Simple tips to
help you take better pictures |
Tips for Better Pictures
This is the third of several
e-tips
on how to take
sharper pictures
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Today's e-tip:
Getting the most out of your
camera's AutoFocus system
for Sharper Pictures
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| I've seen many images where the photographer shows me a picture and exclaims how sharp it is. Often, the reality is, that the image is just not sharp. It doesn't have the "crispness" that is so apparent with a "sharp" image. Most lenses are designed to take sharp pictures. Some lenses are sharper than others, but most are capable of taking a sharp image. If you've ever looked at images in magazines, and wondered why your pictures aren't as sharp, then this series is for you.
Learn to take sharp pictures like the Pros!
The rules for taking sharp pictures are relatively simple, and you can do it!
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Part 3: Getting the most out of AutoFocus
AF (AutoFocus) If you want sharp pictures, you should invest a little time in understanding your camera's AF system. We're going to explain the basics here.
So far, we’ve gone over some other simple basics – like keeping your optics clean and keeping your camera stable. But now, we’re going to focus on “focusing”. You can’t get a sharp picture if your lens isn’t properly focused on your subject. In the “old days”, we only had manual focus. YOU were the “autofocus”. It worked well, and you actually felt like you were more a part of the image-taking process. Today, most of us rely on autofocus. You need to understand your camera’s AF system in order for it to work properly. Depending on your camera, all of what is discussed here may require you to shoot in either A (Av), S (Tv), or M (Aperture Priority, Shutter Priority or Manual) modes. (we recommend shooting in A (Av) mode most of the time, as that mode gives you control over your aperture, which significantly determines how much depth of field you have - but that's a topic for another time.)
We’re going to use terms appropriate to Nikon and Canon cameras. But, most modern DSLR cameras have the same features – they just call it something different. We’ll use Nikon terminology, with Canon terminology in parenthesis – e.g. “AF-C Continuous (AI Servo) mode” - Nikon uses AF-C or Continuous AF and Canon calls the same thing AI Servo AF. Not all of these features discussed are found on point-and-shoot cameras, although some are.
So, let’s get started…
Light and Contrast
Most modern AF systems use “contrast” to determine focus. Thus, you need to have a minimum amount of light and a subject that has reasonable contrast – that is, both light and dark tones. AF systems can’t work on a subject that has a little contrast or is in very low light. In these cases, you’ll have to resort to (gasp!) Manual Focus.
Fully Automatic Mode
You can always just put your camera into full-automatic mode and not have to worry about focusing. In that mode, your camera will try to decide on what to focus on. Many cameras will just focus on the closest object that's behind one of the AF points in the viewfinder. Some will focus on a face if it detects one. Obviously, the camera doesn't know what YOU want to focus on, so it will make an arbitrary decision based on its programming. Sometimes it works well, sometimes it doesn't. You will get consistently better results if YOU make the decision what to focus on. (see below)
AF Points
Most AF systems have more than one AF point. AF Points are small areas in the Viewfinder or LCD screen that you use to tell the camera what you want to focus on. You have to first tell the camera which AF point you want to use and then you place that AF point on your subject and focus (generally by pressing the shutter button half-way). You can also tell your camera that you don’t want to choose an AF point by selecting the “auto selection” mode. If you do that, the camera will try to decide for you. This “auto” mode generally tries to focus on the closest object behind one of the AF points. Sometimes it works well; sometimes it doesn’t. For most photography, we don't recommend the "auto selection" mode.
AF Modes
There are generally TWO modes of autofocus: AF-S Single shot (One Shot) and AF-C Continuous (AI Servo), although most cameras now support a Face-detection AF mode, which focuses on one or more faces. AF-S Single shot (One Shot) AF is used for stationary subjects. This is what you would generally want to use most of the time. I suggest that when using this mode, you almost always select the CENTER AF point (the only time you wouldn’t want to do this is if your subject is moving within a small area and you don’t want to center it – remember, centering your subject is almost ALWAYS a BAD idea - but that’s a topic for another time). Aim the AF point at your subject; press the shutter button half-way and hold it there; and then recompose your image (remember the rule of thirds??). As long as you hold the shutter half-way, focus is locked. When you recompose, focus remains where it was when you first pressed the shutter half-way. Once recomposed, you can then take the shot. This is what most pros do. It’s simple and becomes absolutely “automatic” once you get used to doing it. Remember – keeping it simple will result in better images!
The second mode of AF is “AF-C Continuous” (“AI Servo”) – in this mode, when you press the shutter half-way, focus does NOT lock – instead it continually focusses on an AF point (depending on the AF-Area mode – see below). This is good for a subject that’s moving in the frame. BUT… this mode often means that the camera thinks it’s OK to take an out-of focus picture – unlike AF-S Single Shot (One-Shot) mode which won’t take a picture unless it’s IN focus. Sometimes you can change these default settings in the Custom Settings of your camera.
Some cameras also have an "Auto" (AI Focus AF) mode that will start out as AF-S Single Shot (One Shot) and change to AF-C Continuous (AI Servo) if the subject moves from the selected AF point.
