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, 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 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.
Aperture Stops
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 is 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, third-stop values as well as 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 sensitive surface (a digital sensor or a piece of film). Think of a beautiful mansion with a fence and a gate where a party is hosted: the sensor or film is the mansion itself, the light – guests flooding through the gate into the mansion, aperture – the width of the road leading to the gate while shutter speed is how long the gate remains open and allows the guests through so as to enough guests attend, but the place does not become overcrowded.
As you would imagine, shutter speed is measured in seconds or, rather more often, in fractions of a second. Much like with aperture, there are standard full-stops, half-stops and third-stops. Here is an illustration showing a shutter speed range of 10 stops with values from one second all the way to 1/500th of a second:
Notice how the first shutter speed marking of the scale has a quotation mark – 1“. The quotation mark means that the marked shutter speed in in seconds and not fractions. It is necessary for distinguishing purposes because fractions are not marked as fractions – so the 250 stop is actually a shutter speed of 1/250th of a second. All this has been done purely for convenience.
It is also important to understand that, with today’s digital photography, shutter speed scale is much larger than the ten stops shown in the illustration – photographers often use speeds as low as 30″ (seconds), which is 5 stops slower than 1″ shutter speed that I started with; and as high as 8000 (fraction of a second), which is another 4 stops higher than 500 shown in the graph. Certain cameras can go higher still. In other words, the range of practical shutter speed stops is much wider than that of aperture. Also notice that calculating stops is much easier with shutter speeds than it is with aperture – you just have to multiply or divide the number by two to get the value of the next or previous stop.
ISO Sensitivity Stops
Finally, we get to the last exposure parameter and its stops. As with the other two parameters, we have a comprehensive article on the subject – there is a lot to know about ISO sensitivity. In this article, however, we are talking about exposure stops, so going too in-depth is not necessary. For our purposes, the following explanation is quite enough (but I do suggest you read that article if you want to learn more) – ISO is the level of sensitivity of your camera sensor (or film) to available light. The lower the ISO number, the less sensitive it is to the light, while a higher ISO number increases the sensitivity of your camera. The component within your camera that can change sensitivity is called “image sensor” or simply “sensor”. It is the most important (and most expensive) part of a camera and it is responsible for gathering light and transforming it into an image. With increased sensitivity, your camera sensor can capture images in low-light environments without having to use a flash. But higher sensitivity comes at an expense – it adds grain or “noise” to the pictures.
Ever sensor has a “base” ISO – a value when it does not need to increase or decrease its sensitivity to light. Some cameras start at ISO 200, like older Nikon models and current Fujifilm mirrorless cameras. Others start at (arguably more preferable) ISO 100, and that is where our ISO sensitivity stops illustration starts:
As with shutter speed stops, calculating ISO sensitivity stops is very easy – one just needs to divide or multiply the value by 2 to add a stop or go one stop down. The standard stops are also very easy to memorize. While most modern cameras can go lower than ISO 100 or higher than 51,200, because of the limitations that are imposed when increasing sensitivity too much, the illustration shows the most useful and usable ISO stops with the last two or three highest full-stop values bordering on hardly-ever-used for most photographers.
As with the previous two illustrations, this one also shows half- and third-stop values. I’ve not talked about them yet for one reason – there is a caveat.
Standardization
If you take a look at one of the illustrations again, but focus this time not on the full-stop values, but rather on half-stops and third-stops, you will probably notice that the scale contains some inconsistencies. For example, half-stop between f/11 and f/16 is f/13, but then the first third-stop is also defined as f/13. How could it be, if a third is less than a half? Shouldn’t the value be less, too? Well, theoretically, yes. But from a practical standpoint, manufacturers chose differently.
