Digital photography

In addition to the complicated aspects of exposure and photometry, photographers usually find that depth of field is the most difficult concept to master. This is not difficult to understand, because it is a hypothetical coefficient based on subjective judgment. If some people really pay attention to these technical problems, some people just cram for evening classes, while many people almost completely ignore the technical problems or only absorb what is urgently needed. I have to admit that at first, depth of field seems to be an abstract theory. At that time, the lecture on "diffusion circle" quite explained my attitude towards this issue. Later, I finally realized the practical value of depth of field in solving practical problems and realizing special effects for creation. Next, I will introduce how to control the depth of field step by step in completely practical terms, and add new information in each part.

* Understand the basic theory. Generally speaking, the depth of field is an "acceptable clear area" extending around the adjusted focus. In fact, in any photo, only the plane in focus is really clear. However, in the observer's view, the objects before and after this plane may also appear quite clear. The difference of clear distance is based on several standards, and we will touch on the following parts. Teaching materials often discuss the "minimum dispersion circle" when explaining the depth of field, but that is a technical problem and there is no need for practical application. For example, in the nature zoo, you focus on the eyes of the Bengal tiger in the shadow, and its eyes are the clearest on the negative. At this time, the tiger's mouth and the bark behind it also showed an acceptable and clear image in the final photo. When you look away from your focused eyes, the degree of blur gradually increases. The farther away from the tiger's head, the worse the clarity of objects in the near foreground and far background.

* choose "appropriate" aperture. Aperture Aperture is the basic factor that affects the depth of field. Generally speaking, a small aperture (represented by a large f/ value), such as f/ 16 or f/22, will produce a wide clear focus range. On the contrary, a large aperture (represented by a small f/ value), such as f/2.8 or f/4, produces a shorter depth of field, and the acceptable clear range on the foreground and background is much smaller. To create a work instead of simply taking a photo, the choice of aperture is a basic element. Even when using the program exposure mode, if possible, you should choose the most suitable combination of aperture and speed. However, it should be noted that because of the hand-held camera shooting, if it is exposed for a long time, the movement of the subject or camera shake may blur the photo. Therefore, you have to compromise: choose a larger aperture that is far from ideal to maintain sufficient shutter speed. For example, shoot a scene of a frame formed by a stone arcade in Britain. You can focus on the fountain near the middle lens and press the shutter. Take six pictures, the first one is f/4, the second one is f/5.6, and so on until the second one is f/22. After that, you can carefully observe the printed 8× 10 inch photos. The arcade and the background castle in the first photo taken with f/4 are not clear. All this looks "almost clear" with f/1,but it still doesn't meet your standards. The images of arcades, fountains and background castles taken by f/ 16 are clear enough, and your friends think this is the best photo. However, the image taken by f/22 is blurred, which is caused by the use of low-speed shutter, no tripod and camera shake.

* Choose a longer or shorter focal length. Anyone who uses 28 mm and 300 mm focal length lenses will find that wide-angle photos usually have a wide range of depth of field. On the contrary, those photos taken with long focal length are generally shallow in depth of field. This situation usually leads to a simple conclusion: the longer the focal length, the shorter the depth of field of any known aperture position. If you need a photo with a larger depth of field, shoot it with a shorter focal length. The above seems to be the evaluation of depth of field in practice. But in fact, these statements are quite confusing. It is an optical fact that the depth of field will not change with different focal lengths. Don't you believe it? Then use 35 mm lens and 200 mm lens for specific test. First, take a long shot of a friend standing by a fence full of flowers. Then put on a wide-angle lens and shoot it at close range, so that she can be as big as the previous image in the viewfinder. Look at these slides with a magnifying glass of 10 times, and you will find that the acceptable clear focus range (check those flowers) is similar. Suppose they have different perspectives: the expressions of lines and shapes are definitely inconsistent, and the surface distances of objects at various distances are also inconsistent. Wide-angle lens produces "magnified perspective" and increases the surface distance between objects. This can create an optical illusion that careless observers will think that the depth of field in these photos is different.

