RGB Histogram?

[mmcaleer]

Does anyone use the RGB Histogram? If so, how do you use it? I am used to the standard histogram but now my new camera offers the RGB.

Thanks,

Mike

[yikes]

Try here for starters

http://www.sphoto.com/techinfo/histo...histograms.htm

[mmcaleer]

Perfect. Thanks yikes.

Mike

[MRCPix]

that is some really great information thanks I am also new to the RGB histogram so I am always looking to learn.
thanks for sharing

[tjavery]

Yes, always use it. The luminance histogram only sums up the three channels, and will not indicate that one of the channels is blown out for particular scenes (and neither will the "blinkies", at least for Canon cameras).

For example, say you're shooting a brilliant sunrise or sunset and you "expose to the right" using the luminance histogram (i.e. expose so that the right, or highlight, side of the historgram is just inside the right edge or limit of the graph). When you open your photo in PS, you'll see that the red channel has blown out (but the blue and green channels are within range). If you had used a RGB histogram to begin with, you'd see the red channel by itself and prevent that problem.

[vibhav]

Thanks TJ.

A question: So if you could see that your red is blown out and Blue and Green are OK, then how would you prevent it right there? Reduce the exposure? But won't it reduce exposure for both Blues and Greens as well? So, in my understanding, if there is no way to prevent just the Red channel (in this example) from blowing over without reducing the exposure for Blues and Greens as well, then what good is that histogram to me? Wouldn't the good old b&w histogram also show me that these is something blown over without showing me which channel exactly, thereby prompting me to take the same action?

Thanks!

[tjavery]

Originally Posted by vibhav

A question: So if you could see that your red is blown out and Blue and Green are OK, then how would you prevent it right there? Reduce the exposure? But won't it reduce exposure for both Blues and Greens as well?

Yes and Yes. You can reduce your exposure, or perhaps choose a "cooler" white balance.

Originally Posted by vibhav

So, in my understanding, if there is no way to prevent just the Red channel (in this example) from blowing over without reducing the exposure for Blues and Greens as well, then what good is that histogram to me? Wouldn't the good old b&w histogram also show me that these is something blown over without showing me which channel exactly, thereby prompting me to take the same action?

Thanks!

No, the b&w histogram (aka luminance histogram) sorta averages the three channels, and in doing so, can report a "high" value that is below the "high" value for a single channel.

In other words, your b&w histogram can report that the highlights are very close to being clipped, but aren't there yet, while the RGB histogram will show highlight clipping of one channel for the same scene. This usually occurs in situations where one color is very bright (e.g. the reds in a sunset shot).

It's frustrating that the three channels (RGB) can be so unequal for some scenes. Also, the white balance can affect how "unequal" or "equal" the three channels are. A warmer white balance will boost the red channel more.

I recall an old thread here where someone had used a type of magenta filter and also set their white balance to the "native" (i.e. un-balanced) setting. This was using a Nikon camera. The result was an image where the three channels were approximately equal in appearance in the RGB histogram, and the resulting white balance worked out to be about "daylight".

I think this is all a result of how a Bayer-type digital sensor works. There are twice as many green pixels as there are blue or red pixels, so the color interpolation therefore boosts reds greatly to compensate. Something like that...

[K1000]

[QUOTE=vibhav]Thanks TJ.

A question: So if you could see that your red is blown out and Blue and Green are OK, then how would you prevent it right there? Reduce the exposure? Yes. But won't it reduce exposure for both Blues and Greens as well? Yes. So, in my understanding, if there is no way to prevent just the Red channel (in this example) from blowing over without reducing the exposure for Blues and Greens as well, then what good is that histogram to me? Tells you Red is over-exposed. Wouldn't the good old b&w histogram also show me that these is something blown over without showing me which channel exactly, thereby prompting me to take the same action? No.

.

[akimartymar]

How will choosing a "cooler" WB help anything if you're shooting RAW? RAW doesn't capture info like that.

[tjavery]

Originally Posted by akimartymar

How will choosing a "cooler" WB help anything if you're shooting RAW? RAW doesn't capture info like that.

You're correct. You need not be too concerned about WB when shooting RAW - you select the WB later when processing on your computer.

However, when you do shoot RAW in the field, the histogram (b&w or RGB) that's reported on your LCD after the shot is based on your shooting settings (including WB). So, it helps to have your WB (and also your contrast and saturation settings) selected to what you think you'll like later on when you process. That way, you can get your exposure just right when shooting. Then, when processing your RAW file, you can fine-tune the WB, saturation, contrast, etc.