Ant Smith

Photography

Exposure Primer - or how we draw with light

These primers are intended to provide overviews to the core fundamentals of photography. They're best read kind of quickly at first, then revisited later as practical experience starts to make things click into place. They cover stuff that will become second-nature - which means they cover stuff people won't generally think to bother mentioning! I hope they prove useful to somebody. Someday.

In this primer, you will read

  • The nature of photographic exposure

  • Exposure controls

  • Exposure Stops

  • Exposure Values

  • Choosing exposure settings

  • Exposure modes

  • Exposure compensation control

You must have heard that 'photography' literally means 'drawing with light'? Well, leastways, you have now. Fundamentally then, you need to understand how to control light. Managing and supplementing available light is something we'll look at on another day. Here I'll explore controlling the light that gets into the camera and allows us to record an image. However it gets there...

You may be using a camera with either a digital sensor or maybe a roll of film. There are some practical differences, but the basic exposure control mechanisms are the same. If you're using film then you're probably a little ahead of the curve already and can make sense of what this article covers; so I shall continue specifically thinking about how we control the application of available light in a digital camera.

There are 3 things to think about:

  • Sensitivity - how much light it takes to record an image

  • Transmission - how much of the available light we let the sensor see at any given moment

  • Duration - how long we let the sensor see the light

If there isn't much light available (ie. it's dark) we can still manage to 'draw a picture with it' if we:

  • Make the camera more sensitive to the sensor's signals, OR

  • Make sure we transmit more of it to the sensor, OR

  • Let the sensor see what light there is for a longer time

Conversely, there can be way too much light for drawing with - overwhelming the camera's sensor so we end up with something that's basically just a white image. As you might expect, in these cases we have to:

  • Make the camera less sensitive to the sensor's signals, OR

  • Block-out some of the light from the sensor, OR

  • give the sensor only the briefest of glimpses of the scene

Or some suitable arrangement of those three control mechanisms.

Control Principles

Understanding Sensitivity

I could draw you a picture here of a sensor array and its associated amplification circuits, which would be terribly exciting but perhaps not so useful. Suffice it to say that the camera can be made more, or less, sensitive to the signals the sensor creates from the light it sees by virtue of electronic amplifiers. Like any amplifier, if you turn them up too far you will hear hiss and/or hummmm. Which you'll see in the photograph as 'noise'.

The sensitivity is controlled by the camera's ISO setting. This image shows two quick shots I took of my computer screen. On the left I used a low ISO value (not very sensitive, limited amplification of the sensor's signals) and on the right the highest possible ISO value (extremely sesitive to light, a lot of amplification of the sensor's signals).

Showing effect onnoise of ISO setting

These are not the most interesting photos I have ever taken, but you should be able to see the effect of 'noise' in an image quite well.

There's stuff we can do about noise, which is just as well since every photo has some of it, but it's best to keep things nice and clean at every stage if we want good results at the end of the day.

"How low can you go?"

"humm, about ISO200, maybes 100"

Ya Rick& KDDK Remix, kinda, but not really.

Keep the ISO setting low.

By the way, there are a LOT of geeky pedants in the world of photography who will scoff at the idea that we can 'change the sensitivity of the sensor'. Because we can't really, the sensitivity is fixed by virtue of the construction of the sensor (the size of its pixels). But we can change how much the sensor's signal gets amplified. Which, once you appreciate the pitfalls of signal amplification (noise!), amounts to exactly the same thing.

The same pedants will be horrified that I beseech you to keep the ISO setting low. We'll investigate this a little more in the next article - but it's actually a really fine rule of thumb to start with.

The ISO range varies from camera to camera but 200 to 64,000 is the least you'd expect to see these days. My D750 provides an ISO range of 50-512,000. Seems like quite a lot. Except that, its a geometric scale. Each doubling of the ISO doubles the sensitivity/amplification.

What that means is the camera can record an image at ISO200 with half the light needed to record the same thing at ISO100.

Turn the ISO up when things get darker.

There's one more little fine-scale detail to be aware of... strictly speaking 'exposure' of a light sensitive medium (such as a frame of film or a digital sensor) is determined as a quantity of light striking the medium for a duration of time. Which has nothing to do with the sensitivity of the medium.

