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Posted in : Featured / Photography Principles on February 3rd, 2012.
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One of the questions I most commonly see from new photographers, and one of the things that seems to confuse them the most, is what a stop of light is exactly (understandable though, once you break it down and realise that everything measures stops with a different set of numbers), and how it affects their decision making process when it comes time to capture an exposure. So, here I shall attempt to explain.

It is important, right at the very start of this, to understand what it is not. A stop of light is not a fixed quantity. How much a stop is all depends on how much light you have to begin with.

It is also important to note that there’s quite a lot of information below, for what is a fairly simple and straightforward principle, but once you start to soak it in, it just comes naturally and you’ll be adding and taking away stops of light from your exposure without even thinking about it.

So, what is a stop of light?

Put simply, a stop of light is either a doubling or halving of an amount of light, depending on whether you’re adding a stop or taking it away. This is why, when you have one light at a set distance from your subject, adding another identical light at the same position increases the output by a stop, but adding two does not increase it by two stops.

The fact that it is a ratio of light also explains why changing your ISO, aperture or shutter speed all have a very predictable effect on the images you shoot.

Let’s assume, for a minute, that we have a constant light source (LED, tungsten, compact flourescent, whatever, it doesn’t matter) that outputs X amount of light at maximum power. Exactly how much X is doesn’t really matter, as long as it remains consistent throughout the example.

If our light is set to maximum power (X), and we want to add a stop of light, we can simply add a second identical light. We are now shooting at a power of 2X. If we now wanted to increase this amount of light by another stop on top of that, we would have to add two more lights. Remember, when we add a stop, we double the amount of light, and double of two lights is four lights. We are now shooting at 4X power, or 2 stops brighter.  If we have one light at maximum power and we want to add 4 stops of light, we now need 16 lights instead of our original one!

If our one light is currently at maximum power, and we want to take away a stop of light, we simply dial it down to half (1/2 power). If we want to take away a second stop of light we dial it down to half of a half – otherwise known as a quarter (or 1/4 power).

The same principle applies to flash & studio strobes too. A Nikon SB-900 speedlight at maximum power is nowhere near the output of something like a Bowens Gemini 500 Pro at maximum power, but if you want to dial down the SB-900 by a stop, you cut the power in half. Similarly, if you want to dial the Bowens down by a stop, again, you cut the power in half.  If you’re already at maximum power and want to add a stop, you add another identical light and set them both to maximum power (or replace it with a more powerful light!)

Moving the lights closer to or further away from your subject will also affect the amount of light hitting your subject, but that’s for a whole other article. :)

Ok, I’m with you so far, but what about shooting ambient light?

If we’re shooting ambient light, and we are unable to use flashes or continuous lights to augment the light that’s available, we are at the mercy of what is around us or our subject. So, how do we add or remove stops of light in this instance?

Well, there are essentially three ways common to most all cameras out there. There’s the ISO (or ASA as it was often referred to in the film days), shutter speed, and your aperture. All of these methods of adding or subtracting a stop of light are a doubling of halving of the light that’s recorded.

ISO is the simplest to get to grips with. If we want twice as much light, we simply double the number of the ISO. If we want half as much light, we half the ISO. So, if we’re shooting at ISO400 and we want an extra stop, we switch to ISO800. If we’re at ISO400 and we want to lose a stop of light we switch to ISO200.

ISO/ASA on film was a physical property of the makeup of the film and its coatings. In the digital world, ISO is essentially just a different way for the camera to interpret the light hitting the sensor in order to give the appearance of more or less light. Every DSLR has a “native” ISO, which varies from body to body, and all other ISOs the camera is capable of are essentially simulated, based off the readings at this native ISO.

Shutter speed is a little bit different as speeds are often rounded off in order to simplify the numbers (going from say, 1/8th of a second to 1/15th of a second instead of 1/16th).

If we have a base shutter speed of 1 second, and we want to add a stop of light, we simply double the duration the shutter is open, so we set our shutter speed to 2 seconds. If we want to lose a stop of light, we half the amount of time the shutter is open, and we set it to 1/2 a second.

If we’re have 1/250th as our base shutter speed and we want to add a stop of light, we go to 1/125th of a second.  If we want to lose a stop of light, we go to 1/500th.

Aperture is probably the most confusing to many, as the numbering system used doesn’t follow the typical doubling & halving rule (it sort of does, but not directly). Expand the box below for a full explanation of why the numbers are they way they are, why it makes sense and why the numbers also seem to go the wrong way.

Click to ExpandAperture f/numbers explained (Warning: Contains math n' stuff!)
When you look at a lens, it typically has an f/number associated with it (or perhaps two numbers for zoom lenses). This number is the maximum aperture of your lens at a given focal length.

Let’s go with some easy numbers and have a look at an old classic, the Helios 44-2, which is a 58mm f/2 lens.

The focal length of this lens is 58mm, and the maximum aperture (the most amount of light it will let in) is f/2. The “f” part of f/2 means the focal length of the lens in mm, “/” means to divide by. When we divide the focal length by the number following f/ we are given the diameter of the lens’s aperture. As our lens’s aperture is typically circular (or something close to a circle), the amount of light it lets in is based on the total area of that aperture opening. Double or half the area of the aperture, and you double or half the amount of light able to travel through the lens.

