Knowledge of exposure is an essential part of photography. In this tutorial we will look at what is exposure, how can you control it, and why might you need to control it.
Exposure refers to the amount of light collected by a camera to make a picture.
If the light is too little the image is dark and underexposed. If the amount of light is too much, the resulting image will be bright and overexposed. Correct exposure happens when you allow the right amount of light to reach camera sensor, for your intended picture.
There are three controls in your camera that you may use to regulate exposure.
The f-stop or aperture value is the size of the opening through which light enters through your lens. The f-stop numbers are not logical sounding and appear to be reversed – thus f/2.8 is a large aperture and f/22 is a tiny one – the higher the f-stop number the less is the amount of light it lets in. The aperture values in your camera are spaced at intervals called f-stops. If you move one stop from any f-stop number (e.g. f/8) to the next lower f-stop number (f/5.6) you are doubling the amount of light reaching the camera sensor. If you move from any f-stop number (e.g. f/11) to next higher number (f/16) you are halving the amount of light reaching your camera sensor. The apertures shown in picture are some random examples (not a complete list) to give you an idea.
Please remember that for now!
Shutterspeed refers to the length of time the camera shutter is open to allow light in. This works in quite exact and simple manner. Shutterspeed of 1 second means that your shutter is open for 1 second. And shutterspeed of ½ second means that it is open for half second (that is half the time and half the amount of light entering camera). The difference between adjacent numbers is one-stop. Please remember that.
ISO (International Standards Organization) refers to the sensitivity of your camera sensor. The difference between adjacent numbers is again one-stop. For example, if you go from 100 ISO to 200 ISO you will need half the amount of light for the same exposure – because you have increased sensitivity of your camera sensor by one-stop.
Time now to do an imagination exercise. Imagine that you have a bucket and you place it outdoor on a rainy day. (You may further imagine that the rain falling is LIGHT instead of water). The fill of your bucket with rain water will depend on,
1. The size of the opening or hole of the bucket (aperture).
2. The length of time you stand the bucket out in the rain (shutterspeed).
3. And the size of your bucket (ISO).
You can control the rain water collected in your bucket (or light collected in your camera), by altering one or more of the above three factors. And there you have the gist of it.
You can have a bucket with a larger opening and stand it out for less time to get the same amount of rain water, as you would, if you had a bucket with a smaller opening and let it stand out for a proportionately longer period of time.
Attached in a pdf file is an Exposure Adjustment Chart – please open it alongside this document. This will allow ease of reference.
You need to be shooting in a manual mode (not automatic), that allows you to control these individual factors.
Let us imagine that you are getting the correct exposure for a shot, at f/11 at 1/125 seconds at ISO 100. You may see this in your DSLR, as below, and are happy with this exposure for a particular shot:
-2 I I 1 I I ^ I I 1 I I +2
II (Flashing bar)
1/125 F 11 100
You will also get the same exposure if you move the aperture to f/4 at 1/1000 shutterspeed at ISO 100. You have just changed both aperture and shutterspeed by three stops keeping ISO the same. Check it out. Place your finger at f/11 on the chart and count one- two –three to f/4. Now place your finger on 1/125 shutterspeed and count one- two -three to 1/1000. Its exactly three stops difference, right! By enlarging the aperture you have taken in more light by three stops but by shortening the time the shutter is open you have taken in less light by the same three stops.
OR, you may change aperture to f/22 (two f-stops up from f/11) and change shutterspeed from 1/125 to 1/60 (one stop) and change ISO from 100 to 200 (one stop). The two f-stop change is balanced by one stop change in shutterspeed and one stop change in ISO, to get the same exposure.
At this time we can look at the ‘Sunny 16’ rule. The Sunny 16 rule tells us that if you shoot a front lit subject on a clear sunny day, set aperture to f/16 and shutter speed to the reciprocal of the ISO setting used (that is 1/whatever ISO is used). Thus if you were using ISO 100 you could simply take 1/100 or 1/125 (close enough if that’s on your camera) as your shutterspeed. So you could correctly expose at f/16, 1/125, ISO 100. Now suppose you wanted to expose at f/5.6 instead of f/16. What is the right shutterspeed to get the same exposure? By using the chart you would determine that the right exposure at f/5.6 would be 1/1000 at ISO 100. You have simply changed shutterspeed by three stops to compensate for the three stops change in f/stop. The exposure is the same in both cases but (this is an ahhh! moment), the Depth of field is different at f/5.6 (more on this below).
In the above examples you will have the same exposure because quantity and effect of light on your camera sensor was adjusted to be equally balanced. You can use any other combinations as long as the net effect is fully balanced.
Its really quite simple and will come easily once you practice it a few times.
Download the Exposure Adjustment Chart. I suggest you print it and carry it with you as a quick reference guide, till you get a good hang of the concept.
We shall now briefly review WHY you might want to change the values even though you are going to get the same exposure.
I have cited three examples below.
1. A smaller aperture (higher f-stop number) means greater depth of field (or area in the picture that is in focus). In doing a landscape shot you may prefer a greater depth of field to get more area into focus. Thus you may choose a high f-stop number and balance it with a longer shutterspeed to get correct exposure – along with greater depth of field. (No surprise that landscape photographers tend to carry sturdy tripods around – its to expose for longer shutterspeeds without shake).
2. A bigger aperture (lower f-stop number) means a shallower depth of field. This is great to use in portraits, for example. Because you want the subject (or person in your portrait) to be in sharp focus but to have a blurry or diffused background as that makes them stand out more, and dramatically improves the visual quality of your picture. To make this shot you may want to choose a low aperture number as your camera/ lens will allow. Instead of f/11 at 1/125 for example, you may want to take this shot at f/4 or f/2.8. The 3 or 4 stop difference can be balanced by taking 3 or 4 stops faster shutterspeed, to ensure that you still get correct exposure. See the girl’s portrait as an example. Note that to get a nice blur you may need to do a couple of other things as well. Such as have your subject further away from the background.
3. A fast shutterspeed freezes motion. In shooting a sports scene you might want to take a shutterspeed of 1/1000 for example, to freeze a fast moving player in action. Using the chart can allow you to do that by making appropriate adjustments in aperture value or ISO.
A word of caution about ISO. Higher ISO can mean more ‘noise’ in a picture. Noise is a grainy effect not really desirable unless you are making a special effect type of image. I recommend keep your ISO at 100, vary up to 400, and only take it higher at limited times – for example in very low light conditions. Over time you will learn to use ISO more efficiently and then you can use it more if you wish.
The numbers and values that I have quoted are one-stop values, which can be done in 1/3rd increments or the numbers can be slightly different, depending on your camera. But the concept holds true and remains unchanged, so please don’t let this confuse you. Just adapt it as needed and – Happy shooting!
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