Intensity X Time = exposure

What the camera makes of a light-exposure is affected by the sensitivy, but the exposure per se is not. The exposure happens when the shutter release is pressed. The amplification of the resulting sensor reading (in-line with the ISO setting) happens immediately after the exposure is taken - as part of the process that transfers the sensor reading to the image file. When we think of the camera as a complete system, it is a fair and reasonable simplification to think of 'sensitivity (ISO)' as part of the exposure control - so we will do that. But do bear this distinction in mind, especially if you're reading around the subject elsewhere, as some highly pedantic forums get very confusing in this matter.

exposure X sensitivity = lightness or darkness of the resulting image

We'll also find this distinction is important when we look in detail at 'noise', but until then let's look at the camera as a 'system' where intensity, time and sensitivity determine the resulting lightness or darkness of the image.

Understanding Transmission

How much of the available light we let the sensor see is controlled by the camera lens. Or more specifically by the lens aperture mechanism.

Inside the lens there are several metal blades. These overlap to create an aperture at the centre through which light can pass:

Showing the internal workings of an aperture mechanism

These blades are fixed to a ring which when turned increases, or decreases, the amount by which they overlap - and thus the size of the central aperture that they create.

In this way we control how much of the available light gets through to the sensor.

To be fair the aperture mechanism represented here is pretty crumby; although representative of many a cheap camera lens. Better lenses will use 9, or even more, blades and will shape the blades to make the aperture more circular. Which, trust me, is a good thing.

To control the light accurately and repeatedly we need a scale for this adjustment of the aperture size.

Why These Numbers?

Showing how to caluculate area of a circle
  • An f-stop is calculated by dividing the focal length of the lens by the diameter of the aperture (f/2 in a 50mm lens has an aperture of 25mm)

  • Each f-stop transmits half the light of its predecessor (f/2.8 is one stop darker than f/2).

Which is achieved by halving the area of the aperture Half the area, half the light!

This weird series of numbers arise from how we calculate the area of a circle.

A = π * r2

For our 50mm lens set at f/2 with an aperture diameter of 25mm, the area of the aperture is given by

3.141592 * (25/2)2 = 491mm2

Half this area is 245.5mm2, which we can back-calculate in the same formula to give us a diameter of 17.7mm.

Dividing the focal length (50mm) by this diameter gives us f/2.8 - one stop darker than f/2.

In fact, each f/stop turns out to be a power of the square root of 2 (which is roughly 1.4).

(1.4)2 = 2

(1.4)3 = 2.8

(1.4)4 = 4

(1.4)5 = 5.6

etc...

The numbers on an f-stop scale are not exactly 'user-friendly' - but at least now they are a little bit less mysterious, I hope!

The obvious scale to use might be the diameter of the aperture, but because we want to use the same scale for all lenses, the diameter of the aperture isn't very helpful. So instead we use a scale derived from dividing the focal length of the lens by the diameter of the aperture. There's good science behind this but the upshot is that an aperture setting of f/2 has the same effect in terms of blocking out light whatever the focal length of the lens.

Classic values for an aperture scale are f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22

Notice how they are written: 'f' divided by... this is just a convention but it reminds us of how the numbers are calculated (focal length / aperture diameter) so it is easier to remember that larger numbers mean smaller apertures.

To reduce the amount of light reaching the sensor increase the aperture number towards (or beyond) f/22.

To allow more of the light to reach the sensor reduce the aperture number towards (or below) f/2.

These numbers are called f-stops. Each f-stop (or commonly just each stop) represents a doubling or a halving. So f/4 transmits half the available light that f/2.8 transmits, but double the available light of f/5.6.

It seems an awkward series of numbers to remember. The important thing is: increase the f-stop number in brighter conditions; reduce it as things get dimmer.

Understanding duration

A much simpler concept than with f-stops! The sensor in the camera is covered by a shutter until the moment a photograph is taken. The shutter opens for the given length of time, typically a fraction of a second (unless shooting in very low light).

How long the shutter stays open is set by the 'shutter speed'. Typical shutter speeds you can select are 1/500th (of a second), 1/250th, 1/125th, 1/60th,1/30th, 1/15th, ⅛, ¼, ½, 1 (second), 2, 4... Notice these shutter speeds are arranged so that each value is twice the length of the predecessor; allowing you to double or halve how long the sensor sees the light as you switch between speed settings.

Summary

To summarise all of that

  • ISO controls the camera's sensitivity to the sensor's signals. Doubling the ISO doubles the sensitivity of the sensor.

  • F-Stops control how much light reaches the sensor. Decreasing by 1 f-stop doubles the amount of light reaching the sensor.

  • Shutter Speed controls how long the light hits the sensor. Doubling the shutter speed doubles how long the light hits the sensor.


Exposure

Any given arrangement of f-stop and shutter speed creates the exposure of the sensor to the light it is recording. We'll discuss elsewhere what a 'good' or 'correct' exposure is, but it will always be achieved by these means. The lightness of the resulting image is then determined by the ISO sensitivity.

