:

I vaguely recall that the Highlight Alert on my Canon DSLR's starts blinking at some percent less than 100%. So even though I get the warning in camera, there may still be recoverable highlights there. Of course that only applies to highlights on the margin. And to be safe, it is better to avoid overexposing any critical parts of an image.

The Highlight Alert with Canon cameras have a built in safety factor for JPGs so it usually starts to blink at 95% white. At 97% white you still have details, but just barely and any higher exposure will cause a loss of details. The built in safety factor may be different for different brands of cameras and because of manufacturing variability may be different for supposedly identical cameras.

Since the exposure can only be set in 1/3 stop increments it means that the real exposure can be anywhere within about ± 0.1 stops of the value set on the camera. It is possible that when the Highlight Alert starts to blink you have reached an exposure value as much as 0.1 stop greater than the point at which the Highlight Alert starts blinking and may be overexposing JPGs.

I tested my Canon 7D by shooting a Datacolor ColorChecker Passport and evaluated the values for the R, G, & B channels for the gray squares vs their standard values. I found that for JPGs the value for the 97% white square was sometimes greater than 97% when the highlights started to blink but was always 97% or slightly lower if i set the exposure at 1/3 stop lower. This told me that to be sure white highlights don't lose details I need to use an exposure of 1/3 stop less than when the Highlight Alert starts to blink.

The greater dynamic range of RAW lets me use the exposure at which the Highlight Alert starts to blink and still not lose any highlights.

You should check your own camera and determine how to use it to ensure that you don't lose details on significant highlights.

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Living and loving it in Bangkok, Thailand. Canon 7D - See the gear list for the rest.

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