The myth of digital reds and the concept of highlight headroom
I’ve seen repeated many times on the internet that “reds are difficult to capture on digital” and I’d like to dispel that notion.
http://photonstophotos.net/GeneralTopics/Exposure/Study_in_Red_-_Part_2.htm
On this old page, Bill Claff, a generally trustworthy source of objective data nowadays, notes that with this extremely intense red, he needed to underexpose (read: use -2 stops of exposure compensation) this image of a flower by two stops in order to properly capture its color. He says he based this on the RGB histogram provided by the camera.
While I’m sure Bill is aware of proper technique now, it remains a fact that cameras continue to mislead their users. In this situation, the camera’s sensor is almost certainly not clipping the data for the red channel. By scaring the user into underexposing, the camera gathers less image data relative to the noise, reducing your achievable image quality.
In this article I will go into why this is and how you can change how you shoot in order to optimize your image quality.
White Balance
Many of you may be familiar with white balance correction: adjusting a setting in your editing program in order to ensure that whites stay white. However, you may not be aware of the fact that instead of correcting an image that starts out white for daylight, most cameras start with an image that is extremely green, with the red and blue channels one to two stops darker when exposed to white light.
What does this mean? Let’s get into the concept of highlight “headroom”.
Highlight Headroom
It’s commonly known that when you shoot raw, you can recover highlights that were clipped in the JPEG provided by the camera, thus extending your dynamic range.
This headroom manifests in several ways:
- Highlight reconstruction: a clipped color’s value is extrapolated from the raw values of the color channels that weren’t clipped. This can work very well, or it can work poorly, depending on the subject and the degree of clipping.
- The color channels were actually not clipped, and there’s no guessing involved. This is a lossless process.
We’ll focus on the second type, because it always behaves gracefully.
Effect of White Balance on Highlight Headroom
Because the red and blue channels are darker than the green, they rarely clip. This means that things like the blue sky and red or orange flowers can be significantly clipped on your camera’s histogram and still not be clipped on the sensor. Here’s an example of a photo where the JPEG suggests that the red channel is badly overexposed.
In the camera’s histogram, the photo looks like this:
But when you view the raw histogram, which you can find in a program like RawTherapee, you can see that since the entire right half of the raw histogram is empty, you have in fact a whole stop of highlight headroom.
The white balance multiplier for red for my camera in this situation is roughly 2, which corresponds almost exactly with the one stop of headroom.
The result I got after processing turned out like this, with no lost color information:
You must watch out, however, when your white balance is set to something other than daylight. Under incandescent light, the red channel will be gathering a lot more light in proportion to the other colors, and the blue channel less. Thus, if you’re using a custom WB, a preset tungsten WB, or auto WB in warm light, you may find that the red channel has a very low gain.
Likewise with cool light: the blue channel gathers a lot more light than green and especially red, so it might have gain similar to or even less than than the green channel.
What does this mean?
On the lowest gain channel, whether that be red or blue, you don’t have the lossless highlight recovery that you do in daylight white balance.
How to Expose Properly
So how should you expose in order to maximize your image quality while avoiding clipping?
Low Light (incandescent)
In low light, which I would define as any situation where you must raise the ISO to achieve hand-holdable ETTR, it’s not necessary to expose to the right. Just pick a high-enough ISO and expose as brightly as you’re comfortable. Often there are point light sources in frame, so you will inevitably have to let those clip fully.
Bright light (especially natural light)
Whenever you have enough light to expose to the right at base ISO without risking blur or sacrificing too much DOF, then you should expose to the right. The question is: how do you do this consistently? Ideally, you have a raw histogram or raw zebras which show you where the sensor is clipped. But only Phase One cameras (and Magic Lantern on Canons) give you a raw histogram.
Another method of checking exposure better that used to be popular is to generate a UniWB profile, which is basically a custom WB that used a feedback loop to set all the white balance multipliers equal to 1… but this makes your JPEGs all extremely green, and is really not an option at all for mirrorless cameras where it impacts composing.
What I do is I permanently leave my cameras in fixed daylight white balance except when shooting in dim incandescent light, where I use a custom WB.
This means that except in extremely colored LED lighting, which brings its own problems (with colors that are out of gamut for reasonable color spaces), the green channel is almost guaranteed to clip first.
So if you never let the green channel clip, then you are pretty much always safe.
TL;DR
To obtain maximum image quality with ETTR, use daylight white balance and ETTR based only on the green histogram, and the other color channels should almost never clip.
By doing this you get the most out of your camera’s dynamic range.