There are numerous ways that the desired lightness can be achieved. Eg. if we know that ISO100, f/4 @ 1/60th provides the desired result then we can achieve the same result with:

  • ISO100, f/2.8 @ 1/125th - ie. double the light reaching the sensor and half the duration

  • ISO200, f/5.6 @ 1/60th - ie. double the sensitivity, with half the amount of light reaching the sensor

  • ISO200, f/4 @ 1/125th - ie. double the sensitivity, with half the duration

  • etc...

You should be able to see how these controls work in concert, as they all share a doubling/halving nature. You will set all three to some value to correctly record an image. But you might find, for example, the image is blurry because things in the scene were moving. To solve this, and freeze the motion, you can half the shutter speed so that the sensor sees the image for less time (things won't have had the chance to move so far during the exposure). But because the sensor gets less light in total over the duration of the exposure you could compensate by reducing the f-stop by 1 to double the amount of light received at any given moment.

Each doubling or halving, of any of the three controls, is called a stop. It's just more convenient than describing each control's action in detail. So instead of saying something like:

If the image is too dark either try opening the aperture by 1 f-stop or double the exposure time or double the ISO sensitivity.

We can say:

If the image is too dark try increasing the exposure or sensitivity by 1 stop.

The image below shows the rear display on my camera which at the time was set to ISO100, shutter speed 1/500th of a second at f/5.6:

Camera display showing exposure settings

Exposure Values

You might have noticed that your camera has an 'exposure compensation' setting. This allows you to override the exposure the camera wants to use by a number of Evs. In fact in the image above you can see I have dialed in -0.7Ev of compensation.

Evs are Exposure Values - numbers that represent a combination of f-stop and shutter speed. For example, 1/500th of a second at f/2 happens to be Ev11. As is 1/250th at f/2.8. Doubling the shutter speed and halving the aperture (as we have here) gives us the same exposure, so the Ev is the same.

Evs are represented by a simple numeric scale: ...-1, 0, 1, 2, 3, 4, 5...

Each step represents one exposure stop: a doubling or a halving.

So telling the camera to use +1Ev exposure compensation means 'whatever you think the exposure should be, double it'.

Evs are not just imaginery numbers, arbitarily relating f-stop and shutter speed; they relate to the overall brightness of a scene. 'By Definition' Ev0 is a scene brightness that will be 'correctly' exposed at f/1 for 1s (and this is at sensitivity ISO100).

In the olden days cameras weren't able to take a decent reading of the light in the scene, or any reading at all! We used seperate, hand-held, light meters that reported the brightness in Evs. Unless we couldn't afford the luxury of a light-meter, whereupon we would have to guess. And yes, I did plenty of that. Guessing was aided, somewhat, by standard EV Tables:

(click or tap any table row to enlarge)

SituationEv
Dim interior0
Candle lit close-ups;Christmas Lights4
Campfires, bonfires; bright interiors8
Open shade or heavy overcast exteriors12
Bright sunlight15

That's just a sampler. Now-a-days cameras are excellent at performing light-readings and they suggest f/stop and shutter speeds directly, without any need to think about Evs. So Evs are a bit archaic to be honest. But it is useful to appreciate what they are so that such things as 'exposure compensation' (+/-Ev) make sense.

Note, 1/500th@f/2 is the same exposure as 1/250th@f/2.8 (Ev11) but these two settings will produce different images!

Deciding on how to balance the 3 controls

As we have seen, many different settings can be used to achieve the desired exposure - but there's more to a picture than just the exposure we give it. How we allow the sensor to record the available light affects the aesthetic nature of the image we create.

Eg. faster shutter speeds have the effect of freezing any motion. I'll look at the creative effect of the exposure controls in a later article, as for now I just wanted the underlying principles to be clear - so that later discussions might make more sense.

Briefly, each of the 3 controls have the following effects:

  • Aperture affects the quality of the focus of the shot (can make the background blurry)

  • Shutter speed can freeze, or blur, motion in the shot

  • ISO has a lesser aesthetic impact, but we can adjust it to help us achieve a specific aperture and shutter-speed combination if needs be.

Generally, the ISO is set once depending on the overall shooting conditions, with the shutter speed and f-stop juggled to achieve the correct exposure and photographic effect. If we want a small aperture (f/22 say) and a fast shutter speed we may elect to increase the ISO setting to allow this. Similarly, if we want a wide aperture (f/2) and a slow shutter-speed we may need to reduce the ISO.

As discussed earlier, we try to avoid very high ISO settings.

