Exposure is one of the most important concepts in image creation. Whether photography or cinematography the principles are the same. In either case you are using light to turn your creative vision into reality. The key is learning how to manipulate the light to match your vision.
When you are using a camera, there are 3 components that determine the exposure of your image. The light sensitivity setting of the recording medium (ISO), how much light is reaching that medium (aperture) and for how long (shutter speed). These three settings can be modified to adjust the exposure of your image. Let’s discuss each component in more detail:
The recording medium is you film or the camera images sensor. The sensitivity is rated in a number, the ISO. The ISO is set by the International Standards organization, thus where the name comes from. It was originally used as a rating for film but it is also used today as a rating for the sensitivity of the sensor in a digital camera. This is a standardized number so you know what you are getting when you change from one source to another.
There are a few characteristics that go along with these numbers. The lower the ISO the less noise or grain will show up in the final image. In retrospect, as you increase your ISO settings, you’ll be able to take photographs in lower light but you’ll quickly start to notice and unacceptable amount of noise in your photographs.
Aperture controls the amount of light reaching the sensor. It is made up of a multi-bladed diaphragm inside the lens, where interchangeable lenses are used. The diaphragm opens and closes to create different sized circles allowing differing amounts of light through. The size of the opening is represented by an F-stop number. F-stop is the ratio of the focal length of the lens to the diameter of the diaphragm opening, focal length over opening. It may seem counter-intuitive at first but the lower the F-stop number the larger the opening and, thus, the more light that comes into the camera.
3.) Shutter Speed
Shutter speed controls how long light is hitting the sensor. In most professional cameras it is controlled by a mechanical shutter that opens and closes to let light through. The shutter the shutter speed the crisper images of moving subjects will be. For subjects like insects you will want the higher shutter speed to freeze the action.
Iso is generally rated as ISO 100, 200, 400, 800, 1600, 3200 when you are working with whole stops. Shutter speed is 1/60, 1/125, 1/250, 1/500, 1/1000 and so on again when working with whole stops. Finally aperture is F2, F2.8, F4, F5.6, F8, F11,F16, F22, F32.
Above I mentioned whole stops, what is that? A whole stop is a number that is either twice the number or halve the number on either side of it. Another way of explaining it is Iso 400 is twice as light sensitive as 200 and halves as much as 800. All of the whole numbers of all three components work like this. Many camera these days will allow you to adjust any of these three in thirds of stops. This gives you increased control over the effects you are trying to create in your images.
Say your exposure is ISO 400 F5.6 at 1/1000. Now you would like to move your F-stop to 2.8. Since you added 2 stops with the change of F-stop you will need to pull them from another part of the exposure triangle. Now you might want to do ISO 200 F2.8 at 1/2000. There are any number of ways you can get correct exposure by moving these various pieces around.
I don’t want to go on a side tangent but one of the things you need to know is the dynamic range of you camera sensor. It the camera has a dynamic range of 8 stops then you have 4 stops over and 4 stop under correct exposure before you clip the images. This is important because as you create your image you need to factor in the physical limits of what is possible.
How do we bring all this to work with our focus, macro. First we need to look at what is our subject. Is it a flower and insect or a studio shoot. Is the subject going to completely fill the frame or will there be a background in the shot. If you are working with insects shutter speed will be your most important consideration because you want to stop the action. If you are working with a flower then you probable want to user a smaller aperture to get the increased depth of field. Producing a pleasing Bokah (blurred background) will make a better image and help your subject to stand out.
With this basic knowledge what do you want your image to look like? Play around with different combinations to see the effects they produce. Do you want an over or under exposed image? Remember, when it comes to photography, exposure is just one tool to help you create your vision.