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A PRACTICAL GUIDE TO INTERPRETING RGB HISTOGRAMS

Learn how to interpret your digital camera's thumbnail image histogram to help determine proper in camera exposure
Learn how to adjust tonal range in your scans or existing digital images using the levels-histogram tool
PAGE 4

INTERPRETING YOUR DIGITAL CAMERA'S THUMBNAIL IMAGE AND HISTOGRAM

How well I remember my naiveté when I first got my Canon 10D. I knew I'd miss the spot metering capability of my Canon 1V but I figured the LCD image review would more than compensate for the loss of spot metering. So, I shot about 100 images just looking at the image thumbnail on the LCD to confirm exposure. After all, wasn't I looking at the actual final image? OH BOY this is so easy. However, when I got home and downloaded the images most of those first 100 images were underexposed and a few of them were over exposed. That dang little LCD was next to useless as an aid to judging proper exposure. It became obvious that I needed to incorporate a little science into my DSLR picture taking workflow. Thankfully, the 10D has an option to view an image histogram along with the thumbnail review image. The histogram generated for the images in the 10D is a luminance histogram.

My Canon 1Ds Mark II and some of the other newer DSLR's now offer a three channel RGB histogram option that actually shows a graph for each of the three colors, red, green and blue. A three channel histogram has the full range of values 0 to 255 for each color channel without the 'brightness compensation' of the luminance histogram. The three channel RGB histogram it is a more precise indicator of exposure since it will show you if you have clipping on any of the color channels on either end of the graph. Clipping (pixels at 0 or 255 level on any color channel) may lead to overexposed and blown out highlights or shadows that are nearly 'black'. If your camera has a three channel RGB histogram option, I recommend that you use that in favor of the luminance histogram option.

Digital cameras may use slightly different data to construct the 'in camera' histogram than your imaging program or RAW converter program. We have already discussed the difference between RGB and Luminance histograms. Here is another example. When you shoot RAW your camera's firmware has to set a white point, black point and midpoint to build the 'in camera' histogram for that image. RAW images, by definition, have no white or black point or midpoint setting until they are opened in your RAW converter. Most digital cameras build the 'in camera' histogram for RAW images based on the parameters you have set for JPEG image output even when you do not have the camera set to output JPEG. If you use your camera's supplied RAW converter, there should be no difference between the' in camera' histogram and RAW converter's default histogram. If you shoot RAW, and you use an aftermarket RAW converter, it might be beneficial to check a few 'in camera' histograms against the default histograms for those images in your RAW converter program and see if there if they are about about the same. Look to see if there is a repeatable difference with white point, black point and midpoint placement between the 'in camera' and raw converter program's histogram. If there are repeatable differences, you could take the difference into consideration when checking for proper exposure with your 'in camera' histogram. The bottom line is that the difference between RGB and Luminance histograms and small variations between 'in camera' and RAW converter program's histograms does not detract from the 'in camera' histogram's usefulness as a tool for judging proper exposure.

One thing that became obvious after my first batch of images was that shooting digital was much like shooting transparency film. You get much more flexibility for post processing if you get the exposure close to perfect 'in camera'. This is particularly important if your camera is set to output JPEG. Camera RAW format has more latitude for correcting exposure errors during post processing. Just like shooting transparency film with digital camera photography you have to watch out for blown out high lights. Some photographers say that you get about 5-7 stops of dynamic range in a high end DSLR. I believe the dynamic range available in my Canon 10D and 1Ds Mark II is somewhere between 5 to 6 stops. I see some detail in shadows 2.5 stops under exposure and just a little texture in highlight areas at 2.5 stops over exposure. My current picture taking workflow is to set my camera for 1/3 stop increments in exposure adjustment (either shutter or aperture) shoot in RAW mode and bracket any important shots, when I can, at + and - 1/3 stop.

Below find three image example sets of 3 images each. Each image set was shot at the camera meter's choice for proper exposure and then bracketed at +1 and -1 stop to more fully illustrate the value of histograms and careful choice of exposure. All exposures were made in manual center weighted mode with no exposure compensation or interpretation of scene content. The images were shot in RAW mode. All of the thumbnails below the photo of the 10D's thumbnail image and histogram were generated in Capture One Pro using the program's default color and exposure choices. The images were sized and saved as high quality JPEG using Capture One DSLR Pro. The histogram next to the Capture One DSLR generated image is Photoshop CS's RGB histogram for that image. It was my intention to provide at least one image with a dynamic range that would be hard to handle. We'll start with that one.


