Autofocus on a Digital Camera

There are various ways to focus your camera depending on the situation you are photographing.
When you press the shutter button you feel a slight click halfway down, this allows the camera to focus and locks the focus while you maintain the button depressed, then you press it all the way down to take the photo.(on some DSLRs you may find you have a dedicated AF button). But on DSLR´s and most bridge cameras there are several different focusing options. I will describe them below to help you get the most of your camera and sharper shots. I have described Nikon and Canons options, for other users and for any bridge camera users please consult your manuals.
“AF-S” for Nikon and “One shot AF” for Canon. Here you pick a focus point, one your cameras you should see a little squares on the screen, you can change this and this is where it will focus. It is a pretty simple and straight forward. Although it normally requires you to lock focus on the subject before taking the photo, so if you are taking a photo of something moving the focus may not lock. “AF-C” for Nikon (continuous) and “AI Servo Focus Mode” for Canon. Used for shooting moving subjects, sports, etc. It analyses the subject movement and predicts where the subject will be, placing the focus at the predicted point. The advantage of this is that it will automatically readjust focus if you or the subject moves. All you need to do is continue half-pressing the shutter button or holding the dedicated AF button (if you have one) on your camera and the autofocus system will automatically track any movement.
“AF-A” for Nikon or “AI Focus AF” for Canon. This option switches between the other two options. If the camera detects if the subject is stationary, it automatically switches to Single focus, while if the subject moves, it will switch to Continuous focus. The default method on lower-end Nikon DSLRs is AF-A and it works quite well for most situations. Those with the newer Nikon camera with live view recording there is also a new setting. Full-time Servo AF mode, also known as “AF-F” is a brand new mode that was introduced by Nikon to the latest Nikon DSLRs like Nikon D3100 and Nikon D7000, specifically for recording video in Live View mode. Focus points are the little squares or dots you see when you look through the viewfinder. Depending on your camera you will have more or less of these dots, the more advance the camera the more focus points.
To change between focusing options and to change which focus point you are focusing with you´ll need to check your camera manual. Simple exercises. Set the focus mode to “AF-C” for Nikon (continuous) and “AI Servo Focus Mode” for Canon , and then photograph a moving object. For example, someone walks towards you. Focus on the person by pressing your shutter button half way down (keep it down until you take the shot fully). You should notice the camera re-focusing as the person moves. The important thing here is to not release the shutter button. You need to keep it pressed half way down, then when you’re ready to take the shot, press it down fully.
Now set the camera’s focus mode to “AF-S” for Nikon and “One shot AF” for Canon and photograph a stationary object. Notice this time, it focuses once.
For the third exercise, change the focus mode to “AF-A” for Nikon or “AI Focus AF” for Canon and photograph both a moving and stationary subject. You’ll soon notice, the camera doesn’t always guess the setting for the moving subject correctly. Hence the reasons why photographers like to choose focus settings for themselves. Here are two examples of focusing, in the first I focused on the stream and in the second I focused on the branches. Which is correct, well it totally depends on the intentions of the photographer. But it is important to focus in the correct place if the subject is a person.
Reasons why autofocus won’t always work on a digital SLR camera?
The first thing to check when your camera’s autofocus fails, is that you have your DSLR lens set on AF (autofocus) and not MF (Manual Focus). This may seem obvious, however there have been many times when I’ve used manual focus and forgot to change it back to autofocus when required. If you are able to switch between AF and MF, you will find the switch for this on the side of your actual lens.
Autofocus may have trouble working properly in low light conditions.
Many photographers find autofocus fails when photographing non contrasting subjects. For example, a cloudless blue sky or a wall that contains one solid color.
Photographing highly repetitive patterns like skyscraper windows or cars with reflective bodies can also cause problems.

 

Macro shots can also be challenging
Focusing on overlapping objects can also confuse your lens. For example, if you are trying to focus on a particular leaf of a tree. The lens can find it difficult to know what to focus
What to do when your DSLR lens autofocus AF won’t work?
Sometimes just focusing on something different then returning to what you want to focus on sorts out the problems.
Try focusing on an object within the same distance as the subject you are photographing. Then switch the lens to manual focus MF, recompose and take the shot.
There are times when your only option may be to use manual focusing.

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