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I'm having a problem which is puzzling me.  While shooting with my Canon T2i/550d in Raw+Fine mode in the studio, the background is blown out pure white as I intended.  In the camera's playback with histogram, the background is blinking to indicate this.

I open the raw CR2 image in Digital Photo Professional v3.8.0 and adjust the camera/lens settings (noise reduction, sharpening, or whatever).  In DPP the "View->Highlight Alert" indicates that the background is blown out.  So far so good.  I do "File->Convert and save ... " and save as "TIFF 16 bit".

Then I open the TIF in Photoshop CS3 to touch up specks of dust, etc.  Moving the eyedropper around the background, the info window says:


When finished I do "File->Save for Web and Devices ..." and save with the parameters JPEG/Maximum, Quality: 100, Blur: 0.  Here's where the first problem is apparent.  Moving the cursor around the background in the "Save for Web" window, the bottom of the screen says:


Why is the green channel now indicating 254?

If I save the file and then open the JPG in CS3, the eyedropper now says that the entire background of the JPG file is:


Weird, huh?  Some kind of rounding error converting from 16 bit to 8 bits?  Something not working in T2i/550d raw mode, in DPP or in CS3?

I went to a lot of trouble to get the proper studio setup so I wouldn't have to cut out, isolate, brighten the background, etc. in postprocessing, and the software is misbehaving in a way I don't understand.  Or maybe I'm misbehaving.

That's not what it's indicating, the blinking is indicating that particular area of the image is overexposed based on the settings you've taken the image at, that doesn't mean the background is pure white.

I as well.  Higher than 255 at 0 exposure in the raw converter, and that's what I've experienced.

Cameras as I'm sure you well know don't know that white exists, to a camera a correctly exposed image will have an 18% (or thereabouts) balanced midtone, the highlight alert is to indicate which part of the image the camera thinks is overexposed based on that 18% which it takes from the settings you've applied to take the shot at.

Try it yourself, set up a shot of something white, let the camera auto meter the shot and the result will be that the white object comes out grey, then manually set the camera to overexpose the shot by 1/3 and you'll get the blinking highlights but this is not enough to make the white object 255 it will still be grey albeit a bit lighter, in order to get pure 255 white you'd probably need to overexpose by 1 stop or more.

Sean if you've got your 255 it's because you've correctly overexposed enough to get 255 but you'd still have got the blinking on your camera at 254,253 etc etc if that makes sense.

Something else to consider is that the cameras histogram will not be the same as the image histogram when you go to convert the RAW file, and to be really geeky the cameras highlight alert only blinks if all three channels are 'overexposed'.

To a camera a correctly exposed shot of something white will result in it being 18% grey, an exposure that makes the white thing 16% grey will in the cameras mind be overexposed and it'll blink at you  

Personally I don't use the highlight alert, I go by the histogram.

It says "point sample" in CS3 when the eyedropper is selected.