Hi John Hess here!
This IQBits video is an adendum to our Science of Exposure and Metering video because we
kind of need to talk…
I mean…what the >Facebook< is Middle Gray.
Middle Gray. Middle Gray. Middle Gray. Middle Gray. Middle Gray.
Middle Gray. Middle Gray
Well middle gray is simply half way between white and black… see….
That’s half way - and that’s what they call it middle gray.
I hope this has been helpful for your cinematic journey, go out there and make something…
Oh… you want me to go into more detail on what exactly is middle gray?
Okay, I guess we could do an experiment.
So I’ve got this exposure refrence pop up.
Let’s take a spot meter reading of the black and white portions.
With the black I’m getting 90 cd/m2… with the white I’m getting 1400 cd/m2.
So the middle gray out to be the average of the two right - something like 745 cd/m2 right?
Let’s spot meter the middle gray… not even close - the middle gray 360 cd/m2
That’s because to our perception of color and lightness don’t respond in a linear
fashion - that’s why in photography we always talk in stops or powers of two.
So that means if we want to find the half way point between white and black, we don’t
take an arithmetic average, we take a geometric mean.
We multiply the figures together and take the square root which ends up being 354.9
cd/m2 - really really close to what measured with the spot meter.
This geometric mean approach was how middle grey was determined as early as 1903.
They basically took a geometric mean between a white and black that had a ratio of 60:1.
The middle gray was an equivalent to 12.9% of the white intensity.
But you’ve probably heard of the Kodak 18% gray card.
Where does 18% fit into all of this?
Well 18% actually comes from half tone screen printing process dating way back in 1880s.
Printers used these 18% gray cards to calibrate the ink flow - the 18% meaning it reflected
18% of the light that hits it.
If you think about printing in terms of stops - 18% is the geometric mean between white
paper which 95% reflectance and black which is 3.5% reflectance.
When Ansel Adams and Fred Archer published the Zone System in 1941, they used this 18%
gray card that printers used as a reference for middle gray or Zone V.
Later in the Twentieth century we get even more precise printing color spaces like LAB
colorspace.
In LAB, color is described in terms of L for luminance and a and b for color opposites
red/green and yellow/blue.
Middle gray is literally 50% L value which once you do the math comes to precisely 18.42%
CIEXYZ luminance.
However… that is from the print side.
We’re dealing with capturing light.
When Adams clarified how he used the 18% gray card, he recommended that the card be held
so that it was tilted halfway in the direction of the light effectively darkening the card.
Kodak’s instructions when they manufactured and sold graycards recommend that the readings
using the cards ought to be raised a half a stop which effectively put middle gray at
around 12% reflectance.
And 12.8% is what the ANSI PH3.49-1971 sets as the standard for what middle gray is when
it comes to light meters.
Now 18% or 12%, I really doubt most of you watching would find that big a difference
between the two.
The nice thing about digital is we have a whole host of tools to help further make our
decisions.
Tools like Histograms and Waveforms.
Using a gray card is almost like using an incidence meter with a waveform - put the
card in the shot and get the gray card to hit the middle of the waveform.
Keep in mind that 90IRE is pretty much white and 7.5 IRE is black - so your middle gray
card should sit between 38 to 55 IRE - that’s a big range but it comes down to the camera,
the recording format, and your own personal taste.
In the end middle gray is a pretty elusive thing.
I mean look at this image and tell me which tile is darker A or B?
Truth is, perception is a powerful powerful thing.
Well there you have it… expect to see a bunch of photo nerds arguing over this until
the internet ceases to exist.
Of course that would be nothing new.
I’m John Hess, give this video a like, subscribe, hit the bell and considering popping over
to Patreon and becoming a patron.
Until next time, I’ll see you at Filmmaker IQ.com