Exposure - Philip Yale

Exposure

What Is Exposure?

Exposure is the term that simply means allowing light to reach the light sensor in your camera.  You "expose" the sensor to light from outside the camera that passes through the lens.

That's it.  

Except, of course, there's a bit more to it than that.

It's the job of the computer in your camera to take the light that reaches the sensor and convert that into an image that resembles what our eyes saw as we pressed the shutter.

If only a small amount of light reaches the sensor, it can't pass enough digital information to the on-board computer to let it distinguish different areas of the scene sufficiently.  It can try, but in the end any image it manages to come up with will look very dark, and will contain a lot of digital background "noise".  This is what we call "under exposure".

The opposite happens if too much light reaches the sensor.  Now the computer has so much digital information to process it can't make sense of much of it, especially in those areas of the scene that are naturally bright.  Any resulting image from this will have large areas of complete whiteness with absolutely no detail at all, and other areas that do contain detail but where the colours are pale and washed-out.  This is what we call "over exposure".

So, clearly, somewhere between these two extremes there must be a point where the sensor gets just the right amount of light for the computer to be able to generate a digital image which looks very close indeed to what our eyes saw.

What Do our Eyes See?

This is a very good question, and it's a tricky one to answer without an extensive qualification in opthalmology and neurology.  However, there is one important thing to appreciate:

Our eyes see things differently from the way a camera sees things.

Let me explain ...

It's the Brain, not the Eyes

Our eyes don't see anything.  It's our brain that sees things.  Our eyes simply provide the light information by  focusing light into a real image on the retina at the back of the eye. This is very like a camera lens does onto the camera sensor.  Our brain takes electrical signals from the back of the eye (the retina) and interprets those as images that we are familiar with.  Our eyes are constantly providing this information to the brain in real time.  In fact, they are providing multiple, brief snapshots of vision to the brain.  Our eyes re-position and "see" a scene 3 or 4 times each second, and the brain then combines these multiple inputs and renders them into a single impression to produce an "image".  

Not only that, but the brain is exceptionally good at enhancing shadow areas and reducing highlight areas.  You can look at a scene with a wide range of bright and dark areas, and see the shadow and highlight detail quite distinctly, along with that in the mid-tones.  The brain is infinitely better at doing this than any on-board computer on a camera.

Whilst the physical construction of a camera lens might be similar to that of our eyes, that is where the comparison between photography and human vision ends.

Cameras Can't Match This

A camera takes a single snapshot of a scene.  That's it.  It has to try to record detail in shadow areas, and detail in bright areas, as well as accurate tones and colours across the whole image, all with a single group of settings.

The human eye / brain combination is infinitely more versatile.  The eye is constantly sampling the light from a scene and sending this to the brain.  The brain is able to simultaneously and selectively enhance shadow details, and extract detail from highlights.  

We only know what the world "looks like" because of what our eyes and brain tell us.  A camera is a human invention that attempts to mimic what our view of reality looks like, but it is just that - a reconstruction.  So, a camera is inherently at a disadvantage compared to our eyes because its version of "reality" is man-made.

Why Does This Matter?

So, the challenge is how to consistently set up a camera so that the resulting image, taken from the lens and the sensor and the computer, looks like something very close to what our eyes and brain would have seen at that same time.

All the limitations above matter because somewhere in the whole process of creating an image we have to intervene and help the image coming from the camera in order to get it closer to what the photographer's eyes would have seen.

BOLD STATEMENT ALERT: 

It is not possible to get the exposure for a photograph "right" in the camera.  It has to be edited afterwards on a computer if it is to match what the photographer saw at the time that the shutter was pressed.

Why is this?

Hopefully, after we've discussed how a camera controls exposure, it will become a little clearer.


Controlling Exposure

Three camera settings work simultaneously and jointly to control the exposure of a camera image.  These are:

- Shutter Speed

- Aperture Size

- Sensor sensitivity (or ISO setting)


Any of these can be changed at any time, and can have the effect of increasing or decreasing the exposure, depending on the nature of the change(s).

Changes in overall exposure are often described using a unit called a "stop".  This is a convenient measurement, and it means an overall "doubling or halving of the amount of light reaching the camera sensor".

So, if your current exposure settings are correct, and you then make the sensor twice as sensitive to light as before (by doubling the ISO setting), you have increased your exposure by 1 stop.  The image will now appear rather bright, so you need to reduce the exposure by 1 stop to compensate.

From this new setting, if you now make the shutter speed twice as fast, this will halve the amount of light reaching the sensor, reducing the exposure by 1 stop.

The combined effect of both changes is to increase exposure by 1 stop (using the ISO), and then reduce the exposure by 1 stop (using the shutter speed), so the overall change in exposure is zero.   We'll examine why you might want to do this shortly.

Firstly, let's look at the three exposure controllers in more detail ...

Shutter Speed

The shutter covers the sensor, and opens briefly to let light reach it.  

