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Anonymous asked:
Could you please explain to us what light ratios are ? Heard it in one of your videos :)

Ok. Light ratios.

Ratio - the quantitative relation between two amounts showing the number of times one value contains or is contained within the other:

There are two “kinds” of ratios that I consider when shooting. The ratio of flash and ambient and then the ratio of one flash/strobe compared to another flash/strobe in a multi flash/strobe set up.

Imagine shooting a portrait at night in a dimly lit street scene. Your camera is set to f 4 at 250th of a second at ISO 100. Your flash hits the subject, exposes for them, but 250th at ISO 100 is killing all of the ambient light in the street scene behind your subject. Your exposure is 100% flash and 0% ambient. Now let’s say you slow your shutter speed down to 1/2 a second. Now some ambient light is mixing in. Maybe it is mixing in so the final exposure made by the camera is 50% flash and 50% ambient. How do you know what this mix is? By using a light meter. You can sort of guess what the mix is but a good light meter will tell you what the ratio is.

Now you’re in a studio situation. You have one strobe (main, or “key”, light) 45º to the side of your subject. It fires and you properly expose for that light. There is a dark shadow on the side of your subject’s face opposite of that strobe you are using. You can fill that dark shadow in with another light if you want.

How much do you fill that shadow with light? Light ratios!

Let’s say you meter your main light and it is f 5.6. You then turn on your fill light and meter it and it is also f 5.6. You have a 1:1 light ratio. Your main light and your fill are the same exact exposure. The resulting photo shows very little, if any, of the shadow from the main light because you are adding an equal amount of light to the shadow from your fill.

Let’s say you want to keep some of that shadowed area on their face but you don’t want to completely fill it up as in the 1:1 ratio. You can dial your fill down one stop. Your main is still at f 5.6 but now your fill is at f 4. The fill is one stop less than the main. This is a 2:1 ratio. Some say it’s a 1:2 ratio. Either way you are saying your fill is half of your main. 

If you dial your power down to two stops less than your main then you have a 4:1 light ratio. Your main is f 5.6 and your fill is f 2.8. You set your camera to f 5.6. You always expose for the main. Not the fill. The main light is… you know… the MAIN light. Expose for the main. If your fill is suddenly brighter than your main then it’s not a fill any more. It just became the main. The main, or key, light is the brightest light on your subject and you expose for that brightest light. 

Let’s say you have a main (or key) light, a fill, and a hair light and they are all metering in at f 8. That is a 1:1:1 light ratio. You then have all sorts of crazy configurations of 2:1:1 or 1:1:2 or whatever. That’s too much maths for me.

You’ll find photographers who prefer specific light ratios over another. IF I’m going to use a fill, and that isn’t all that often, I prefer something around a 8:1 ratio. I want my fill to be at least three stops under my main. 

Basic ratios of fill to main ::

1:1 = Equal
2:1 = 1 stop
3:1 = 1.5 stops
4:1 = 2 stops
6:1 = 2.5 stops
8:1 = 3 stops 

Note that some folks do different maths and say 1:2 is equal. I’ve never learned it that way and every photographer I ever worked with called a 1:1 ratio an “equal” mix of main and fill. If you search the Interwebs on this topic you’ll find some discrepencies on exactly what is a 3:1 (or 1:3) ratio. I call a 3:1 light ratio a difference of 1.5 stops.

You can only know ratios by using a light meter. Otherwise you just sort of wing it. Fire your fill. Does it look exactly like your main? Then you’re probably around a 1:1 ratio. Are your shadows just starting to get filled with a little light? You’re probably around an 8:1 ratio.

Hope this helps. Mark Wallace has a video on youtube you can watch as well.


ETA - Just answered another question from a reader regarding something about this. You can read it here.

50 Notes

  1. bidonvillestudio said: Perfect. Thanks Zack.