You see, only the full-stops are completely standardized and manufacturers are doing their best to stick to such full-stop values, be it the ones you see in the aperture, shutter speed or ISO sensitivity illustrations. But for third- and half-stops, some rounding up was unavoidable. More than that, different camera manufacturers choose to round up differently, too. For example, the first third-stop between f/5.6 and f/8 on my Nikon D700 is specified as f/6.3, as shown in the illustration, but my Fujifilm specifies f/6.4 instead. In this particular case the margin of error appears because, mathematically, you need to multiply an f-stop value by the square root of 2 to add a stop to it, and square root of 2 is not a round number (1.414 is just the start). So, theoretically, the third stop between f/11 and f/16 should actually be marginally less than f/13, while half-stop should be marginally more. For convenience, manufacturers round up the values, and so the marked values are the actual values used during exposure. In practice, this is hardly relevant. Bottom line is, third- and half-stops may appear differently in your camera than in these illustrations. That is perfectly normal.
Compensating One Parameter Change With Another
All three of the parameters have exactly the same effect on exposure. Increasing either one by one stop will let in twice more light to the sensor (not strictly true with ISO sensitivity, but you get my meaning), while decreasing will have an opposite effect. This also means that an increase of one parameter by a stop can be compensated by decreasing another by the same stop, or two others by half-stop each. So, for example, if you are doing some sports photography and coming up with slightly blurry images while your current settings read f/5.6, 1/250, ISO 400, you need to speed-up the shutter to capture quick motion more crisply, but keep the overall exposure the same. What’s the best way to do it? Provided that the lens allows this in the first place, opening up the aperture by one stop to f/4 and bumping up the ISO value by another stop to ISO 800 will let result in four times more light coming in, which in turn gives you two full stops to compensate with shutter speed and normalize the exposure of the image. So by opening up the aperture by a stop, increasing sensitivity by a stop and increasing the shutter speed by two stops as a result of that, you end up with f/4, 1/1000, ISO 800, a sharply captured images and a correct exposure, at the expense of slight increase in noise and slightly shallower depth of field. There is always some trade-off, but it’s not always a meaningful one.
From Theory to Practice
Now that we have the whole theory before our eyes, time to do some basic experiments to see if it really does work that way. Coming up with unpretentious photographs for illustrative purposes proved to be simple – I found an immeasurably cool, green and old Land Rover Defender to show what a roughly two-thirds of a stop and over one stop of difference in shutter speed does to exposure, and some archways to help me out with compensating a change in one setting with another. Let’s start with the latter.
The following image was taken at f/1.4, 1/800, ISO 200:
X-E2 + XF23mmF1.4 R @ 23mm, ISO 200, 1/800, f/1.4
Here is how it compares to the same scene (don’t mind the slight framing changes, I shot hand-held) captured at f/2, 1/800, ISO 400:
As you can see, after compensating a one-stop increase in ISO sensitivity with one-stop decrease in aperture size, the overall exposure (or brightness) of the photograph remained exactly the same. Even though closing down the aperture by a stop ensured twice less light was making its way onto the sensor, a one-stop increase in sensitivity did the exact opposite. Now, let’s make a comparison with shutter speed change:
The before image was captured at f/1.4, 1/800, ISO 200, while the after image was captured at f/1.4, 1/1600, ISO 400. Again, compensating a one-stop faster shutter speed with a one-stop increase in ISO sensitivity proved to render the exact same exposure.
It is important to understand that, in practice, photographers (and the cameras, too) rarely stick to full-stops. It is not too likely that the correct exposure for a particular scene will require standard full-stop values from ever parameter, is it? And so the sample images that you will see now will showcase larger or smaller adjustments than a full-stop.
A side note: I took these sample images without using a tripod. Any weird perspective changes you might notice are a side-result of using Photoshop’s Layer Align tool.
The following photograph is what I (and the Fujifilm X-E2) consider to be well-exposed:
X-E2 + XF23mmF1.4 R @ 23mm, ISO 400, 1/680, f/1.4
Now let’s see what a shutter speed change from 1/680 to 1/1700 (roughly 1.3 stops of difference) does to the brightness:
The difference is significant – the sensor definitely received over two times less light due to the shutter being open for a much shorter period of time. Now let’s see what happens if we decrease the shutter speed by two thirds a stop instead – from 1/680 to 1/420:
The difference is, again, clearly noticeable, although nowhere near as prominent as before. Still, this proves that even two thirds of a stop is a lot. What it also proves is that “correct” exposure is very subjective – one could prefer any of the three images in terms of exposure.