An understanding of “stops” is essential for anyone who wants to be involved in film making, especially in the areas of cinematography and lighting. Stops can seem confusing at first, especially since the word is often used in slightly different ways in different contexts (“stops”, “f-stops”, and “t-stops”).
But it’s really not that hard — it can’t be, or else most camera crews wouldn’t understand it (just kidding!). In this article we’ll take a look at the basics of what we mean by “stops”. For the purposes of this article, I’ll assume you understand the principles of lighting and metering.
There are many different types of units used to measure the actual brightness of lights (lux, lumens, foot candles, etc), but “stops” are not a unit of measurement as such. They are a relative way of measuring brightness.
By “relative” I mean that using stops you can describe a light’s brightness compared with another light source, or measure changes in a light’s brightness. So while it makes sense to say “The brightness of this light is 500 foot candles”, it doesn’t make sense to say “The brightness of this light is three stops”, since you haven’t told us what you’re comparing it to. On the other hand, you can say “This light is two stops darker than that light”, or “Dim that light about a stop”.
So how much brighter or darker is a stop? Simply put, each stop represents a halving or doubling of the amount of light. So if a light doubles in brightness, then it is one stop brighter than it was before. If one light is half as bright as another light, then it is one stop dimmer than that light.
Remember, each stop is a doubling or halving of brightness. So if a light is two stops dimmer than another, that’s another way of saying that it’s four times dimmer, or one quarter the brightness (halved twice). If it’s three stops brighter, then it’s eight times brighter (2x2x2=8).
Here’s a diagram showing this relationship – the middle bunch of lights is our starting light, and the other lights’ brightnesses are described in stops (compared to the start light):
Why do we use this doubling and halving method of talking about light’s brightness? Well, it has a lot to do with the way we perceive changes in light brightness. Simplifying it a tad, humans perceive relative brightness in in an exponential or logarithmic way.
For example, say we have a light and we increase it’s brightness by 100 foot-candles, and make a mental note of the change in brightness. If we wanted to further increase it’s brightness by what appears to be the same amount again, we’d have to increase it by about 200 foot-candles this time.
This would appear to us as the same increase as before, even though it’s actually twice as much. If we actually increased it by the same amount as before (100 foot-candles), we would only perceive it as a smaller increase than before (about half).
Seems a little confusing at first, but just remember this — each stop difference in light levels is either double or half the amount of light. So how do we use this on a shoot? See the next article on f-stops and t-stops (coming soon).
Originally written for About.com’s Filmmaking page.