Since almost all modern cameras, point & shoot or DSLR have autofocus built in, why blog about focus? Well, a couple of reasons.
- If you look at almost any photo gallery, either independent such as Flickr, or picture posting sections of individual & shared web pages, you will see out of focus images.
- It can be extremely frustrating to shoot once in a lifetime images and find after the fact that they are soft or out of focus.
- In many cases making a few changes in either the settings or operation of your camera can cure the problem.
The first thing to do is determine if the problem is focus or camera shake.
If you look at the sharp edges of the robes, etc. you can see the signs of camera shake in this image. This was taken at 1/6th of a second with a 70mm lens. There is a limit to how steady anyone can hold a camera. A basic rule is to use a tripod or other method of steadying a camera if the shutter speed is longer than the reciprocal of the focal length of the lens. For those who have been away from math for awhile, a reciprocal is the number over or divided by 1. For example, if your zoom lens is set to a focal length of 150mm, your shutter speed should be at least 1/150th of a second. If the shutter speed is much longer than this, most photographers will shake the camera enough to blur the image. The exceptions to this are cameras or lenses with built in image stabilization (IS) or vibration reduction (VR). The best way to identify camera shake is look closely at the image. Hard edges will often appear as multiple images. In other cases they may appear stretched or spread wider than they should be, but in both cases, still fairly sharp.
A poorly focused image will be blury throughout the area if interest. This is a good time to point out that a good photographer often uses focus to “focus” the viewer’s attention to parts of the image. You don’t need to have every part of your image tack sharp. The problem arises when the important part of your image is soft or blurred. Some reasons this happens include:
- Attempting to photograph a subject too close to the camera. All lenses have a close focusing distance. If the subject is closer than that distance, the image will be blurred. Some cameras will not allow the user to take a picture unless it is focused; some will take a photograph focused or not, and some can be set either way using the menus. If you like to take close ups or macro photographs, you need to learn the limits of your camera or lenses. In general, the longer the lens (telephoto) the longer the close focus limit. Some specialized lenses (macro lenses) are designed specifically for close focusing. Many cameras have a “macro” mode that can be used for closeups. Even if your camera doesn’t, close up attachments or add on lens may be available camera. Check this Digital Photography School link for suggestions for macro photography.
- Many point & shoot as well as most DSLRs have multiple focus points. Generally, they appear as brackets in the viewfinder or on the LCD. They are often selective – the photographer can choose which bracket is determining focus. Be sure the highlighted bracket is placed on the part of the image you want in focus. In this case the camera focused on the zipper rather than the baby’s face. Some cameras have face recognition built into the operating software that will automatically focus on faces. If your camera uses this technique, be sure that the subject’s face is what you want to be in focus. If not, you will need to shut off the feature.
- Many cameras provide “active” focusing, that is they will lock on to the subject under the selected bracket when the shutter release is half pressed, then follow the subject as it moves within the viewfinder so that it will be in focus when you fully press the shutter button. An example of where this is useful is if you were taking a photograph head on of someone on a swing. As they move towards & away from you the camera adjusts to keep them in focus. This is a feature that is usually turned on & off through your camera’s menus. Be sure you understand how it works so that it can help you keep things focused rather than create problems. The more advanced DSLRs have many different focusing modes – again it is important to understand what each mode does & decide if it will help or hinder your photograph.
- Cameras use a couple of techniques to determine focus. Phase Detection and Contrast Detection are two of the methods most often used. Phase detection is faster while contrast detection is often more accurate. While a few cameras can use either method, most use phase detection. A limitation of phase detection is the speed of the lens. Slow lenses (for example a slow f: 5.6 lens compared to a faster f: 2.8) will take longer to focus, or may not be able to lock focus at all, particularly under low light. When shooting with slow lenses under low lighting conditions, the photographer will often need to switch to manual focus.
- Although a good photographer often chooses his or her depth of field to determine the portion of an image that is in focus, when light levels are low or you need fast shutter speeds you may find your depth of field is so shallow that important elements of your image will be soft. Because choosing a larger f: stop to provide more light decreases your depth of field, low light can cause problems. Solutions include adding flash, or, if the subject is stationary, stop down to a smaller f: stop with a longer shutter speed and using a tripod to steady the camera. This photograph of a touch me not would be better if I had chosen a smaller f: stop to increase the depth of field so that the entire flower was in focus.
- Finally, use your camera’s magnified playback feature (most have it) to check the focus while still at the location. Better yet, if the image is truly a once in a lifetime photograph, use a laptop or other large screen to review the image to insure overall quality. The small, low resolution LCD screens used on most cameras are not all that useful for checking focus, even when magnified.
Anyone can improve their images by spending a little time learning how your camera’s focusing systems work.