The Urban Legend of 18% Grey
|18% grey as recorded by the Epic, 5D MKIII, F65, GH2, & Alexa|
Ever since I picked up my first camera I've been on a pursuit to create visually compelling imagery. A big part of my education has come from taking classes and reading books written by those who have come before me. One of the early lessons I learned was about the importance of 18% grey, and how that value fit the Zone System in obtaining proper exposure. But what if what I was taught wasn't quite as solid as I was led to believe? Read on to find out why I now think 18% grey is an urban legend, and how that impacts us in todays digital age ...
The Origination Of The Legend ...
Our eyes perceive brightness in a logarithmic fashion. Because of this phenomenon, as brightness levels increase we have a harder time perceiving small changes in light. This perception of brightness also translates over to the print world. You would think that when we look at printed tones from black to white, where black is at 0% and white is at 100%, middle grey would be in the center at 50% reflected brightness. Instead middle grey is at 18% (17.5%). When grey is printed at this value we see it as being in the exact middle between black and white. It is from this perception of middle grey that we get the value of 18%. We have the print world to thank for this "standardization" of what value middle grey should be. And consequently, it is what is repeated time and time again in our photography books and classes. (I know I've shared this "knowledge" when teaching others). It is what we accept as the correct placement of middle grey, especially when exposing according to the Zone System.
Why It Is An Urban Legend ...
18% grey is an urban legend because it is a "standard" taken from one medium and applied to a different medium where it may not appropriately fit. Taking the lead from the print world may have been very useful for photography, especially as the printing of photographs was the only method for delivery for the vast majority of photographs. However, with the advent of digital technology, what does this value really mean to us practically? If I record an 18% grey card at the correct exposure, and display it on my computer screen, will it be presented at 18%? What about if I project it? View it on my iPhone or iPad? On my plasma TV? What about on an old CRT monitor? When will 18% grey be viewed at 18% grey, and how will I know?
Furthermore the published standards for correct reflectance values of middle grey (ANSI PH3.49-1971) actually put it at 12%-13%, which is about 1/2 stop under 18%. And it seems that most meter and camera manufacturers agree with this standard, as many of the reflected meters will determine middle tone as a 12%-13% middle grey value. Yep, companies like Sekonic and Canon, among others put middle tone at 12%-13%, not 18%. It seems that even Kodak, THE go to maker of 18% grey cards agrees, as Kodak's earlier instructions on using their grey card read: "Meter readings of the gray card should be adjusted as follows: 1) For subjects of normal reflectance increase the indicated exposure by 1/2 stop." The instructions have since been updated to: "Place the card close to and in front of the subject, aimed halfway between the main light and the camera." Angling the card like this has the effect of reducing the reflectance of the card, which in effect gets the meter to increase the exposure by about a 1/2 stop.
If you want to get into some high level math, and get into the nitty gritty of why 12%-13% is correct, I highly recommend the following articles:
18% grey is what I have always known as middle tone. It is what I've been taught, and what I've relied on. So I need to see this in action for myself, as I wasn't quite convinced that 18% grey was a myth. I took my trusty Sekonic Grey Card, as well as my Kodak Grey Card and put it to the test- and here are the results: (Thanks for Nic Rios for use of his 7D, 5D MKIII, & Zeiss 50mm lens for these tests.)
These cards were shot with 2x 1k Arri Open Face Lights, and the light was kept within 1/10 of a difference from edge to edge on both the Sekonic and Kodak Grey Cards.
The Sekonic Grey Card (Shot perpendicular to camera):
|Sekonic 478D Readings: 1/50, ISO 160, F5.6 0/10|
|Camera In Full Auto = 478D reading. [1/100 (-1), ISO 100 (-1/3), F3.5 (+1 1/3)]|
The Kodak Grey Card (Shot perpendicular to camera):
|Sekonic 478D Readings: 1/50, ISO 160, F5.6 5/10|
|Camera In Full Auto = 478D Readings. [1/160 (-1 2/3), ISO 160 (0), F4.0 (+1.5)|
The Kodak Grey Card (Angled Per Instructions):
|Sekonic 478D readings: 1/50, ISO 160, F4.0 8/10|
|Camera In Full Auto = 478D reading. [1/125 (-1 1/3). ISO 160 (0), F3.5 (+1 1/3)]|
|Exposure Compensated To Put Histogram In The Center|
Great, but what is the take away from these pictures? Well, if you look at the Sekonic meter readings and the Canon meter (the little tick mark at the bottom of the screen) you'll see that they agree in all instances. Now take a look at where the histogram falls. It is left of center, or slightly underexposed. This shows that the Sekonic and Canon meters are conforming to the ANSI standard of 12%-13% for middle tone. That makes sense, since the 18% grey card is brighter than 12.5%. The meters are looking for a darker tone and this brighter tone is telling the meter to under expose the image slightly. In the very last example, I opened up the aperture to put the histogram in the center. Now the histogram is showing a correct exposure, but the meters are saying it is slightly over exposed. In all of these cases, if I want the histogram to be correct, I need to open up my exposure.
