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|1 Acadia National Park, Maine|
It’s easy to take autofocus for granted. I know, I do. This technology is amazing—your camera has to figure out what should be sharp in a scene, focus the lens and take the picture, all in a fraction of a second. We expect our cameras to do this frame after frame without fail. Of course, autofocus does fail us at times. As much as the camera manufacturers would like us to believe that their autofocus (AF) technologies are beyond compare, it sometimes has problems and we get images that aren’t focused properly. Sometimes we believe manufacturers’ hype and expect cameras to be perfect in an imperfect world.
You can make autofocus work better for you. There are steps you can take with any camera you own that will get you consistently sharper pictures with autofocus. You could spend a lot of time studying how autofocus works and then compare cameras, but in my experience, this won’t help you get better pictures. Frankly, having used all sorts of camera models, I can tell you that no matter what the manufacturers tell you about their systems, each one seems to have its strengths and weaknesses. It’s true that certain pro cameras are designed for speed, including AF speed. If you’re photographing subjects like especially active wildlife, that could be a critical need, but if you’re photographing landscapes, it won’t matter.
The key is not to find the absolute “best” autofocus, but to get the best autofocus from your system. Here are some tips to follow.
|2 St. Augustine, Florida|
1 Lock Focus For Manual Focus
There are situations when the camera wants to keep changing its point of focus in continuous focus or it wants to shift focus points every time you press the shutter when you’re on single-shot autofocus. There are also times where you may find it hard to focus the camera manually, perhaps because you’re using a wide-angle lens and you need to be sure it focuses on a specific part of the scene. In any of these instances, you can use your autofocus to get you close by locking focus on a key part of the scene and then turning off the autofocus so you just have manual focus. You’ll be focused on a specific spot and, in essence, you’re using autofocus to help you with manual focus.
2 Lock Focus
It’s important to be sure that focus is in the right place in any scene, whether that’s a landscape or a flock of birds. When focus is off, it’s all too obvious and can ruin a perfectly good picture. Nature photographers often shoot in low-light conditions that can some-times make it hard to see to focus. We become dependent on autofocus in such situations. Watch the AF lights in your viewfinder as your camera finds focus. This will tell you where the camera is focusing. If the camera isn’t focusing in the right place, move your camera slightly as you press the shutter release halfway until the right place is highlighted. Keep the shutter release pressed to lock focus (or hold the AF button down if your camera has one) as you move the camera back to the composition and then press the shutter release all the way to take the picture.
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|3 Acadia National Park, Maine|
3 Change Your AF Point
Most cameras allow you to select a specific AF point. The standard way that autofocus works in cameras is for the camera to choose what it thinks is the best focus point from an array of AF points across the scene. This works in a lot of situations. But if you have a subject that must be sharp in a specific part of the composition, you’re best off changing this default setting to a specific AF point where you need sharpness. One example is an animal that continually comes to a specific area in the composition. You need to be sure the animal is absolutely sharp, yet the subject may be at slightly different distances from the camera so that manual focus can’t be used. Simply select an AF point where the animal is likely to be.
4 Know Your Camera’s Idiosyncrasies
It’s great fun for photographers to get together and debate the relative merits of different cameras. And there’s no question that there are differences in the way that cameras handle autofocus. While the debate may be fun, it doesn’t help you get better pictures with your camera. You need to learn the idio-syncrasies of autofocus with your camera. What does it do best? Where does it seem to have problems? This comes from using your camera in all sorts of situations. One of the great things about digital photography is that you can shoot lots of pictures in order to see what your camera can do, yet there’s no cost to shooting those pictures.
|4 Shenandoah National Park, Virginia|
Sure, we’d all like the latest and greatest of the newest cameras, but until we can afford that new purchase, we need to understand how to get the best from the equipment that we do have. Every time that I purchase or test a new camera, I always take it out and play with it before I have to use it seriously. I want to know the camera’s idiosyncrasies regarding autofocus and other controls. It’s important to work with the camera in your hand and get the most out of it rather than worry that it can’t do something it wasn’t designed for.
5 SAF Vs. CAF Vs. Hybrid
Your camera offers at least two options for autofocus: single shot (SAF) and continuous (CAF). Single-shot AF locks down the focus and won’t allow the camera to shoot until focus is confirmed. This is an important type of focus for most standard nature scenes where focus doesn’t change (and shouldn’t change). Continuous autofocus allows the camera to focus continually as you take pictures, updating the progress of a moving subject, for example.
|5 San Elijo Lagoon, California|
A third option on many cameras is a hybrid. This type of autofocus allows the camera to decide when to use single-shot autofocus and continuous autofocus. This has never been a choice that I’ve liked. It seems like the camera is always choosing the wrong type of autofocus and screwing up my focus as I shoot. I’d rather choose a specific type of autofocus based on the subject and movement of the subject.
6 Watch For Bright Light
Bright light in your composition, especially the sun, can confuse your AF system. A dramatic way of photographing a landscape with trees is to shoot it against the sun so the sun creates a starburst pattern through the trees. But that dramatic effect also can cause problems with autofocus. Try moving the camera to autofocus without the sun and then reframing the composition. You also might have to change the camera to manual focus for scenes like this.
|6 Shenandoah National Park, Virginia|
7 Beware Of AF Up Close
Autofocus often has trouble when you’re dealing with close subjects. Depth of field is so shallow that even a slight change in focus can make the difference between a good picture and one for the trash. Very often the camera will choose the wrong point for focus up close. For this reason, many of the best macro shooters use manual focus for close-up work. There’s also a trick to using autofocus up close. Move your camera around and lock focus on an important part of your subject. Keep that focus locked and gently move your camera toward and away from the subject until you have exactly the right spot in focus. Take the picture. Another thing that drives you crazy up close is when the camera starts focusing to infinity. Many lenses have focus limiters for just this reason. If yours has such a switch, set it so that it only focuses at a close distance when you’re doing close-up photography.
