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Sunday, December 27, 2015

••◊ 18% Gray Cards and Camera Exposure

With my recent interest in log gamma curves I've been having to do some self-education on the meaning of 18% gray cards.  One of the issues I've run into is that I can't justify the cost of true cinema lenses, so I'm using Zeiss ZF.2 lenses.  I like the sharpness of the glass, but the lack of aperture measurement in T-stops makes getting accurate exposure with S-log a real chore.  Those lenses also make my life difficult because I can't just use a factory calibrated light meter.  See my previous post and tutorial video about why this is.

The first thing I learned is that 18% gray became a reference standard by accident.  The history of it goes something like this...  Kodak originally produced R-27 reference cards in the 1970's with the instructions that you were supposed to take an exposure reading off the card, then open up the aperture 1/2 stop more.  Somehow Kodak's manufacturing organization lost those last instructions for 20-some odd years, so a whole generation grew up believing that the exposure reading was exactly what an R-27 card read in your exposure meter - making everyone's readings 1/2 stop off!  Then it seems video cameras came along and adopted a "standard" that didn't exist.

The "official" ANSI reference is really 12.5% gray, which is 1/2 stop brighter than an 18% gray card.  One thing I forgot to mention is that 18% gray is a rounded number.  An 18% gray card is really 17.7% gray.  The difference between the ANSI/Kodak standard and the vendor specific adoption of a "18% gray standard" is where the confusion lies.  There are much more notable people on the web who have written longer diatribes on why this mythology is confusing...here, here, and here.  I recommend reading all three links to get a more in-depth understanding - after you finish my article!

So as a personal educational experiment I took my Sekonic L-758DR light meter and measured both an incident meter reading of sunlight as well as a reflected reading off an 18% gray card.  They matched exactly.  So my light meter appears to be calibrated to 18% gray.  If it was calibrated to 12.5% gray then the reflected reading would be 1/2 stop more closed than the incident reading.

The good news is that a Kodak R-27 card (18% gray) is still very useful if you use it correctly.  An 18% gray card is also perfect for calibrating your light meter to the particular camera system you own, as vendors shift around the meaning of "middle gray" quite significantly.  Middle gray can mean anywhere from 32-50 IRE in my recent experience, whether you're using rec.709 or a log gamma.  It's camera manufacturer specific.  So while the strict mathematics works out to 44 IRE for 18% gray (assuming 0-108.5 IRE range), the camera manufacturer may choose to put 18% gray at 50 IRE, for instance.  I see a part 2 and 3 of our camera exposure tutorials already!

So here's the other experiment I did.  I took both the Lastolite and R-27 card and measured their actual reflectance using a Gretag Macbeth SpectroEye.  This color science instrument accurately measures color and is used in the print and textile industries.  First, let's look at the R-27 18% gray card.


I know the instrument is measuring in LAB color space, but just stick with me here.  I created a spreadsheet to calculate the reflectance from the L*a*b* numbers.  If a* and b* are 0 this would be an absolutely neutral reference since those are the color coordinates.  So we can see here that the R-27 card is pretty much perfectly neutral.  The L* value of 49 translates to a reflectance of 17.6%, which is almost perfectly the 17.7% reflectance target for an "18% gray" card.  So this inexpensive purchase from Amazon is actually quite good quality, despite being painted on cardboard.  This is a reference chart I can trust to get any future measurements right.

The next experiment I did was with the Lastolite Ezybalance fold out 18% gray/white "card".



This 18% gray "card" is nice to have since it folds up for travel.  Here we can see that the gray side measures an L* value of 45, which translates to a reflectance of about 14.5%.  That's a difference of about 0.3 stop between it and 17.7% gray.  So any reflected light meter exposure measurements will be about 0.3 stop too bright - which is nothing really unforgivable when using a log gamma curve such as S-log3.  It might even be a good thing considering that some of the new Sony cameras have dark noise that could benefit from a bit of over exposure.  If you're really worried about it you can either subtract 0.3 stop from your exposure measurement, or set an offset in your light meter (Sekonic lets you set an offset value).  One other minor issue is that the fabric material appears to be just slightly blue from the a* and b* measurements - but this is just slight.  Probably nothing significant to fret about.  Still, this isn't as dead-on perfect as the R-27 cards.

There's also the option of using the white side of the chart as a reference since camera manufacturers specify both the 18% gray and 90% white targets (at least Sony and Canon do).  We can see here that the white side has an L* value around 90.4.  This translates to a reflectance of 77%, so while we could use this side for white balancing, it appears to be too dark to use as a 90% reflectance chart.

This is all I have for now, but I'm sure we'll be producing some more camera exposure tutorials in the next few months.  I know these items were confusing to me at some point, so there's a need for clear in-depth information.

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