The two principal factors that you have to understand to use the camera are:
Even more important is composition, but that’s a whole other subject. Here, we’ll stick to getting familiar with the camera.
There are three variables you need to consider to get the exposure right. Whilst it is true that you can correct a wrong exposure to some extent afterwards in the computer, you will get much better results by getting it right in the camera, so it is worth giving it some attention.
The three variables are:
ISO (pronounced eye-ess-oh, not eyezo!). This is the modern digital equivalent of film-speed and adjusts the sensitivity of the camera’s sensor to light. In general, you want to use the lowest ISO setting for the best quality image. Each camera has a native ISO at which it delivers the best quality, probably ISO 100. Higher ISO settings are useful for low light conditions or for fast-moving subjects, but image quality is compromised, though modern DSLRs can use remarkably high ISO settings satisfactorily. I recommend you use ISO 100 for most static objects.
Aperture – this is the size of the hole in the lens through which the light passes to strike the sensor- the larger the hole, the more light passes through. Aperture is measured in “f numbers”, sometimes called “f stops”. You need to remember that the larger the f number, eg f22, the smaller the aperture, and the less light gets through. Large apertures will have numbers like f2 or f1.8.
Shutter Speed – this determines how long the camera lets light through the aperture. In a DSLR, the shutter comprises two curtains that pass very quickly in front of the sensor, and the time between the passing of the two curtains is the shutter speed. Shutter speeds are measured in fractions of a second. A typical shutter speed might be 1/125 of a second. Faster shutter speeds, such as 1/400 are needed to capture moving subjects. If the shutter is only open for a short time, there is less likelihood of the subject moving during the exposure and becoming blurred. You might use very slow shutter speeds, perhaps 2 or 3 seconds, to intentionally blur a subject such as moving water, but in such cases you would have to use a tripod to hold the camera steady and a remote release to fire the shutter.
The amount of light reaching the sensor is determined by the size of the aperture – the f number – and the length of time it is open – the shutter speed. These two have to work in conjunction with the ISO number to get a correct exposure. Whilst it is important to understand these three things because you may wish to to be able to adjust them manually later, your camera can use its internal light meter to control some or all of them for you and get a perfect exposure in normal lighting conditions.
Your camera will have very clever and very fast autofocus facilities and I suggest you start by using them. It helps if you understand a little about how the various autofocus modes work, and they vary from camera to camera, so we’ll deal with those separately. Note that to use autofocus, you may have to select “autofocus” on both the camera and the lens for it to work properly. Again, how to do that varies from model to model. It is helpful to know which part of your subject the camera is choosing to focus on. For example, if you are photographing someone’s face, you will want to make sure their eyes are sharp, so you want the autofocus to focus on the eyes. As cameras vary in how they do this, I can show you on your camera.
Getting your image sharp
Whilst you might in certain circumstances want an image that is soft or blurry, let’s start by aiming to get your images sharp. Two things influence sharpness: focus and blur.
We have looked at focus above – you have to allow the camera to focus properly on the subject. An autofocus setting will do that just before it takes the picture. When you first press the shutter very lightly – not all the way down – it will focus. (We’ll return later to why this is helpful to know.) When you press it completely down, it will take the picture.
Blur is caused by movement. It can be movement of the subject during the exposure (the time that the shutter is open). You would be surprised how much movement can take place – eg of a flower moving in a gentle breeze – during even a short exposure. This will ruin your picture, unless blur is what you want!
Blur is also caused by the camera moving during the exposure – ie by you not holding it steady while you press the shutter. I will show you the generally accepted way to hold the camera to keep it steady. You should breathe in or out gently before you first press the shutter and hold your breath until after you have taken the picture. The effect of blur can be reduced by using a shutter speed that is not too slow – try not to use slower than 1/125 unless you’re using a tripod. You can sometimes help hold it steady by leaning against something like a tree, a wall or a doorway. I’ll show you how. Your camera probably has an anti-blur function that helps – sometimes called “image stabilisation”. Nikon call it Vibration Reduction, or VR.
This kind of blur – caused by you moving the camera, however slightly, during the exposure, is the single most common cause of poor images. To reduce blur, I
- Use the tripod and remote release whenever I can, or
- Lean against something firm, or
- Stand steady and hold my breath while taking the picture
Aperture and Depth of Field
Depth of field is the amount of the picture that is in focus, from the area nearest to the camera to the area furthest away. When you look at a scene, your eye constantly adjusts its focus so that the whole scene looks sharp. The camera cannot do that. It can only focus once. The depth of field depends on the aperture selected. The smaller the aperture (larger f number), the greater the depth of field. Conversely, the larger the aperture (smaller f number), the less the depth of field.
Let’s look at an example. If you were photographing a landscape, you might want a large depth of field, with objects in focus from the foreground near the camera right to the horizon. To achieve that, you need a small aperture, say f16 or f22. If you wanted to take a picture of a flower in a busy garden, you might want to get the flower sharp, but throw the background out of focus to help the flower stand out. Here you would use a large aperture (small f number) to restrict the depth of field.
Being able to adjust all these variables is what gives you control over your image, and is what makes the DSLR the perfect camera for taking more than just snaps.
(c) Colin McLean, August 29 2013