Live View AF Modes
If you are shooting in Live View mode, the AF options are often named differently with names that loosely correspond to either AF-S Single Shot (One Shot) or AF-C Continuous (AI Servo). For example, newer Nikon cameras have a Face-Priority AF mode that detects and focuses on faces. They also have a Wide-Area AF where you select the focus point and a Subject-Tracking AF where the AF system tracks a subject as it moves around the frame. Newer Canon cameras support Face-tracking, Flexizone-Multi (where focus is selected automatically from among 31 AF zones), Flexizone-Single (where you use a single AF point to focus), and a hybrid which switches between modes automatically to track moving subjects.
AF Area Selection Method
You can also select an AF-Point Area Selection method. This tells the camera what to do if your subject moves from the initially-selected AF-Point. Frankly, I like to keep things simple, and often choose NOT to have the camera make that decision for me. Instead, I prefer to use one point – and it becomes MY responsibility to make sure the subject is always behind that AF point. But, if the subject’s motion is very erratic, you may need to select a dynamic method, where the camera dynamically decides where in the frame your subject has moved to. Dynamic modes sometimes work well; sometimes they don’t. Be prepared for many out-of-focus pictures with a Dynamic AF Area Selection Method - so take LOTS of pictures, hoping to get a reasonable amount of images in focus.
Combining the various options
You can mix-and-match the above options to create an AutoFocus system that does what you want it to. Below is a chart that shows how these can be mixed.
Nikon/Canon System Options (Note: other cameras will have similar options)
| AF-Area | AF-S Single Shot
(One Shot)
(You select one AF point & AF can be locked on that point by holding the shutter half-way) | AF-C Continuous
(AI Servo)
(One or more AF points can be used and AF continuously focuses until the picture is taken) | Auto Mode
(AI Focus AF)
(Single Shot/One Shot is used until subject moves, and then Continuous/AI Servo is used) | Single Point
| ...
A single AF point is used and focus locks when shutter is pressed half-way. Focus does not change once locked.
... | A single AF point is used and focus continuously focuses until picture is taken | A single AF point is used. The camera decides if the subject is moving or not. If stationary, Single Shot is used. If movement is detected, the mode changes to Continuous. | Dynamic
(AF Point Expansion) | You select a single AF point. The camera will attempt to follow the subject and maintain focus if it moves from the selected AF point to another AF point. You can often select how many points to use to follow a subject in the Custom Settings. | ...
You select a single AF point. The camera will use Single Shot AF initially, but will change to Continuous AF if the subject moves. If movement is detected, multiple AF points may be used
.... | Dynamic w/3D Tracking
(n/a) | You select a single AF point. The camera will attempt to follow the subject and maintain focus if it moves from the selected AF point to another AF point. All of the AF points are used and the camera's AF system will use the subject's color and contrast to track it to other AF points. | | Auto Area | The camera will focus on the closest object that is behind one of the AF points. | The camera will focus on the closest object that is behind one of the AF points and will continuously focus on it and will track it to other AF points if it moves. |
How to set your AF settings?
It's pretty simple to decide how to set your Autofocus settings:
- Decide whether to use AF-S Single Shot (One Shot) or AF-C Continuous (AI Servo) mode
- If the subject is moving, use AF-C Continuous (AI Servo)
- If the subject is stationary, use AF-S Single Shot (One Shot)
- Decide if YOU can track the subject with a single AF point
- If you can, use Single AF-Area mode
- If you can’t, use a Dynamic AF-Area (AF Point Expansion) mode
Custom Settings
Many DSLRs have options in their Custom Settings to customize how the AF system works. Often, the default settings works best, but you should become familiar with the settings that may help your photography. One significant custom option is whether or not to allow the AF system to take an out-of-focus picture when in AF-S Single Shot (One Shot) and AF-C Continuous (AI Servo) modes. Nikon cameras default to requiring the AF system acquire focus before a picture can be taken in Single Shot mode. That means, if the AF system doesn’t think the picture is in focus, it won’t take a picture. Nikon cameras also default to allowing an out-of focus picture when in Continuous mode. If you’re photographing a once-in-a-lifetime event, you may want this, but, many of us will want to change this setting to require the image be in focus.
What's the best mode to use when?
Here are some guidelines:
| Type of picture | AF Mode | AF Area | Comments | | Portraits or stationary subjects | AF-S Single
(One Shot) | Single AF Point | Always focus on the eye of a person/animal. Select a single AF point (I almost always just use the center AF point). Press and hold the shutter half-way to focus and then recompose and press the shutter fully to take the picture. | Sports
(slow moving) | AF-C Continuous
(AI Servo) | Single AF Point | Best for slower moving subjects. Select a single AF point and press the shutter half-way to focus on your subject. Hold the shutter half way and follow your moving subject keeping the selected AF point on your subject. While the shutter is held half-way, the camera will continuously focus on your subject, but it will NOT track it if it leaves the selected AF point. | Sports
(fast moving) | AF-C Continuous
(AI Servo) | Dynamic
(AF Point Expansion) | Best for fast moving subjects that you can't follow with a single AF point. Select a single AF point and press the shutter half-way to focus on your subject. Hold the shutter half way and follow your moving subject. While the shutter is held half-way, the camera will continuously focus on your subject and will track it if it leaves the selected AF point. |
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