Getting started with depth of field

In addition to the complex aspects of exposure and metering, Photography enthusiasts often find that depth of field is the most difficult concept to grasp. This is not hard to understand, because it is a coefficient of assumptions based on subjective judgment. If some people really pay attention to these technical issues, others simply stick to their backs to cope with night school courses, however, many people almost ignore technical issues or simply absorb something in urgent need. I have to admit that at first, the depth of field seemed like an abstract theory. At that time, the lecture on "dispersion sphere" showed me my attitude towards this problem.

Later, I finally realized the practical value of depth of field in solving practical problems and achieving special results for creation. Next I will gradually introduce how to control the depth of field in fully practical terms, and add new information in each section.

* Understand basic theories. In general, the depth of field is the "acceptable clear area" that is extended before and after the adjusted focus ".

In fact, only the focused plane in any photo is truly clear. However, in the viewer's opinion, the objects before and after the plane may also appear quite clear. The difference in clear scope is based on several standards and we will be exposed to it in the following sections. Textbooks often discuss the "minimum dispersion circle" when interpreting the depth of field, but it is a technical problem that is not necessary for practical application.

For example, if you place the focus of the lens on the eyes of the benghu in the shadow at the natural zoo, its eyes are the clearest on the negative film. At this time, the tiger's mouth and the bark behind it also showed acceptable clear images in the final photo. When your eyes are removed from the focus, the Blur will gradually increase. The farther the object in the near foreground and in the distant background is from the tiger's head, the worse the definition.

* Select an appropriate aperture. Aperture calibers are the basic elements that affect the depth of field. In summary, a small aperture (represented by a large F/value), such as f/16 or F/22, produces a wide range of clear focusing ranges. Conversely, a large aperture (represented by a smaller F/value), such as f/2.8 or F/4, produces a short depth of depth, the acceptable clear range of prospects and backgrounds is much smaller.

The choice of aperture is a basic element for creating a file instead of simply taking a photo. Even when using the program exposure mode, you should select the most appropriate combination of aperture and speed when feasible. However, it should be noted that if you take a photo with a hand-held camera for a long time, the movement of the subject or the jitter of the camera can blur the photo. Therefore, you have to compromise: Choose a larger aperture that is far from ideal to maintain sufficient shutter speed.

For example, in the UK, you can take a photo of a scene formed by a stone arch corridor. You can focus on the fountain close to the central scene and press the shutter. 6 real shots, F/4 for the first shot, and F/2nd for the second shot, and so on until F/22 for the second shot. Then you can carefully observe the 8x10 inch photo.

The images of the arch corridor and the background castle on the first photo taken with F/4 are not clear. It seems "almost clear" when I use "f/11", but it still does not meet your standards. The arch corridor, fountain, and background Castle images taken with F/16 are clear enough, and your friends think this is the best picture. The image captured with F/22 is blurred because camera jitter is caused by the use of low-speed shutter and no tripod.

* Select a longer or shorter focal length. Anyone who uses a 28mm or 300mm focal length lens will find that wide-angle photos usually have a wide range of depth of field. On the contrary, those photos taken with a long focal length generally have a very low depth of field. This usually leads to a simple conclusion: the longer the focal length and the shorter the depth of field on any known aperture gear. If you need a picture with a large depth of field, you can use a shorter focal length.

The above seems to be an evaluation of depth of field in practice. But in fact these statements are quite confusing. The depth of field does not change due to different focal lengths. This is an optical fact. Believe it? Then a specific test is performed with one 35mm shot and one 200mm shot. First, use a long camera to photograph a friend standing next to a long fence filled with flowers. Then she switched to the wide-angle camera and approached the shooting, so that she was as big as the previous image in the viewfinder. Observe these slides with a 10x magnifier and you will find that the acceptable clear focus ranges (checking those flowers) are almost the same. Suppose they are different in perspective: the line and shape are definitely inconsistent, and the surface distance between objects is also different from each other. The wide-angle lens produces an "extended perspective", increasing the surface distance between objects. This produces an optical illusion that careless observers think that the depth of depth is different on these photos.

* The focal length used for the transformation at the same position. To be fair, the long focal length does reduce the depth of field, while the wide-angle lens expands the depth of field-but only when your shooting distance is the same. In fact, the size of the image affects the depth of field, regardless of the lens used. Whether you use a long focal length lens from a distance or move forward close to the subject and use a short focal length lens to fill the screen, the depth of field will be shallow.

For example, you can sit down on a lawn across a street from a historic building and take a photo of its entrance at f/28mm with a 5.6 lens. The image is still in the same position. However, if you use a 300mm-100mm zoom lens, you can take another three images at the focal lengths of 200mm, 300mm, and 5.6, F/is used. Check the photos and you will find that the doors of the building appear larger and larger on these photos, while the acceptable intensity of focus is obviously reduced with the extension of each focal length.