CAMERA'S CORRECT EXPOSURE
For this early morning contrasty image the sun was behind and to my left. The tall spike is the sky area and the graph area to the right of the spike represents the boats. The area to the left of the spike represents the bay water and green grass and other darker tones. The in camera luminance histogram seems to indicate that we have fairly well balanced exposure with our tonal range biased just a little on the dark side. However, when we look at all the pixels in the RGB histogram below we find to our chagrin that we have quite a few pixels with 0 color value on at least one color channel. The in camera 'compensated brightness' luminance histogram does not show us the entire 3 channel tonal range. It looks like we may have benefited from just a little more exposure.


RGB Histogram

We can do a levels adjustment to shift the histogram but the pixels that originally were 0 or black on all color channels, even if moved to RGB numbers of 10 to 40 on all color channels, will still not have any texture or detail.


PLUS ONE STOP IN CAMERA
Adding a full stop of exposure gets rid of a bunch of those black pixels. The body of the in camera luminance histogram looks to be nearly centered. However, we are now climbing up the right side of the RGB histogram with a few too many pixels with white or 255 on at least one color channel. Some of those boats may have gone pure white. Since the boats are an important part of this composition, we don't want them blown out. Plus 1/2 to 2/3 stop more exposure from our camera's original metering choice may have given the best exposure for this scene. If I had paid attention to exactly centering the in camera luminance histogram, I would have nailed the exposure for this particular image. In this case it would be better to have a few more 0 pixels than to have some of the boats blown out to pure white.


RGB Histogram

 


MINUS ONE STOP IN CAMERA
Minus one stop for this scene doesn't give us what we want. The in camera luminance histogram is shifted way over to the left. There are very few midtone or highlight pixels in either the in camera luminance histogram or Photoshop RGB histogram. This image would be difficult to fix properly with a levels tool or any sort of tonal adjustment. We could expand the tonal range and reset the output level for our lowest number pixels up to 10 to 15 but we would still have dull, detail-less lower midtones and shadows. Also, if we attempted to fix this image as an 8 bit JPEG, the tonal range (histogram) would have had a lot of blank tonal areas and spikes. A little more tonal information could have been saved if we worked on this image in its original RAW file (16 bit) state.


RGB Histogram

 


CAMERA'S CORRECT EXPOSURE
It is pretty obvious that the camera's selected metering is not going to cut it with this midday beach image. The luminance histogram indicates that almost all of our tonality is left of midtones. Looking at the image content we can see that this should not be the case. This is a typical reaction for a camera meter to a high key scene. The tallest spike in the RGB histogram is the sky area and is just barely above midtone level. If we visually divide the histogram into 5 equal sections left to right, light midday skies should usually fall somewhere between the beginning and middle of the last 20% of the graph. Looking at the RGB histogram below even the second spike, which includes some of the white surf, hovers around 175-185, not very 'white'. However, the tonal range of this image could be expanded to look quite nice. We only have a few pixels climbing up the left side of the histogram graph


RGB Histogram

The right hand end of the small flat spur of highlights just to the right of the second spike could be set close to white and we'd be in the ballpark.


PLUS ONE STOP IN CAMERA
Plus one stop from the camera meter's choice is pretty darned close. The in camera luminance histogram is fairly well centered and the sky/highlight area represented by the tall spike looks to be about in the right place on the graph for a side lit pale blue sky. Looking at the RGB histogram below we see the sky area represented by the thicker spike to the right and the surf and highlight areas represented by the thinner spike to the right are also about in the right place. However we do have a few pixels in the 0 or black area of the graph. One and 1/3 stop more exposure from the camera's original metering choice would have got rid of the black pixels. Doing this may have moved the sky, surf and highlight pixels a little too far to the right. Did I say... ...BRACKET....:^)


RGB Histogram

 


MINUS ONE STOP IN CAMERA
One stop under the camera meter's choice is way too dark. Again all of our pixels are below midtone level. If this was your only image from this scene, you'd be hard pressed to fix it properly. Are we seeing a trend here? You bet! With contrasty scenes get as much  exposure as is possible without blowing out (pixels climbing up the RH side of the graph) the brightest area of your image that needs to hold detail.