We try to avoid the extremes of the aperture setting (the very first or last f-stops) as these can introduce optical effects. eg. a very wide aperture on a wide-angle lens might reveal significant fall-off of light in the corners of the frame (vignetting). Very small apertures (f/22 or beyond) on a long lens might cause diffraction effects (light will bend when passing through very small openings).

We try to avoid very long shutter speeds, as there's a fair chance the camera itself will shake during the exposure.

Exposure Modes

Because there's so much flexibility in setting the controls for the desired exposure, cameras offer some 'modes' to help us out.

Some of these modes are very camera-specific, sometimes called 'scene' modes. Eg. you tell the camera you're shooting a portrait and then let the camera make all of the settings it wants. Sounds great. But I never, ever, use such settings. They're essentially a short-cut to ignorance. You never quite know what the camera is doing, and you're never quite in control of the aesthetic result.

On the other hand, there are 4 classic modes (not 'scene' modes) that any half-decent camera will provide, which are pretty useful. They are called the PSAM modes and you generally find them on a dial on the camera body.

Showing a PSAM mode dial on a Nikon camera

The PSAM modes are:

  • Manual: The camera gets completely out of your way, you set the aperture and shutter speed yourself for every shot. The camera takes a light reading and advises what it thinks about your settings. (the big 'M' at the top-left of the picture of my camera display above indicates Manual PSAM mode is selected).

  • Aperture Priority: You set the aperture and the camera sets the shutter speed based on the light reading it takes.

  • Shutter Priority: You set the shutter speed and the camera sets the aperture based on the light reading it takes.

  • Program: The camera sets the aperture and shutter speed based on the light reading it takes.

You can also typically tell the camera to control the ISO by selecting Auto-ISO, usually somewhere in the menu system. It may not be surprising to hear that I never use Auto-ISO nor the Program PSAM mode - both these modes reduce the freedom I have over the aperture and shutter speed settings. And as these two settings have a significant impact on the aesthetic nature of the image, I want to decide what they should be.

These modes all rely on the light reading the camera takes of the scene. So I will often use them in combination with the camera's exposure compensation dial. This allows me to bias the camera's view of the world by dialling in a stop or so of compensation. We'll be looking in more detail at how to find the 'right' exposure in a later article. The exposure compensation dial, though, is a very convenient control that allows me to shoot in aperture or shutter priority most of the time, without having to resort to full manual mode.

I've talked a lot about doubling or halving exposure and shown the 'classic' values for f-stops and shutter speeds that support this. Doubling and halving is quite a gross step-change though, often we want a subtler level of control. To support this modern cameras typically offer not just full stop changes (f/2.8 to f/4, or 1/500th to 1/250th) but rather provide ⅓ stop changes (maybe ½-stop increments on lower-end cameras).

So the change of aperture from f/2.8 to f/4 will likely step through f/3.2 and f/3.5 on its way to f/4.

Similarly, the change of shutter speed from 1/500 to 1/250 will likely step through 1/400th and 1/320th on its way to 1/250th.

To be honest, once I've set-up the basic exposure I pretty much ignore these weird and wonderful numbers. If I want to make a balanced adjustment I count how many steps I add to one control, and then reduce the other by the same count of steps. Because counting is easy.

Oh, the exposure compensation setting will typically be in ⅓ Ev steps as well, instead of counting 0, 1, 2... we get 0, 0.3, 0.7, 1, 1.3, 1.7, 2...

In overview

  • There are 3 basic controls effecting the lightness of the image:

    • Sensitivity expressed as ISO

    • Aperture, using the f-stop scale

    • Shutter speed, in seconds or fraction of seconds

  • The controls are set by 'stops' (or by increments of ⅓ of a stop)

  • We can add one-stop of brightness to the resulting image by

    • Doubling the ISO

    • Decreasing the f-stop by one full value

    • Doubling the shutter speed

  • We can subtract one-stop of brightness from the resulting image by

    • Halving the ISO

    • Increasing the f-stop by one full value

    • Halving the shutter speed

  • The camera's light meter gives guidance on the required exposure

  • This guidance can be biased by Ev Compensation of +/- some number of stops

  • Aperture and shutter speed (exposure) affect the aesthetic quality of the image

  • ISO can help us to achieve the specific aperture and shutter speed settings we want to use

  • We don't use automatic 'scene' modes.

  • Nor do we use the Program PSAM mode.

  • Nor do we use Auto-ISO.

  • Exposure Compensation is a convenience when using the semi-automatic PSAM modes (aperture and shutter speed priority), so we don't need to use full-manual control quite so often

That's it for this primer - if it doesn't make the greatest of sense, come back and read it again after we've explored more about actually taking photographs in my later articles.


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