Photo Seven

From a technical point there is such a thing as a correct exposure, however does the correct exposure give artistic effect. In this post I am going to ignore artistic intention and just deal with the technicality of correct exposure.

So what is correct exposure. Technically correct exposure for a scene is where the average of any given scene is calculated to be 18% Grey. And what exactly is 18% grey?

18% grey is the mid point between black and white. On the digital scale where 0 is black and 255 is while, 18% grey is an RGB value of 127 for Blue, Green and Red. So 18% grey looks like this.

In the printing industry it is standardised as the ink density which reflects 18% of incident light, however cameras seldom adhere to this. This topic deserves a discussion of its own, but for the purposes of this tutorial simply know that each camera has a default somewhere in the middle gray tones (~10-18% reflectance). Metering off of a subject which reflects more or less light than this may cause your camera’s metering algorithm to go awry– either through under or over-exposure, respectively. (Source)

This means that your camera, when calculating exposure is analysing the scene in an attempt to find the 18% grey point. To explain, when your camera looks at any scene it finds the darkest and lightest points in the image, for example black and white. Your camera knows that the darkest point should be black and the lightest should be white. So technically correct exposure is where the average of an image is calculated to be 18% Grey or a value of 127 for each of the RGB values.

On the left is a correctly exposed image, the right is the 18% grey averaged from the same image. You can see the spike on the histogram, this is the 18% spike averaged from the histogram.

Why is this 18% grey so important? There are two reasons why this is important. Firstly if you have an 18% grey card you can calculate correct exposure for any situation, but there is an easier way to calculate correct exposure. Secondly you can also use an 18% grey card to calculate correct colour balance.

Thankfully correct exposure has already been calculated for us and we don’t need to re-invent the wheel, so to speak. Photographically speaking correct exposure is called the sunny f16 rule. So what is the sunny f16 rule? The sunny f16 rule is a estimation of correct exposure under specific conditions. So how does this work?

The basic rule is: on a sunny day set your aperture to f16 and your shutter speed to your iso.

For Example:

• On a sunny day if your ISO is 100 set your aperture to f16 and your shutter speed to 100th.
• On a sunny day if your ISO is 200 set your aperture to f16 and your shutter speed to 200th of a second.
• On a sunny day If your ISO is 400, set your aperture to f16 and your shutter speed to 1/400th or the nearest of value.

The sunny f16 rule also changes as your light changes.

You can use the sunny f16 rule in most lighting conditions, set your shutter speed to your nearest ISO value and the f number according to the following table.

But why would you want to know and remember this rule when your digital camera has a automatic light meter. To put it simply, the automatic light meter in your digital camera does not work how you want it to work. In a camera, especially a digital camera the light meter works significantly well if there is a significant spread from light to dark reflected light, then the average reflection will be roughly 18% grey and the camera will expose correctly. Unfortunately most scenes do not have an even spread of lights to darks. Take for example a white animal in a white environment, i.e. a polar bear in snow. Here there is not an even spread of lights and darks.

Most people who have photographed in snow will know that their images do not look correct, that is because too much light in the scene. This means that the light meter can not correct evaluate the exposure. In most cases the snow exposure is over by between 0.7 and 1 stop.

This leads me to my final point about correct exposure. To get “correct exposure” use your histogram. This I will explain at a later date, but to summarise, if you use the sunny f16 rule you can get a technically correct exposure, but is this what the image really requires, in some cases yes, but for a artistic and aesthetic point, sometimes the best images are those that do not conform to the rules.

As you get further into Digital photography there comes a point when your inbuilt flash will no longer fit your needs. This can be for any number of reasons including to red eye, lens shadow and throw distance. I will explain some of the disadvantages of inbuilt flash and how having a external flash can help solve some of these problems.

Red Eye

You may have noticed that when you take a photo using a flash that there is often a red spot in the middle of the subjects eyes. This is appropriate called Red Eye. So what is Red Eye? Red Eye is where the flash from your camera bounces off the back of the subjects retina and back into the camera. Because the back of the retina contains blood it appears to be red. This will only occur when there is a small angle between the camera and the light source. To reduce redeye the simplest this is to increase the angle between the light source and the camera lens.

In this setup the light will travel from the flash directly into the eye and then back to the camera. The yellow indicates the light coming from the flash head and the red indicated the returning light back form the eye. Eliminating the red eye is very simple. All that we need to do is move out light source further away from the camera lens.