Here is another approach for getting proper exposure. If you can determine where your photograph's actual midtones are located on the histogram graph, try to center them by adjusting exposure until the midtone area of the photograph is located close to the center of the histogram graph.


RGB Histogram

If you can't determine which part of the graph represents your midtones, adjust exposure until the entire body of the histogram is fairly centered within the histogram graph.
Either of these approaches will generally get the exposure very close to correct, if not spot on.


CAMERA'S CORRECT EXPOSURE
Here's an example of an 'easy' image. It is heavily biased toward midtones so the camera's meter, which wants to read everything as a midtone, is pretty accurate with its default setting. The luminance histogram is fairly centered as we would expect in an image consisting mostly of midtone content. The second spike is obviously the sky area. In this case there are no surprises with the RGB histogram. The more serrated area to the right of the main hump in the RGB histogram represents the sky and some clouds we can barely see in either thumbnail image. We could add just a little contrast to this image by setting new white and black points with our levels tool. Since there are no real black areas in this image, we would not move the black point slider all the way to the start of the slope on the left side of the graph. We would put the new black point just at or about half way up to the short flat line of dark pixels on the left side of the graph


RGB Histogram

Same for the new white point. About half way to the right hand start point of the graph would do nicely for a new white point.
We might want to fine tune the midtone slider depending on where our midtones ended up after white and black point adjustment. Again, we'd want to do these adjustments during RAW conversion or in 16 bit mode in Photoshop.


PLUS ONE STOP IN CAMERA
One stop over for this image shifts the tonal range a bit too far to the right. Most of the midtones are now actually above midtone. Looking at the RGB histogram below we see that the brightest sky areas are approaching washout. Looking at the RGB histogram below we can see the little tail end on the left side of the graph starts at about tonal level 20. The darkest pixels in this image start at 20 and there are very few pixels between 20 and 45. This image would be improved by setting a new black point close to the start of the thin line of pixels on the left hand side of the graph. This action would move the thin tail of the graph closer to 0 and shift the midtone pixels down to a more natural looking earth tone range


RGB Histogram

 


PLUS 2 STOPS IN CAMERA
I added this rendition at 2 stops over to show a nice feature that many mid to high end digital cameras have. Notice the little bit of speckle at the top of the image. This is the camera letting me know that I have 'clipped' those areas of this image out of color range and into 255R+255G+255B or pure white. This visual 'clipping' notification is available on many mid to high end digital cameras. Clipping notification usually takes the graphical form of turning the clipped pixel areas to a blinking red or black in the review thumbnail. With some cameras this feature must be turned on in the camera's features menu and with other cameras it is automatically active when you are in histogram view mode. The clipped area is represented by the last spike in the histogram that is climbing the right hand side of the graph. Take another photo, this one is not fixable...


RGB Histogram

 

It takes a little practice to be able to identify which parts of the histogram correlate to the different visual components and tonal range areas of the image. It is a skill well worth learning and not too different from the old black and white zone system. After a while you'll have an intuitive feel for where sky tones, clouds, dark or light earth tones, and bright highlights should fall in your image's histogram. You'll also know how much a 1/3 or 1/2 stop increase in exposure will shift the histogram to the right. Or, exactly how much a 1/3 or 1/2 stop decrease in exposure will shift the histogram to the left. Watching the endpoints of your histogram for stacking and using the visual tonality of the thumbnail image to determine where your midtones should fall is key to producing properly exposed digital camera images. Lastly, if your camera has a spot meter and you are shooting a stationary subject, place the spot on the very brightest light colored area of the scene that you want to hold a little detail. Then set +2 stops of exposure from that reading...and BRACKET + and - 1/2 or 1/3 stop. You'll have a winner in that set somewhere, just check out those image histograms...:^)

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LINKS:
Histogram and RGB Tonal Range Explanation - Page 1
Histogram and RGB Tonal Range Explanation - Page 2
Digital Image Tonal Range Adjustment Using a Levels Tool

WEB SITE LINKS
Steve Hoffmann's Nature and Landscape Photography - Gallery Index & News

PHOTOGRAPHY AND DIGITAL IMAGING INFORMATION
Photography Tech Tips
Beginners Level Digital Darkroom Slide Show Tutorial
A Practical Guide to Interpreting RGB Histograms