* The focal length used for transformation in the same position. To be fair, telephoto lenses do reduce the depth of field, while wide-angle lenses enlarge the depth of field-but only if you shoot at the same distance. In fact, no matter what lens is used, the size of the image will affect the depth of field. Whether you take a full picture with a long focal length lens from a distance or a short focal length lens before you walk to the main body, the depth of field will be shallow. For example, sitting on the grass across the street from a historic building, taking a picture of its entrance with a 28 mm lens at f/5.6. It's still in the same position, but it's changed to a zoom lens of 100-300mm, and three shots are taken, with focal lengths of 100mm, 200mm and 300mm respectively, all of which are f/5.6. Looking at the photos, you will find that the doors of the buildings in these photos are getting bigger and bigger, but the acceptable focus range is obviously reduced with the extension of each focal length. So, can a long focal length lens really produce a smaller depth of field than a wide-angle lens? Yes, but this can only be used at the same shooting distance. This is because the magnification of the subject image increases, thus reducing the acceptable focus range.

* Change the distance between the camera and the subject. As I 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 at a long distance, the depth of field will be larger, and the depth of field will change with the focal length. Tip: When taking close-up photos, such as macro photography, please remember that the depth of field will be in inches. The camera should be placed so that its film plane is parallel to the plane of the subject-perhaps the wings of the monarch butterfly. You don't have to use a small aperture like f/32 to keep the butterfly completely within the depth of field. With a larger aperture, a higher shutter speed can be used, which will reduce the risk of image blurring caused by camera or subject movement. Another advantage is that most lenses can provide higher resolution when an intermediate aperture is used. For example, if you use a 300 mm lens and the nearest focus distance is 8 feet, suppose you choose the aperture f/8. You found a flowering cactus in the Sonora desert. You took some photos in the car. At this time, you still feel too far away, in the infinite position of the lens ruler. After that, you continue to take the second photo until you are 8 feet away from the subject. In the photo of 1, the cactus in the beautiful scenery is quite clear, at least it can be recognized; The cactus in the background also looks very clear. However, in recent photos, only your favorite cactus is clear. The cactus in the background becomes a light green block with no outline, while the cactus in the foreground is blurred by the shallow depth of field. In both cases, the farther the cactus is from the focus, the less clear it is.

Remember: when you are close to or away from the subject, the other two factors-the size and perspective of the subject in the frame will also change. For example, the shape and line appearance at 100 feet and the surface distance between objects are quite different from those at 8 feet (this can be seen in the viewfinder. )

* Determine the "correct" focus. As common sense, the depth of field has the following distribution modes in the usual 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? Then set the focus roughly at 1/3 of the big scene. Especially when shooting with an autofocus single-lens reflex camera, it is easy for the camera to set the focus. Although "hasty shooting" may be necessary to capture fleeting dynamic moments, it is also important to control the accurate focus if the environment permits. Even with an autofocus camera, it's simple-usually just press the shutter button to lock the focus when you re-compose.

For example, at the racing scene, you see three high-performance sports cars parked nearby, with a black Porsche in the foreground, a red BMW in the back and a white lotus in the background. At this time, you find a celebrity leaning against the nearest car, and his blue eyes are clearly visible. You must make a decision immediately: focus on the BMW car near the middle scene to get the maximum depth of field and make the three cars clear at the same time; Or carefully focus on the person in the foreground, so that you can take a complete picture of him and show it to your friends? When you take a clear target in the scene as the main shooting object, you should usually focus on the most important places: the eyes of the characters, the letters on the side of the yacht, or the rock paintings on the cave walls. Although you mentioned the control of depth of field in other parts, depth of field has become a secondary issue at this time.

* Select hyperfocal distance to focus. There is a special point, which can produce the maximum depth of field-focus on this "hyperfocal distance" position accurately. Even the nearest focus can still keep the object clear enough at infinity, so that the depth of field extends from half of the focal length to infinity. Of course, due to the different aperture and focal length of the lens, the depth of field will be different. However, this technology does increase the depth of field of your photos, and you don't have to minimize the aperture, so the image will be blurred because of camera vibration or subject movement. (For hyperfocal distance's calculation formula, please refer to the fourth issue of this year on page 72 of Selected Letters from Readers-Editor's Note).

