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| p.2 #1 · 5dmk2, problems with pure reds (and sometimes blues) |
First, I have seen it with green - though, it is rare. The problem most often manifests in the red channel. It is not limited to the 5D2, so anyone thinking that this is an issue related to a specific camera is barking up the wrong tree.
You are on the right track when you note that some images or image elements might provide luminosity almost entirely to a single color channel. The issue is simply that this channel gets "swamped" (i.e. - over exposed) even though the overall image luminosity seems relatively low. Although this isn't strictly an accurate way to describe it, it is essentially that you overexpose the red channel and underexpose the others. In the same way that highlights (say clouds in the sky) blow out when you over expose an image, here the reds blow out - and in the same way that cloud highlight detail is lost and you get pure white, red highlight detail is lost and you get essentially pure red.
The notion that the camera should always be able to correctly interpret and expose for any complex situation that is thrown at it, without intervention or judgment by the photographer is unfortunate. Cameras can do this in many, many cases, but sometimes we still need to make some decisions ourselves when confronted by challenging lighting and other situations. Here, I have learned that when the scene is very "hot" in one color channel that a) there is a risk of blowing out that channel, b) the averaged luminosity histogram display will not tell you this since it averages the three color channels, c) it is a great idea to use the RGB histogram display the separately shows the three color channels, and d) you should, in most cases, err on the side of underexposure rather than overexposure.
I believe that current sensors still have twice as many green sensing photo sites. However, I think your explanation, while it has some interesting merit, is not getting at the essential problem or the relatively simple solution to it.
Actually I was thinking about this late last night and thought of something, is this what is going on?
When the part of the photo is pure red, that means it has no green or blue component. Meaning it will only show up on the red part of the bayer array, and not the green and blue part which will be zero or close to zero. Meaning, one third (or less if the bayer doesn't favor red) the advertised resolution, and less luminence data to average over or interpolate from. The raw conversion software does the best job it can, but it can only do so much since 2/3rd or more of the data that is normally there isn't.
Because with a bayer array sensor, isn't it normally interpolating color data in order to get the full resolution and even luminence precision?
It only happens with pure red, or blue (rarer). I've never seen it with green. Red may be the worst case because the bayer favors green over blue and blue over red. I know in the old days bayers sometimes had two greens for every one blue and red, but modern day I believe the patterns are more complicated than that.
And out of habit now, I only turn on HTP if I think it's going to be necessary.