Understanding Auto-focus

Learning about your cameras auto-focus system is one of the most important aspects in wildlife photography.

A cameras auto-focus system adjusts the camera lens to obtain focus on the subject. This is probably the most important part of the entire process because out of focus shots = poor photos. We have one simple goal, sharp focus on the area we are focussing on. The photographic subject can have an enormous impact on how well your camera autofocuses—and often even more so than any variation between camera models, lenses or focus settings. The three most important factors influencing autofocus are the light level, subject contrast and camera or subject motion. Depending on the camera system, we have different auto-focus options to use. Some of the newer cameras on the market have very advanced auto-focus systems. All cameras have the best auto-focus in the centre of the view finder. It is advisable, if you are struggling to get an accurate focus, to use the centre AF point, utilise the focus lock and the recompose the image. Otherwise it is advisable to us a lens with a wide aperture. For SLR cameras, the number and accuracy of autofocus points can also change depending on the maximum aperture of the lens being used, as illustrated above. This is an important consideration when choosing a camera lens: even if you do not plan on using a lens at its maximum aperture, this aperture may still help the camera achieve better focus accuracy.

Auto-focus points

Your camera gives you the opportunity to set where in the view finder you would like to focus. The image above shows an example of a few different options from different manufacturers. We have the option to set a single focal point, a group of focal points or all of the focal points depending on the type of auto-focus we want to achieve. Below is an example of the Canon 7Dmk2s auto-focus points (based on 19 selection points) and how they work: 

Different Auto-focus options

¨ Single point Spot AF (manual selection) For pinpoint focussing. Great for macro photography

¨ Single-point AF (manual selection) Select one of the available AF points for focus

¨ AF point expansion (manual selection) A manually selected AF point along with 4 adjacent AF points above, below, left and right. This gives more leeway in focus with moving subjects.

¨  AF point expansion (manual selection) A manually selected AF point along with the surrounding AF points are used to focus

¨ Large and automatic AF selections are great when you have a high contrasted scene with fast moving subject which are far away, for example, birds in flight.

Auto-focus modes and “back button” focus

There are two types of focussing available, single shot and continuous. Your camera starts the auto-focus process when the shutter button is pushed half way down or when the AF button is pushed. When using the AF button, the auto-focus process is not hindered by any shutter activity as it utilizes a separate AF button. Using this very handy function will definitely improve your auto-focus, especially with moving subjects. However, you need to use the correct auto-focus type for different subject types.

One Shot or Single AF  One shot (Canon) or AF-S (Nikon, Pentax etc.)

This AF mode simply focusses and locks the focus whenever you auto-focus. It is great for portraiture, landscapes or scenes where there are static elements. Most cameras will emit a “beep” sound when the focus has been reached. If you keep your shutter half way down (for AF) or the AF button depressed, the focal distance will be locked by the camera until the shutter is released. Once the photo has been taken, the auto-focus process has to start again. It is handy for tricky compositional situations where you want to recompose a scene but lock your focal distance. Simply focus, recompose and then take the photo.

Continuous Auto-focus or AI Servo  AI Servo (Canon) or AF-C (Nikon, Sony, etc.)

Continuous auto-focus modes allow the auto-focus system to focus on the scene non-stop. This is a great mode to use when there is movement in the scene. AI Servo or AF-C tracks focus on moving subjects so that the auto-focus always stays accurate regardless of whether the subject has moved or not. Using your back button AF in conjunction with this is a very useful technique in getting sharp, moving subjects. In the burst below, the hippo charged us and covered a good 50 meters in a flash. I captured the series of images using continuous AF and managed to get every photos focus sharp even though the focal distance was continually changing. I have only shown 6 images here for illustrative purposes. This is the most important AF mode in wildlife photography due to the unpredictability of the subjects.

Even though this Hippo was charging at full speed (therefore changing the focal distance), the cameras ability to continually focus made sure that all the images were sharply focussed.

For more information on your cameras auto-focus settings and the ability to utilise them properly, why not join us for a course. If you need more information, please contact us using the forms on our website.