Mark Pemberton
The Digital Curmudgeon
I love digital. I love everything about it. That’s why it is so disappointing when you discover that all is not perfect with your true “love”.
However, before I get to the problem with my “love” I’d like to quickly discuss a very important aspect of digital photography and histograms. A typical histogram from an image containing a full range of tones will look something like the example shown below:
However, sometimes the subject matter does not contain the full range of tones from black to white. A histogram from that image might look like this:
Many people are familiar with the concept of “exposing to the right” (ETTR). If you aren’t, it is the practice of exposing the image such that the histogram is forced to the right hand side, but without actually touching the “wall”. This can be accomplished by dialing in positive exposure compensation if you are using Auto Exposure (AE) or by opening up your aperture if exposing manually. That histogram should look as follows:
Why would you want to do this? The amount of information carried by light is proportional to the quantity of light. Every time we reduce the amount of light striking the sensor by 1 f-stop we cut the amount of light, and thus the amount of information in half. A 12 bit RAW image can theoretically produce 4096 (212) different tones. Those tones are not evenly divided across the typical 5 f-stop range of the histogram. The 4096 tones are actually arrayed as shown:
It is entirely to our benefit to shift the histogram as much to the right as possible to take advantage of the greater number of tones available there. This is also why it is critically important not to under-expose your images in digital photography.
So what does this have to do with my alleged disillusion with digital photography? The other day, Cathy and I were shooting close-ups of flowers. Everything was perfect. The lighting was good. We carefully metered the light and checked our histograms and then began to shoot. After a while we took the images and loaded them up to Lightroom. To our surprise, the petals were blown out, completely lacking detail. Yet the camera’s histogram was perfect and there were no “blinkies” on the highlight display.
The answer was completely obvious in Lightroom. Lightroom’s histogram is a true RGB histogram, showing all three color channels independently. It was clear that the green channel was “blown out” and the red channel was just about to start “climbing the wall.”
Why did this happen? The answer lies in the fact that the basic histogram displayed by virtually all digital cameras is a “Luminance” histogram. It displays only the brightness data. The Luminance histogram can differ significantly from a true RGB histogram. Our problem was exacerbated by the fact that we were shooting close-ups of flowers with just a single color predominating.
To document this effect further I went back into the yard and took a few more photos of a yellow-orange rose. These are shown below. Below each photo I have included the associated histograms. The top histogram is a luminance histogram, similar to that found on the camera. The lower three histograms are for each of the RGB channels.
Figure 1
The first image was shot with a normal, metered exposure. Note the significant difference in each of the three RGB channels and how the red channel extends much farther to the right than the Luminance histogram.
I then over-exposed the flower by one f-stop to move the Luminance histogram to the right. (Remember ETTR?) This was probably the perfect exposure for this situation. Note that none of the histograms is touching the right-hand side.
I took a final exposure at + 2 f-stops. Even at +2 stops the luminance histogram was not touching the right-hand side. However, the green channel is now “climbing the wall” and the red channel is about to start. This is the situation that Cathy and I experienced earlier that lead to the blown out petals.
So the camera’s histogram failed to alert us that there was a problem. But what about the “blinkies”? The following two images were shot from the back of the camera.
Figure 2
The first image is a typical view of the Highlights display on the back of our camera. The “blinkies” are based upon the combined RGB luminance data. No “blinkies” are visible. Our Nikon D300, however, allows you to monitor the individual RGB channels for “blinkies”. When I changed from RGB to the Red channel the screen lit up with “blinkies”, indicating gross over-exposure.
This is all academically interesting but is it really important to obtaining good images?
To test this out I took the two over-exposed RAW images from Figure 1 and adjusted the exposure down in Lightroom and compared them to the correctly exposed image. I also took a 2 f-stop over-exposed JPEG image and corrected it in Lightroom.
Figure 3
A – Correctly exposed RAW image
B – 1 stop over-exposed RAW image corrected in Lightroom
C – 2 stop over-exposed RAW image corrected in Lightroom
D – 2 stop over-exposed JPEG image corrected in Lightroom
These results show that even though the Green channel was blown out in image C, I was able to recover the image in Lightroom with no sign of significant loss in detail. However, the JPEG image did not fare so well. Although I was able to produce an adequate result in Lightroom the loss of the data from the Green channel was significant, causing the color to shift from a light orange to yellow. There is also some loss of detail in the upper petals.
Conclusions
This was a fun set of simple experiments. However, it reinforces certain concepts.
1. Always shoot RAW. It has more flexibility and latitude.
2. If your camera has the capability, turn on your RGB histogram display, AND LOOK AT IT! My camera is set to display all three channels. Unfortunately, when we set up Cathy’s camera, we forgot to turn that feature on. It’s on now.
3. Go ahead and expose to the right. Just keep an eye on those RGB histograms.
Further Reading
Not surprisingly, this topic has been covered by others. There are a number of good articles on Luminous Landscape and other sites on this and related subjects.
http://www.digitalphotopro.com/technique/camera-technique/exposing-for-raw.html
http://www.luminous-landscape.com/tutorials/expose-right.shtml