8 Faster Lenses Help AF
A fast lens is a lens with a wide maximum aperture, such as ƒ/2.8. If you need fast autofocusing, you need a fast lens. Most zooms are slower lenses with maximum apertures of ƒ/4 or so, especially the compact zooms. If you have an extended range zoom that’s also compact, you can find that the lens speed gets very slow. For subjects such as landscapes or flowers, that’s not a big deal. For fast-moving wildlife, a slow lens can have a big effect on how quickly you can get the animal in focus.
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|7 Shenandoah National Park, Virginia|
9 Working With Teleconverters And AF
Modern teleconverters that are designed for specific focal lengths are very good. They might not match a single-focal-length lens in absolute sharpness, but on a cost-benefit basis, a teleconverter’s value can be huge. The problem is that they significantly reduce light to the focal plane of the camera and that results in less light to AF sensors. Sometimes this means that the autofocus doesn’t work at all with a teleconverter and a lens. In this case, especially, you may be required to use a fast lens with a teleconverter to coax your AF system to work.
10 Help Out Your AF System
It sometimes can take a fraction of a second to find an object and lock focus. Give your camera and lens a head start by starting the autofocusing early. If you wait until the last minute, you may find that you can’t get the photograph you want because focusing will be trailing the action. This is especially important for moving subjects, such as flying birds. Start by pressing the shutter button lightly, which engages your autofocus. If your camera has a specific AF button on the back of the camera, use it to start your autofocus (the camera is only focusing and not setting exposure or setting off the shutter).
Lenses and systems that allow manual focusing at the same time that your autofocus is on can help, too. Do a little prefocusing manually before you need your autofocus to work. That way, the AF system doesn’t have to search for something to focus on as you have already given it that information.
|A number of D-SLR (and 35mm SLR) manufacturers point out that some (and with a few models, all) of the AF sensors in their cameras are cross-types. Early phase-detection AF cameras used line-type sensors, which could only read focus with subject lines perpendicular to them: A horizontal sensor could read vertical lines but not horizontal ones; a vertical sensor could read horizontal lines but not vertical ones. A cross-type sensor, as you might suspect, can read both horizontal and vertical lines and thus is able to focus on a much greater number of subjects than a horizontal or vertical line sensor. Some cameras even employ diagonally oriented cross-sensors, so that they can autofocus on diagonal lines in a scene or subject.|
|Phase-Detect AF Vs. Contrast-Detect AF|
|With phase-detection AF, a portion of the light entering the lens is diverted to the AF module, where it’s split into two parts, which are directed onto a pair of CCD line sensors. The points where the two beams strike the sensor tell the camera’s AF computer whether the image is in focus or not, and if not, in which direction it’s out of focus and by how much. A phase-detection system can thus establish focus with a single reading and adjustment, making it quicker than contrast-based systems and much better for moving subjects (and in dim light).
All of today’s D-SLRs use phase-detection AF systems for normal shooting. Contrast-detection AF measures contrast at the image sensor, the idea being contrast is at its maximum when the image is sharply focused. A contrast-based AF system must take multiple readings to determine and set focus: After the first reading, focus is adjusted and another reading is taken. If the contrast is greater, another adjustment is made in the same direction and another reading is taken, and so on, until contrast starts to decrease. If the second reading shows less contrast than the first, an adjustment in focus is made in the opposite direction, then another reading is taken, etc. The result is that contrast-detection AF requires multiple readings and adjustments, taking longer than phase-detection AF to establish focus. But it can be more precise, and there’s no disruption of the live image during focusing as there is with phase-detection, so it’s excellent for tripod-mounted live-view work, where speed is not of the essence. Some D-SLRs offer both types of AF in live-view operation.
|AF In SLRs: A Timeline|
|1981 Pentax introduced the ME-F, the first interchangeable-lens SLR with TTL autofocusing capability. It took all Pentax SLR lenses, but autofocusing was possible only with one special AF lens: a 35-70mm zoom, which contained both the AF motor and the four AA batteries it needed to operate.|
|1985 Minolta introduced the Maxxum 7000, which really started the AF SLR revolution. The body contained not only the AF sensor, but a focusing motor as well, so AF worked with all lenses. Caveat: Only the new Maxxum lenses could be used with the camera; the Maxxum cameras couldn’t use previous Minolta system lenses.|
|1987 Canon introduced the EOS system, with the EOS 620 and EOS 650 models. EOS AF SLRs don’t contain focusing motors; rather, each lens contains its own motor optimized for its requirements. The drawback was that longtime Canon users couldn’t use their earlier lenses with EOS cameras; only EF (and today, EF-S) lenses can be used on EOS bodies. Other manufacturers put the AF motor in the AF SLR body to retain compatibility with previous as well as new AF lenses (only the AF lenses would autofocus, but users could use their existing lenses with manual focusing). Today, most D-SLR manufacturers offer higher-end lenses that contain their own focusing motors. (Nikon has even introduced entry-level D40 and D60 bodies that don’t have focusing motors and thus must be used with the AF-S lenses that do contain AF motors if one wants autofocusing capability.)|
|2006 Olympus introduced the first D-SLR with a Live-View monitor. The EVOLT E-330 had two Live-View modes, one that used the same phase-detection AF as was employed for normal shooting and a full-time Live-View mode with manual focusing.|
|2007 Nikon introduced the D3 and D300 with Live-View modes, featuring both types of autofocusing. In Handheld mode, phase-detection AF is used; in Tripod mode, contrast-detect AF is used.|