Almost every modern DSLR camera and lens will have an auto focus mode which allows you to half press the shutter button to allow the camera to select the focus itself. How much of the image is in pin-sharp focus is up to you depending on a number of settings and other factors (like using a tripod).
When you half press the shutter button you will hear the motor in the lens shifting backwards / forwards as it seeks out that sweet point to give the best focus on the chosen part of the image. What the camera actually puts into focus in the final image is under your control even with auto-focus selected.
Your camera should have something like the following modes for focus:
- AF-A = the camera will have one pass at focussing if the image is stationary or will track a moving image and continually re-focus until the image is recorded
- AF-S = this is a single pass at focussing and the focal point is locked when the shutter button is pressed half-way
- AF-C = used for moving subjects the camera will continually refocus the image while the shutter button is pressed half-way as it tracks the object
Many cameras will also have a combination of the number of points of focus they use to determine the perfect focus for an image. This is known as SINGLE POINT (one point, normally the centre of the image) or MULTI-POINT (many points arranged in a pattern across the image). See the image below as an illustration:
You can see here that there are many points of focus available but the camera is in SINGLE point mode as only the middle focal point is active, indicated by it turning red. As you press the shutter button half-way to lock in the focus and exposure, etc. the set number of focal points will light up (normally red) to show they are active.
When there are this many possible focal points available there is normally a varying number of points that can be set from SINGLE to all of them and a number of combinations in between.
As an advanced technique some photographs will benefit from the focal point not being at the centre of the image. This can be achieved by putting the main element in the centre of the image, pressing the shutter button half-way to lock in the focus (with a SINGLE point of focus) and then reframing the image before pressing the shutter button all the way to capture the image.
Autofocus is great and makes a brilliant job of finding the right level of focus for the chosen focal points. However, it does have its restrictions:
- In images with little variation in colour (i.e. snow) the autofocus system will struggle to find the right focus and you will hear the motor searching backwards / forwards as it tries to hone in on the correct focus
- When there is a strong element near the front of the image but the desired focal point is deeper in the focus system can focus on the wrong point – for example taking a picture through a window can cause the camera to focus on the window rather than the object on the other side of the glass
- In low light or at night time the camera can often become confused and unable to focus effectively
The alternative to AUTO FOCUS (AF) is MANUAL FOCUS (MF) where you have complete control over the focus of the image. This is achieved by twisting a slider control around the lens and by looking through the viewfinder you can see when the image comes into focus. There is often also a visual indicator of when the camera judges the images focal point to be in clear focus, normally shown as a circle that lights up when in focus.
However, sometimes a perfectly focussed image is not the intention! Consider the following image:
This night time street scene was deliberately taken out of focus to capture dots and circles of light. The subjects here are either street lights or car headlights and you can just about make out the edges of the road and a couple of the vehicles.