For example, you shoot Times Square in the middle of the road with a 50 mm lens. You want to shoot the whole area clearly, but you can't reduce the aperture to more than f/ 1 1 (even a smaller aperture requires a slow shutter of 1/30 seconds, which will blur the moving car). With hyperfocal distance's method, you can set the focal length to 30 feet, which replaces the autofocus system focusing on the blonde in the foreground. On the viewfinder screen, most images seem to be out of focus, which makes them look unpleasant. You insisted on taking this photo, and found that all the scenes from 15 feet to infinity in the printed photo were quite clear, that is, they were all within the depth of field.

* Use infinite focus less. I seldom use infinite focus when shooting wild and urban landscapes, because in this case, any foreground part may be blurred, and such subjects often play an important role in the picture-they can add three-dimensional effects to two-dimensional photos. Furthermore, in this case, most of the depth of field will lose its practical value to the picture after infinity, and "waste" the depth of field. However, when shooting sports, racing cars and wild animals, it may be useful to focus at infinity if the subject is far away. In this case, you can tolerate some blurred images in the viewfinder-because your main purpose is to shoot an absolutely clear subject. Skills of shooting waterscape Photographers who like rain often enjoy themselves in rivers, lakes and seas by boat. Shooting waterscape is also an interesting activity, but it also has its own unique shooting requirements. Generally, when shooting waterscape, you can't be underexposed. Because the natural water source is often set off by the sky, the reflection is strong, just like a huge luminous body. When shooting, if you only rely on the camera's photometric instructions to deal with the exposure, there will often be underexposure, because the brightness of the sky is very different from that of the general scenery. When shooting a waterscape with sky reflection, at least 2 exposures should be added according to the camera metering instructions. Attention should be paid to the use of light when shooting the picture of wave ship shadow, and the subject should be highlighted according to the relationship between light and shade in the picture. In order to make the water features have special effects. One method is to use a high-speed shutter, such as11000 seconds, to condense moving water, such as waves. Proper framing and exposure may get the effect of spraying beads and splashing jade, and we can often see similar techniques in some photos showing waves.

Another method is to obtain the illusion and confusion of running water in the picture with the help of a slow shutter. The photos taken in this way are very distinctive in artistic features, and the flowing water often presents a feeling as ethereal as a cloud. The attached picture was taken at the entrance of a village in Jiangxi Province. This place is actually not very good, but through the slow shutter action, the flowing water of the stream has a new charm, making an originally ordinary mountain village scene interesting.

It must be pointed out that in order to reflect the sense of movement of running water, the shutter speed needs to be selected according to the speed of running water. If the running water speed is fast, the shutter speed should be high and the shutter speed should be low. When choosing a low-speed shutter, it is best to use a tripod to stabilize the camera, so as not to shake the camera and affect the picture clarity. If you have a telephoto lens, you can also use it to compress the space and shoot ripples in the water. When the radio wave is compressed by the telephoto lens, it will show a very rhythmic line, which is wonderful.

Shutter answer:

Readers with certain photographic knowledge know the relationship between shutter and imaging: the faster the shutter speed, the less likely it is to paste. Since the shutter is called shutter, "fast" is very important, especially when shooting or shooting moving objects by hand, the speed of shutter and the clarity of image play a decisive role. However, have you ever heard the expression "slow door"? Whether it is a SLR camera or a small DC with manual function, special visual effects can be obtained by skillfully using the "slow door".

The "miraculous effect" here refers to peculiar and special visual effects. For example, the running water is like silk, the car river is like a beam of light ... all these effects need to be achieved by using the "slow door". What is a "slow door"? As we all know, cameras have shutter speed, and the shutter speed of ordinary digital cameras is mostly within 1/2000 seconds, which can basically cope with most daily shooting. The so-called "slow door" is actually not a technical term, but refers to a relatively low shutter speed, which is relative to a high-speed shutter, and usually means that the shutter speed is slower than 1/30 seconds.

