There is so much duality in photography. On one hand, it’s the light and the subject, it’s the story we tell and the story the viewer sees, it’s a feeling, an emotion, a state, a symbol, a metaphor. Sounds poetic, doesn’t it? On the other hand, it’s pure science, every single bit of it – from the said light traveling through a complex lens design, all the way to the scene being imprinted whether on a piece of light-sensitive film or, temporarily, on a digital sensor. And that scientific part of photography brings all sorts of terms with it, terms that may not be necessary for the creative process, but as far as skillful execution goes, you can’t do without understanding them for very long. A painter needs to know his brushes at some point, right?
And so we are back to covering basics, something you surely must have noticed. In this article, I will talk about yet another, a confusing-at-first-encounter term used in photography, more specifically – exposure stops. I will try to explain what they are and how stops of different exposure triangle parameters – shutter speed, aperture and ISO sensitivity – correlate, as well as give you examples of what are considered to be regular stop values of each parameter, and what are full, half and third-stops.
Let’s Start From the Start
As most of you know, how much light or information a digital sensor or film receives during exposure to light (capture of an image) depends on three things – the shutter speed, aperture size and light sensitivity of the surface on which the image is captured. More than that, every one of these parameters is exactly as important.
To make them directly comparable and to be able to compensate a change in one parameter with a change in another easily, something common had to be found between how long the light-sensitive surface is exposed to light (shutter speed), how much light is hitting the sensor at any given moment (aperture) and how sensitive the surface is to light in the first place (ISO value). A number, a measuring unit needs to be assigned. In other words, there has to be some sort of correlation between the three parameters, where a certain increase of one must equal a certain decrease of another in order to preserve the same overall exposure or brightness of the photograph captured.
Now, I say “needs to be found”, but it’s really quite obvious. You see, if you expose a piece of film of a certain sensitivity through an aperture of a certain diameter for, say, one second, and then expose another piece of film under the same circumstances of the same sensitivity through the same aperture diameter for two seconds, the second piece of film will receive twice more light, simply because it was exposed to light two times as long. Likewise, if you expose two identical pieces of film under the same lighting conditions for one second, but use a twice-as-big aperture (area-wise) for one of the pieces, that piece is, again, going to be twice as bright as the first one. Finally, if you expose two pieces of film through the same aperture for one second under equal lighting conditions, but with one piece of film being twice as sensitive to light as the other, the more sensitive piece of film will contain an image that is – you guessed it – twice brighter.
Have you noticed a pattern? Increasing any one of the parameters twice increases the amount of light hitting the surface twice (and, although technically changing sensitivity does not change the actual amount of light hitting the surface, it still has pretty much the same effect on exposure). And it doesn’t matter which parameters you change for the two pieces of film, too – increasing the size of the aperture twice for one piece of film is the same as exposing another piece of film for two times as long, the resulting exposure of the image, all else being equal, will be the same. It’s not like compensating a 2x increase in exposure time requires a 7.4-or-any-random-number times smaller aperture, correct?
That is the correlation we are looking for and a clear answer to what exposure stops are. So, a stop is two-times increase or decrease of light gathered during exposure. Adjusting any one of the three exposure parameters by one stop results either in twice more or twice less light captured. As such, a stop is a very convenient way of relating three different parameters that have different measurement units assigned to them by emphasizing not the measuring units, but the effect on exposure.
Making Sense of Numeric Values
Now that we know what a stop is in theory, it’s time to get acquainted with the numeric values and learn to compensate a change in one parameter with a change in another.
To those of you who are yet unfamiliar with the definition of aperture, or f-stop, in photography, reading our article on the subject is the very first step to take before continuing. Simply put, aperture is the opening that the light goes through before reaching the sensor or film. The size of the aperture (its diameter) is controlled with diaphragm blades. The lower the numeric f-stop value, the larger the aperture, the more light goes through at any given moment and vice versa.
Aperture size is defined by the so-called f-stops (as I’ve already mentioned, the lower the f-stop number, the larger the aperture opening). Now, the physical size of the aperture depends on the focal length of the lens as well as the actual f-stop, but for the purposes of this article that is largely irrelevant. What is important is that, in order to double the amount of light coming through, the area of the opening and not the diameter must be doubled. That is why calculating numeric aperture stop values are a tiny bit more difficult than that of shutter speed or ISO sensitivity, as you will see, and memorizing the numbers is perhaps more practical (if, arguably, unnecessary in most cases). Just as with shutter speed and ISO, there are certain f-stop values that are considered to be “default”, “round” or “standard”. Here is an illustration showcasing standard full-stop, half-stop, and third-stop values as well as a graphical representation of the size of the opening itself:
The illustration shows standard full-stop apertures values ranging from a very-large f/1.4 to really-rather-tiny f/32, with f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16 and f/22 in-between the two values. In total, the diagram spans the range of 10 full stops, but that does not mean that is all the stops you get. One stop wider than f/1.4 is f/1, go further than that and you will reach f/0.7, which is extremely large. Lenses with such parameters are extremely rare and exotic, however, so including them in the illustration really is not necessary. The same goes for the other end of the scale where aperture size gets smaller – f/44 or f/64 (not to mention even smaller apertures) are hardly ever applicable in today’s photography and there are few lenses that even allow such a setting (those that do are mostly designed for medium or large format film cameras).
Given that a difference of one-stop results in twice more or less light going through, calculating the difference in the amount of light going through between the two extremes of the scale is quite simple – f/1.4 opening lets in twice more light than f/2, eight times more light than f/4 and 512 times more light than f/32, at any given moment. Yes, that is a lot.
Shutter Speed Stops
I will not go into too much detail when talking about shutter speed and what it is, exactly – we already have a great article for that purpose and if you are yet unfamiliar with the term, I recommend you read it before continuing. In short, shutter speed defines the period of time during which light is allowed to go through the optical element (the lens) onto the light