We use our camera histogram to verify that the exposure is correct when shooting raw files. We shoot RAW to get the greatest dynamic range and image quality from each file. Houston, we have a problem, camera histograms are slightly flawed in displaying accurate exposure data from the RAW. Let me explain further.
I am going to keep this explanation low tech, but will give you a link to the tech weenie version if you wish to read 6 pages that discuss this problem in much greater detail, by Andrew Rodney. Those of you who know of him, he needs no introduction, and is highly regarded in our industry.
The histogram we see on the cameras LCD is actually created from a JPEG thumbnail produced by the RAW file. This JPEG is gamma-corrected and has a s-curve applied to it…..thus it is NOT the LINEAR RAW data we see. We only see an interpolated view. Digital camera sensors capture light as linear data and display monitors are non-linear, so a gamma correction is used to display the data.
Actually, to display the RAW file on the camera LCD histogram several things happen. First the image is de-mosaiced, then a color-space is added, then the gamma correction, and finally the JPEG curve settings that we imposed is applied. And that image is what we see on the camera histogram LCD…hmmm.
We can avoid much of this problem by using a light-meter and shooting tethered to a laptop or desktop computer. When viewing the RAW files in conversion software such as LightRoom, Aperture or Adobe Camera RAW, the linear raw data is correctly displayed and accurate on the big monitor. When we are out in the field however, it is awkward to always shoot tethered so we need a work-around.
Fortunately we have some control over the JPEG curve when we set up our camera in the menu system. I am referring to the settings we can adjust for the shooting menu. You will need to look at your camera owners manual and figure out how to get to the setup menu. Here we can adjust saturation, sharpness, tone compensation, color-space, and hue among other items.
I adjust the saturation all the way down. Image sharpening to “none”. And the contrast or tone compensation also, all the way down. For what it is worth, I set the white-balance to the scene that I am shooting. This will create a flat RAW file only for the camera histogram preview, yet the file looks normal when it is opened in your RAW conversion software.
What this accomplishes is this. Yes the raw file has not changed physically, but the JPEG file created for the histogram data is different now and this effects the way I set my exposure. You may notice that all of a sudden we see a smoother curve in the LCD window, we have simply removed much of the RAW in-camera JPEG thumbnail processing. My camera shows that I have more high-light and shadow detail available to work with. Now I can adjust the exposure and lighting so that the curve fits neatly within the cameras histogram LCD display.
I should point out that this method is very similar to processing negative film. From the film days we learned to “expose for the shadows and develop for the highlights”. This rule applies to digital as well. In RAW conversion software, I now have the shadow detail and I pull back a bit in highlight recovery if needed. Fortunately I rarely if ever have to pull back the highlights because I either light flatly in the studio or I shoot landscapes under quality sunlight. Also, my Nikon D2x gets about 9 stops of dynamic range when I expose properly. I expect the same and better from the new Canons and Nikons, you probably have 10 stops to work with or more.
Maybe not a perfect solution, but now the camera histogram more closely resembles what I see in LightRoom’s histogram. I read about this technique 4-5 years ago on a review site about my camera, and the author had noticed the same results. Been doing it ever since on all my digital cameras…seems to work just fine.
People comment on how I get such smooth tones in my images. This is done with lighting and proper exposure, not hours of Photoshop manipulations. Shoot RAW, process clean data rich 16 bit ProPhoto colorspace, and practice clean lighting and balanced exposure. It is when all these individual elements come together that the image quality really improves.
A few people have had success with a file called UniWB and NearUniWB. Essentially it loads a whitebalance preset into the camera that gives all three RGB channels a 1.0 coefficient. Personally I have not used this file but I wanted to mention this for the adventurous.