Some novices pay too much attention to the clarity of photos, never try to use the "slow door", and even many people are too lazy to use a tripod ("slow door" needs a tripod), which is too much trouble. In fact, taking pictures with the slow door, because of the tripod, the still objects on the picture are still static, while the moving objects can take pictures of the combination of reality and reality because of dragging, which is often more dynamic than the high-speed shutter.

The picture 1 is a picture of a fast-moving tour bus, and the shutter 1/8 seconds. The combination of static and dynamic pictures is more vivid and beautiful than a completely still picture. Moreover, the pillars supporting the roof disappeared because of the movement of the car. The shops and people were clearly visible through the glass, and the picture was very interesting.

Beginners can also try to shoot moving objects at a slower shutter speed. Fig. 2 shows the tail effect of the car lamp caused by shooting the vehicle at high speed with a shutter of 20 seconds. M-stop manually sets the exposure parameters. Because the exposure setting is based on the headlights, and the car body is dark, the exposure is seriously insufficient, so there will be no interference from the car body (unless the car speed is very slow), only the bright headlights on the screen form the feeling of pulling the wire. Isn't it interesting? As for the exposure time, it depends on the speed and density of the vehicle. Fast and dense vehicles are short, and slow and sparse vehicles are long. The aperture is also set according to the brightness of the scene.

In principle, you should use a small aperture to shoot night scenes. Take Figure 3 as an example, the aperture is F 18 and the exposure time is 10 second. Under a small aperture, the fixed light will have a dotted effect, which is caused by the diffraction of light at the edge of the aperture blade (the larger the aperture, the closer to the circle, and the smaller the aperture, the more obvious the corner). Generally speaking, the star-awn effect is better, and the lighting effect under large aperture is halo. Unless you particularly like the halo effect, use a large aperture.

Door B is also an application mode of "slow door". As long as you want, you can manually control the exposure time and end the exposure at any time, which is more flexible. Figure 4 is a photo of fireworks taken with door B. Because it is difficult to predict the blooming time of fireworks, it is more handy to use door B. Press the shutter at the moment of blooming, and let go when the fireworks bloom to the maximum, and it will be over. Of course, in order to avoid the impact of the fuselage shaking, in addition to the tripod, you should use the cable release or remote control, and try not to press the shutter by hand.

It is much better to use the "slow door" to represent the trajectory of a moving object than to take a completely clear photo. For example, let the vehicle freeze on the photo, but it is best to let it drag, which is more dynamic; Shooting dancers shaking ribbons on the stage can make the actors' arms and ribbons move and make the picture more active. Need to emphasize the peak passenger flow. On the one hand, it solidified the background building of the station, on the other hand, it deliberately let the crowd drag, and the crowd really "flowed".

Usually, we use shutter priority to reduce the shutter speed. If you shoot in bright light during the day, the shutter speed often cannot be reduced. At this time, filters such as ND or CPL can be used to dim the light, but it is easier to use the "slow door" method in night photography and stage photography. The photo we introduced today is the "slow door" technique used in night scenes.

What is gate b

B-door is a function that allows you to be exposed for a long time. The exposure time of door B is the time from when you press the shutter to when you release the shutter. It can be exposed for tens of minutes or even hours and can be controlled at will. The basic requirements for using B-door photography are a camera with B-door, which can open the shutter for long exposure, a stable tripod and a manual cable release.

"slow door" focusing skills

The "slow door" is often used to shoot fast-moving objects, so it is difficult to focus. When autofocusing, either the focus runs to the background or the lens "pulls the bellows" for a long time (the lens will move repeatedly when the focus cannot be found in a monochromatic background such as the sky). Then, when the car passes by, fireworks bloom and other focusing conditions appear, you can quickly autofocus, and then change the focusing mode to manual focusing without rotating the lens, so that when you shoot the next car and fireworks again, you can omit the autofocus process. The above methods are only applicable to SLR cameras. When shooting with a small DC with autofocus function, you can only finish shooting quickly while focusing, and take more pictures to improve the success rate.

Shutter priority exposure mode is often used in "slow door" shooting, but when the light ratio is strong (especially bright and dark), the camera often misjudges. At this time, you can increase the exposure compensation, or set it yourself with the M file, increase the exposure time or open the aperture.