Day 20, Learning 20: Light Ratios

Whew, 20 days into my project.  Overall, it's been pretty energizing.  I still recommend everyone grab an empty journal and start their own journey.  There's not a photographer out there who couldn't benefit from 30 days of learning.

Ages-o-go back on Day Two I wrote about Reciprocals, the relationship between aperture, shutter, and ISO.  The Big Three.  But this is just one side of the Rubik's Cube.

Let me wax philosophical for a moment.

If you spread 20 great images on a table you wouldn't be able to tell if they were shot with a Nikon or a Canon, at 16MP or 30MP, or whether they spent $100 on their lens or $1000.  But you would be able to describe how the light created the image-- it's location, size, continuous or flash-- because the light is more apparent and important.  Camera makers barrage us with advertising to draw attention to their product, but if we're going to be spending money (and mind you, I never encourage that) it should be on light.  We should be learning how to sculpt the sun, because you can.

I'll come back to the Rubik next week, but for today it's about the final effect, understanding light ratios.  The "light ratio" in simplistic terms is the relationship between the lit and shadow areas.  This is true if we ignore the background, because we are typically referring to the ratio of light on the subject.  So a ratio of 1:1 means that there is no difference.  They are evenly lit.  A ratio of 2:1 (key to fill light) means twice as much light (or the equivalent of an f-stop) is striking the bright area compared to the "dark" area.  This seems like a big difference, but it's actually quite subtle; a ratio of 3:1 is traditional for classic portraiture.  16:1 is in the realm of "noir," where the shadow is almost devoid of detail.

Certain light ratios produce a more masculine or feminine look.  1:1 is flat and boring for both genders.  A 2:1 or 3:1 ratio has a very open, gentle feel that works well for more "feminine" portraits.  The higher contrast of 4:1 or more brings out a darker, more closed quality that tends to feel masculine.

Now consider the "equivalent of an f-stop" remark.  If you were to measure ratios with a light meter it would translate thus:

1:1  =  f/2.8: f/2.8
2:1  =  f5.6:f/2.8 (or if f/2.48:f1/4, which is the same ratio)
4:1  =  f8:f/28

And so forth.  Why is this helpful?  Because if you know what look you're going for, you can adjust accordingly.  And it's not just the Big Three.  Say you want a fairly classic portrait look for the company boss.  He's male, very masculine featured, so maybe that's a 4:1.  You've got your fancy TTL flash system so it'll do all the thinking for you, right?  Nope.  The TTL function will give you the right light for the key, but you'll have to tell the second flash what's right for the fill.  Since you know that it's a two-stop difference you can adjust your flash's exposure compensation to -2. 

Or maybe you couldn't afford the pricey TTL flash?  Set one flash to 1/2 power and the other to1/8 power and keep them at the same distance from your boss.

Light Ratios seem to be "beginner" level material, but they're really the entry into an advanced understanding of lighting.