If we take our 58mm lens at f/2, our aperture has a diameter of 29mm. If my old high school maths serves me correctly, you get the area of a circle using the formula π. If our diameter is 29mm, then our radius is 14.5mm. According to the π equation, our aperture has an area of approximately 660mm².

If we now turn our aperture down one stop from f/2 to f/2.8 the result of the equation changes and the area of our aperture opening is now around 337mm². If we stop down to f/4, we get 165mm², f/5.6 is 84mm², f/8 is 41mm², and so on.

As you can see, each time we stop down the lens, the area of the lens is approximately halved (give or take a few mm²).

This is why, unlike ISO & shutter speed, the lower numbers let in more light, because you’re dividing the focal length by a smaller number and increasing that final value in the equation.  It’s also why the numbers don’t double and half every stop as they do with ISO & shutter speed (although the area of the aperture opening does!)

As the amount of light traveling through the lens is a function of the focal length and aperture combined, this also means that f/ numbers are consistent across all lenses (in theory anyway – in the real world, manufacturing tolerances may mean that they vary a little from lens to lens). But this essentially means that f/2.8 on a 24mm lens would let in exactly the same amount of light and give you the same exposure as f/2.8 on a 50mm, 70mm, 105mm, 200mm or 300mm lens.

Right, that was way too complicated, is there an easier way to understand this?

Here’s the short version. This is a set of numbers that you should learn and become familiar with.

f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32.

f/1.4 is our brightest setting in this list, and f/32 is our darkest setting. Not all lenses go as bright as f/1.4, and not all go as dark as f/32 (and some lenses go brighter than f/1.4 or darker than f/32).

Each of the numbers in that list is either a stop brighter than the number than it’s neighbour on the right, or a stop darker than its neighbour on the left.  If, for example, you’re at f/4 and you want to lose a stop of light, you go to f/5.6, if you want to lose two stops, you go to f/8. If you’re at f/4 and you want to increase the light by a stop, you open up to f/2.8, and if you want to increase it by two stops you go to f/2 (assuming you have a lens capable of it).

Your camera may show other numbers in between those, these are either 1/2 stop or 1/3rd stop increments.  For now, you don’t really need to worry about those, but if you understand what’s been said above, you’ll probably understand that too. :)

Cool, got it!  So, is there anything else that can affect the amount of light my camera sees or records?

Why, yes, there is.  How fortunate you asked!

Pretty much anything that you put in front of a lens or a light source will affect the amount of light that is ultimately received by your camera.  For the sake of consistent and predictable results, however, let’s have a look at neutral density, or ND filters (and please, no arguments on whether they technically are or aren’t classed as “filters”).

One thing you need to remember about ND filters is that they can only take light away, they cannot increase the amount of light put out by a light or seen through your lens.

Why might I want to use one of them then?

There are two common situations where you might wish to use an ND filter.  One is the ability to shoot long exposures, and the other is to allow you to shoot in very bright conditions with a wide aperture in order to get a shallow depth of field.  In either situation, without ND filters, your camera could be seeing far too much light and just result in a pure white image with everything blown out.

ND filters work on exactly the same principle as above.  If you put a 1 stop ND filter over your lens, your sensor sees 1/2 the amount of light through the lens.  If you use a 2 stop ND filter, it sees 1/4 of the light, a 3 stop ND is 1/8th of the light, a 4 stop ND is 1/16th and so on.  Commonly available commercial ND filters go all the way down to 10 stops, which is 1/1024th the amount of light – which would allow you to shoot long exposures at very small apertures even in the brightest sunlight.

If you are using flash, and you’re already at minimum power, ND gels will also cut your flash power by even more.  SB-900 speedlights go all the way down to 1/128th power, that’s a reduction of 7 stops below their maximum power output.  If you’re shooting macro and your flashes are very close to your subject, even lowest power might be too much.  A 1 stop ND gel over the end of your flash will essentially give you 1/256th power.  A 2 stop ND gel will bring you down to 1/512th power.  You add however much you need to prevent the flash from blowing out your subject.

 Anything else?

Yes.  As mentioned above, pretty much anything you put in front of your lens will affect the amount of light the camera sees.  This includes almost any filter you put in front of your lens, although things like UV filters reduce the light by such a small amount it’s not even worth mentioning, especially high end top quality UV filters.

Circular Polarising filters (CPL filters), on the other hand, can decrease the amount of light your camera sees by as much as 1-3 stops.  Typically, the more expensive and higher quality CPL you purchase, the amount of light lost is reduced.  Sometimes this is an advantage, sometimes it is not.

And there you have it.  As mentioned at the top of this rather long article, it’s a pretty straightforward principle that can look very confusing until you figure out exactly what it is, what affects it, and how.

Many of the things mentioned above deserve complete articles all on their own, and most of those things will be elaborated upon in future posts, so stay tuned!