So can a long focal length lens produce a smaller depth of field than a wide-angle lens? Yes, but it can only be used at the same shooting distance. This is due to the increase in the magnification of the subject image, which reduces the acceptable range of focusing clarity.

* Change the distance between the camera and the subject. As I have mentioned, the depth of field is affected by the distance from the camera to the subject. When you focus on something very close to the lens, the depth of field is very small. When you focus on a distance, the depth of field will be larger, and the depth of field will change due to different focal lengths.

Tip: Take close-up photos, such as macro photography. Remember that the depth of field is calculated in inches. The camera must be placed so that its film plane is parallel to the plane of the subject-perhaps the wings of a dark-spotted butterfly.

You do not need to use small aperture such as f/32 to keep the butterfly completely within the depth of field. You can use a large aperture with a high shutter speed, which reduces the risk of blurred images produced by moving the camera or the subject. Another benefit is that most lenses provide a high resolution when using a mid-range aperture.

For example, if you use a 8 feet lens with a recent focusing distance of 300mm, assume that the aperture f/8 is used. You found a blooming cactus in the sounola desert and captured a few images from the car. At this time, you still think it is too far away, in the infinite distance of the camera ruler. Then you go up until you start shooting 8 feet times at a distance from the subject.

The cactus in the beautiful scenery is quite clear, at least recognizable, And the cactus on the background looks very clear. However, in the recent photo, only the cactus you see is clear. Those cuts on the background become pale green color blocks that cannot see the outline, while those in the foreground become blurred by the shortest depth. In these two cases, the longer the cactus is from the focal point, the clearer it is.

Remember: When you approach or stay away from the subject, two other factors-the size and perspective of the subject in the context box have also changed. For example, the representation of shapes and lines at 100 feet and the surface distance between objects are much different from those at 8 feet (this point can be seen on the viewfinder .)

* Determine the "correct" focus. As a common sense, the depth of field is distributed in the following ways at the normal shooting distance: the depth of field extends about 1/3 to the front of the focus, and about 2/3 to the back of the focus. Do you want a large depth of field? In this case, the focus is roughly located at 1/3 of the major scenarios.

In particular, it is tempting for the camera to set the focus when shooting with a single-lens reflective camera with auto focus. Although "hasty shooting" may be necessary to capture transient dynamic moments, it is important to control the exact focus when conditions permit. It's easy to use an automatic focus camera-usually you can lock the focus by clicking the shutter button when you recompose the image.

For example, when you are at a racing scene and see three high-performance sports cars parked nearby: a black Porsche parking space in the foreground, a red BMW car behind it, A white lotus car is in the background. At this moment, you find a celebrity leaning against the nearest car, his blue eyes are clearly visible. You must make a decision immediately: whether to focus on the BMW car close to Zhongjing to get the maximum depth of field, so that the three cars are clear at the same time; or to carefully focus on the people in the foreground, in this way, you can take his photos and present them to your friends?

When you take a clear goal at the scene as the main subject, you should usually focus on the most important place: the eyes of the character, the letters on the yacht ship's side, or a rock painting on the cave wall. Although you do maintain some degree of control over the depth of field as described in other sections, the depth of field has become a secondary issue at this time.

* Superfocal focus is used. There is a special point that can generate the maximum depth of field-accurately aligning the focus to this "superfocal length" position. It is still able to keep the subject in an infinitely distant distance clear enough recent focus, so that the depth of field will extend from half of the focus distance to infinitely far. Of course, the depth of field varies depending on the selected aperture and the focal length of the lens. However, this technology will indeed allow you to increase the depth of field of the screen you shot, and you do not have to shrink the aperture to a minimum, so that the image is blurred due to camera vibration or moving the subject. (For details about the formula for calculating the superfocal length, refer to the 4th issue 72 pages of this journal's "selection of letters from readers"-Editor's note)

For example, you shot times square in the middle of a street with a 50mm lens. You want to make the whole area clear, but you cannot shrink the aperture to more than F/11 (a small aperture requires a 1/30-second slow shutter, this will blur the car on the road ). With the superfocal length method, you can set the focus to 30 feet, which replaces the autofocus system that will focus on the foreground kingfa beauty. On the Scene screen, most of the images seem to be out of focus and will not be pleasing to the eye. You insisted on taking this photo and found that all the scenes from 15 feet to infinitely far on the printed photo were quite clear, that is, they were within the depth of field.

* Use less infinitely far focus. I rarely focus infinitely far on the field and city scenery, because in this way, any foreground part may be blurred, this type of subject often plays an important role in the image-they can add three-dimensional effects to two-dimensional photos. In addition, most of the depth of field in this method will be behind the infinite distance, and the actual value of the screen will be lost, "Wasting" the depth of field.

However, when shooting sports, racing cars, and wild animals, if the subject is located far away, infinitely far focus may be useful. In this case, you can tolerate a portion of the images in the viewfinder being blurred-because your primary goal is to produce an absolutely clear subject.