[SIDE NOTE: Taking a spot reading off of an angled grey card is rather tricky, as the spot readings varied depending on what side of the card I took my reading from. I did my best to use the 5 degree spot meter of the 478D to read the same area that the camera was reading. But the face of the card did vary in value by about 1/2 of a stop from edge to edge when it was angled.]
Okay, so maybe 18% grey doesn't work for us in the digital age, but that's why we have waveforms. All I need to do is to put middle tone on the 50% IRE line, and I'll be good to go, right? Unfortunately, the answer is not that easy. The recommended placement for middle tone on the waveform varies from 38% - 55% or more depending on the camera system, recording format, and even personal taste.
If you want to dive more deeply into the relation of middle tone and IRE, here are links for further reading:
- Sony's S-Log Manual (Recommends 38% IRE)
Camera manufacturers are not helping us out any, as they are placing middle tone all over the map, as you can see in the following waveforms:
|GH2 - Middle tone is at about 55 - 60 IRE|
|EPIC - Middle tone is at about 39 - 41 IRE|
By all accounts, it looks like 18% grey and 50% IRE are not so "standard" after all ... Yes, I realize we are talking about a Log file, verses a Rec 709 file. And that is my point. With all of the ways to view and record digital images today, "standard" is not so "standard" anymore ...
What To Do About It ...
If you are at all like me, you may be feeling like you just had the rug pulled out from under you. If my meter doesn't see 18% grey, my camera's meter doesn't see 18% grey, and IRE values for middle tone are all over the map, is everything just up for grabs? Does "correct" exposure even exist? Should I even worry about it? Before you give up and throw away your meter and camera, here are my tips on how to handle the situation:
1. If it ain't broke, don't fix it.
If your current metering methodology is yielding consistent, predictable results, more power to you! Don't sweat the fact that 18% really isn't 18%. In the end it is about the results you produce, not the technicalities.
2. Do Some Informed Testing.
Now that you know that 18% grey doesn't really exist, spend some time testing out your camera system and place middle tone at different values and see how that affects the final image. It is important to take the image all the way through the post process, including a final grade. I'd recommend starting by exposing at 1/3 stop increments. As you do so, watch how the camera responds in the highlights and shadow areas. You may find that you can place middle tone at a lower value (underexpose the image), giving your more highlight protection while still keeping good middle tone & skin values. (Just watch out for noise in the shadows).
3. Combine Testing With Custom Profiles.
This is where I think a software program like Sekonic's DTS system comes in really handy. (Learn how to use this system on my Sekonic Page). By using this system in conjunction with your meter, you will be mapping the middle grey value of your camera, to the middle grey value of your meter. So you know that if you expose at the reading of your meter, your camera will respond accordingly. But that is only half of the power of this system. Let's say that you are not happy with where your camera system places middle tone. If you use DTS, you can remap middle tone, and the clipping points of over and under exposure. Here is an example of what I have done for the Canon 5D MKIII:
|Original Profile Created By DTS - Note the middle tone & clipping points.|
|My Modified Profile - Note how I lowered the middle tone & changed the clipping points.|
By using DTS, not only can I see the whole dynamic range of the camera system, but I can map it to my meter according to my needs. This gives me complete control over the entire camera system and exposure. It is a way to get consistent, repeatable results based upon an informed decision, rather than an arbitrary feeling. What I also like about this approach is that I can create in-camera looks or grade the footage in post, apply those looks to the chart, and after running it through DTS I will have a custom profile that allows me to pre-visualize where values will end up after it has gone through the final grade. That can be very helpful if I am creating an extreme look, and I need to make sure that I retain detail in a specific part of the image while I'm lighting a scene on set.
So take a deep breath. The sky isn't falling. Knowledge is power. And knowing how our cameras respond to light, and figuring out where we want to place middle tone will open up new creative opportunities to us. Crafting a compelling image is more than just exposing "correctly" - it's about intelligently choosing where to place your exposure within the limits of your camera system.
Until Next Time - Get Out There And Shoot!
Ryan E. Walters, Cinematographer