The longer it stays open, the more movement it records in the subject, so you need a very brief (fast) shutter speed to freeze movement.

It is measured in fractions of a second  

(On the camera, 250 = 1/250 sec,  500 = 1/500 sec  and so on)

Generally, the faster the shutter speed the better, unless you’re after special motion effects.

Historically, shutter speed settings have been such that each value is either double or half the speed of the one next to it on the dial

i.e.         1, ½ , ¼, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000 etc.

Doubling, or halving, the shutter speed allows you to change exposure in  “1 stop” increments described above.

Modern cameras, though, tend to display many more intermediate shutter speed values than this to allow for finer control over exposure. These are typically ½-stop or 1/3-stop increments.


Aperture

The "aperture" is literally a “hole” part-way down the lens which can be adjusted in size to let through more or less light.  

A small hole lets through less light, so you need a slower shutter speed to compensate (the shutter needs to stay open longer).

With a wide hole (aperture) more light passes through the lens, so you can get away with a faster shutter speed.

Aperture is measured in “f” numbers.  

The smaller the number, the wider the aperture & the more light enters the camera.

    e.g.  f2.8 is a wide aperture.  f4 is a smaller aperture.  f5.6 is smaller still …… to f22  

Using the smallest number possible lets you use a faster shutter speed (good), but makes accurate focusing harder and degrades image quality at the edges (bad).

As with shutter speeds, traditionally f-stop values were set so that each one either halved or doubled the amount of light passing through the aperture compared with the adjacent value (“1-stop changes”)


Front view of a lens showing the overlapping blades that form the aperture.

They only close up like this very briefly as the shutter is pressed. 

At all other times, they stay wide open to allow the maximum amount of light to pass through and make the viewfinder as bright as possible for the photographer.

What is an f-number?

Honestly, you don't need to worry about this.  

I know the numbers appear weird and rather daunting, but just accept them for what they are.

Just remember that the smaller numbers let more light in through the lens, and the bigger numbers let in less light.


Depth Of Field

This is also affected by the aperture size, and is a very important subject, so I'll deal with that completely separately from this "Exposure" section.


Sensor Sensitivity (ISO)

You can make the light sensor very sensitive to light, or less-sensitive to light, by changing it's ISO value. 

So why choose a low ISO value?  Why not set the sensitivity as high as possible so that you never have a problem with shooting in low light again?

Well, as you might have guessed, nothing is free.  With high sensitivity comes increased digital "noise" ( speckled patterns in dark regions or plain areas),   inaccurate colours and lack of shadow detail.  Lower ISO values will give you better quality images all round, so if you have enough available light to use a low ISO value you should do so.

Still, modern camera sensors and software are now better than ever at minimising noise at high ISO values, and no doubt they will continue to improve year on year.  Higher ISO is rarely a problem below about ISO 1000, and I tend to use ISO 800 as a default (unless it's a very bright day).

The Effect on Exposure

So what's the effect on exposure of changing the ISO value?

As with shutter speeds and aperture size, it's best to think of ISO in units of "stops".

If you double your ISO setting, this makes the sensor twice as sensitive to light as before.  This will over-expose your image by 1 Stop (unless you compensate with one of the other setting options).

e.g.  Changing ISO from 400 to 800


If you halve your ISO setting, this makes the sensor half  as sensitive to light as before, and so your picture would be under-exposed by 1 Stop.

e.g.  Changing ISO from 1600 to 800


What Does ISO Mean?

"ISO" is a hangover from the days of film, and stands for "International Standards Association", previously known as "ASA" (American Standards Association). 

With film, an entire roll would react to light at the same rate, and the ASA / ISO rating of the film gave the photographer an indication of how fast that was. 

Common variants of ASA / ISO were 25, 64, 100, 125 and 400 ISO.  There was rarely anything in-between, or higher.

With digital, we have much more freedom to mimic this.  Not only are there many more ISO settings available, we can change them for each individual shot.  We can also use ISO values undreamt of by film photographers, with ISO 125,000 and higher being freely available on many cameras now.


You Can't Get the Exposure "Right" In-Camera

So, back to what I called a BOLD STATEMENT earlier in this article. 

Why can't you get the exposure right in-camera?  Why is it always necessary to edit an image afterwards?

As we've seen, the camera looks at the entire scene, and has to decide what the correct exposure should be for a single shutter speed and single aperture setting and a single ISO setting.  It gets once chance - that's it.

In reality, different parts of that scene require different exposures if they're to be close to what your eye/brain saw.  Your brain can apply local exposure adjustments to shadows, highlights and mid-tones simultaneously.  That's not possible for a camera.  However, it can and shouldbe done in post-processing afterwards.  (Lots more about post-processing to come in other posts) .


Having said all that, you can get the exposure very close to accurate in the camera.  It's important to understand, though, that it cannot be perfect and will always require some degree of editing to get the absolute best from an image.


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