In this setup the yellow line indicates the light coming from the flash head and the red line indicates where you would see the red eye. Notice that these two lines are coming to and from the subject at the exact same angle. As such the simplest way to remove redeye is to move your light source.

Lens Shadow

What is lens shadow. Lens shadow is where you flash is hitting the top of your lens and creating a shadow on you image. The result is a darkening most often at the bottom of your image.

To solve this probem the simplest method is to lift the flash further away form the lens. This is done using a external flash on the how shot mount. This increases the angle from the flash to the lens and this eliminates the lens shadow.

Flash throw

Flash throw is the distance that the flash will take affect. I often look at people with digital cameras at sporting events and watch them all taking flash photos, the best example is the opening of the Olympic games. I can’t help but enjoy the wonderful light show off all over the stadium, it is so pretty and yet completely useless for the person taking the picture. The flash on your camera only has a certain range that is will be effective. This range is determined by your ISO and your focal length. If you refer to your user manual it will give you a table that shows the effective range of your on camera flash. My old canon 10d for example, the effect range of the on camera flash is as follows.

The canon 580 EXii has a vastly more effective range than the built in flash. For example the at ISO 100 my old 10d is just under 4 m not the 580 ex will light up to 58 m at a telephoto range. Canon lists the effective range of the 580 EX ii as between a 13m max at wide angle to the 58m previously mentioned at full zoom.

These are just a few if the problems that having a external flash can help solve.

It is all very fine to understand what a f stop or a stop is, but how does that knowledge allow you to get a correctly exposed image. Well before I get into the deep dark secrets of exposure it is important to under stand the factors that make up exposure

Exposure is controlled by ISO, Aperture and Shutter speed. the question that I get asked all the time is “what are the correct settings to use”?. The simple answer is that there is so such thing as a correct setting. You camera setting allow the recording if the image in a specific way. Take for example you want to create a image that has a lot of blurring in it. This indicates that you will need to use a slow shutter speed. another example would be where you want to have a very fine depth of field, then this dictates that you are going to use a small aperture like f2 for example. Now what this means is that you can then manipulate the remaining two setting to get a correctly exposed image.

This diagram may help in the explination.

The three sections of exposure are connected but also fluid. As you fix one of the points the other two can be adjusted to get correct exposure. This correct exposure is also called the Exposure Value. Wikkipedia gives a excellant defination as

exposure value (EV) denotes all combinations of camera shutter speed and relative aperture that give the same exposure.

Back to the original question, what is a stop, and my addition how does this relate to exposure. Well to recap, so far we know what a sop is in aperture terms, for example f5.6 and f8 are stops. There also one stop difference between f5.6 and f8. There is also one stop difference between f8 and f11. However there is two stops difference between f5.6 and f11. Hopefully you brain has not exploded yet. Stop also refer to shutter speed and ISO. Again the full stops are 100, 200, 400 etc. There is also one stop difference between 100 and 200 as well as 200 and 400, with two stops difference between 100 and 400. For ISO it works in the same manner. One stop between 100 and 200, two stops between 100 and 400. Brains still working?

So how does this work with exposure. Well to explain this you will need to have a look at your camera. On your camera you should find a light meter. It will often look like one of the following two image.

This image comes for inside a Canon 5d but all digital SLR cameras will have one similar. It may be located at the bottom of your view finder however. SO what does this mean, this is a graphical representation of correct exposure. It is a scale form under exposed to over exposed with correct exposure in the middle. Digital SLR cameras often also have the numbers -2, -1, 0 , 1 and 2 also indicated on the bar.

This is the forth instance where stops comes into play. If your light indicator is at position 0 that means that you have a correctly exposed image. If it is at -2 then your image will be under exposed by 2 stops. Similarly if it is at 2 then your image will be overexposed by 2 stops.

The next question is how do you fix this problem. Remember our exposure triangle. to fix the problem all that you need to to is adjust one of the three values, Shutter speed, aperture or ISO. As a example, you are taking a image of a friend of yours and you have set your ISO at 400 and your shutter speed at 100th of a second. You have also set your aperture at f8. When you do your metering you notice that your light meter is reading -1. To fix this problem you will need to move one of your setting by a single stop, so aperture would go from f8 to f5.6 or shutter speed from 100 to 50 or ISO for 400 to 800. Any one of these options will fix the exposure problem.

Next I will explain how exactly a light meter work in your camera.

If you have any questions feel free to ask.