ttl闪光灯控制（TTL flash control）

ttl闪光灯控制（TTL flash control） ttl闪光灯控制(TTL flash control) TTL (through the lens) flash control is a special flash function, also known as TTL flash automatic control. It not only requires the flash to have this function, but also requires the camera to have the corresponding mechanism, cooperate with each other in order to play a role. The principle is similar to that of an automatic dimming flash, but metering is accomplished by a special metering element in the camera, which is located at the bottom of the reflector case. In the flash exposure, the light measuring element in the camera directly measures the reflected light (OTF mode) of the film plane. When the exposure is sufficient, the camera will contact the body through a special shoe to send the control command to the control circuit of the flash, cut off the capacitor discharge circuit to flash lamp, turn off the flash, which is guaranteed not to excessive exposure. The TTL flash control is similar to the camera's "measured light" mode. When the lens is replaced, the camera's metering system can still measure the changing light, so the control of the flash lamp is more accurate. The signal flow chart of TTL flash control is shown in figure 5-6. In TTL flash control mode, as long as the object is in the flash range, operators do not need to consider the relationship between flash index and aperture coefficient, can be absorbed in shot composition, so the way has become a standard feature in the high-end camera. In the TTL flash control mode, although the background of the image is reflected on the film plane, the metering element also reflects the brightness. But the most important thing is that in this way before the flash does not automatically control the flash output until the flash metering element receives the amount, reflecting after to flash control, so it is a kind of passive control method. Because of its principle limitations, the TTL flash control is not perfect. As we mentioned in the third chapter on the photometric system, metering systems are unintelligent, as in TTL flash control. Such as when the background is black and in addition to the flash range, the subject is near the camera, because there is no background reflecting light back flash may be released from all the energy, the subject may be overexposed. This is more common at large dances or at night outdoors. On the contrary, if the background is bright and flash within the effective range, be taken between the subject and the background is close, because the background returns to light more, easy to cause the subject is underexposed, such as standing in front of a white wall photo shoot, we often see this situation. The solution is to use automatic exposure locking, which uses the same method as does not use flash. Measure the target body first (approaching the subject when necessary), then press the auto exposure lock button, then the flash of the flash will be locked, and then re - compose the shot. As with the Minolta 7000 dedicated flash AF2800, you can achieve automatic exposure with flash lock. As to whether the reader's own camera can be locked with such automatic exposure, please test yourself. Test method: dedicated flash connected to the camera and is switched on, the first lens for light bright and locked, then look in the mirror (preferably a big mirror, such as the mirror, the distance to the 1. More than 5 meters), press the shutter release button to flash exposure, observe the flash brightness with the eyes, because the original light region is bright, the brightness of the flash should be relatively small; then aligned shadows (preferably black at) photometry and locked, and press the shutter release button on the mirror. If the brightness of the two flashes is different, it shows that the camera can realize automatic exposure locking with flash; otherwise, the camera does not have the function. At present, most dedicated flash is inserted in the "hot shoe", or by cable from the machine to achieve TTL flash control. Minolta Dynax 7xi created from the TTL machine wireless flash control. The principle is: use and special flash 3500xi (not inserted in the shoe on), Dynax 7xi the first micro built-in flash flash once, triggering flash synchronization through the built-in 3500xi camera, when the optical element measured enough beta flash, built-in flash and micro flash once received second times in 3500xi, automatic shutdown after flash flash. The working principle is shown in figure 5-7. Packed flash control Many people may think that light is only used when light is not enough. In fact, you can still use flash in sunny and sunny days, and you can also take pictures with excellent results. But this method of use is different from the normal way of the flash. Use the fill flash described in this section. This control method also called light, when photographed picture contrast is very strong (such as backlight), can use the flash to fill the shaded area. But to maintain the natural sense of the picture, the flash will have some restrictions, flash brightness and live light to have a certain proportion. If the output brightness flash occupies the main position in the picture, the pictures may be very strange, the effect is not natural, such as in the sunny day under the shade of portrait photo, face for flash brightness and pale. The human eye has a strong adaptability to a variety of brightness occasions. As with a bright window and sit back when talking to people, we can still tell him (or her) the details of the face, unaware he (she) is at the position of backlighting that person's eyes, with automatic backlight compensation function, while the brain is compensation computing machine. Unfortunately, the film does not have the automatic compensation function, if the center weighted average metering by exposure or similar photometric value after the shooting, often appear in silhouette, don't distinguish facial details. If you increase the exposure, the background turns white because of overexposure. Therefore, many people dare not shoot such occasions. When in use, the position of the subject is usually required to be replaced. When outdoors, the subject is asked to be oriented to the sun, which is ideal for photography. The sun's exposure to the face can accurately record all the details. But for the subject, it is a little uncomfortable. If the sun is strong, the eyes may squint and the facial expression is unnatural, as if it is painful and difficult to obtain vivid pictures. The fill flash, you can avoid the above problems, the subject can be in a position to be photographed backlit, brightness emitted by the flash to fill the foreground of the dark part. The principle of filling flash is that the brightness of the subject is enhanced and the details are fully displayed on the premise that the background exposure is normal. Therefore, the light value at the highlight of the background should be set. Manually set the general approach is as follows: the shutter speed is set to the maximum sync speed, then according to the flash index and subject distance, according to the index formula to calculate the aperture value, then down one or two stops to set the actual (equal to or less than a second exposure). Specific settings as backlighting (i.e. degree of contrast degree) and stop down more, there will be negative effects. It's difficult to manually set the parameters of a packed flash, which, like manual exposure and selection of exposure combinations, requires a lot of experience to set it right. In most cases, the parameters that many people set expose to overexposure. Now many dedicated electronic flash flash control program has filled the can before exposure according to the on-site measurement of light and contrast situation picture, according to the program automatically set the flash amount and field ratio of light to flash in the conditions, to keep the scene light just perfect object natural atmosphere. This is called flash program automatic exposure (Canon called A-TTL control mode). Fill flash control and TTL flash control difference: fill flash control mode in flash before it has taken the picture according to the light, the flash output, so is the active control mode. Since the filling type flash control involves the consideration of the background, it is closely related to the metering mode on the camera. The advantages and disadvantages of the metering method also affect the effect of the filling flash control. At present, because the divisional metering method can better deal with the relationship between the subject and the background, so the effect of filling flash control is ideal. Another difference is that the filler flash control is not necessarily TTL. For these reasons, some of the camera names confused many users. Such as the Nikon F4 on the matrix balance and center weighted two fill flash mode, the flash control actually filled balance matrix is associated with matrix metering mode, photo screen photometric value by matrix metering and filling type; and the central focus of the weighted flash control is associated with the center weighted the average metering mode, the photometric values given by the center weighted average metering mode. The difference between the two filling flash controls is simply the difference in metering methods. Other manufacturers do the same thing, because most of the existing AF SLR machines have a zoned metering method, and the effect of the associated flash control is similar to that of the matrix balanced filling type. So we don't have to be confused by a specific noun. We know that the factors that control exposure include shutter speed, aperture, and flash, as well as in fill flash control. Most users use flash to control the exposure when they use flash photography, but in fact they can adjust the shutter speed to control the exposure. If the shutter speed is slow at the same aperture, the background will be more exposure (assuming the background is outside the flash range), and the picture will be more natural. This is also one of the differences between fill flash control and TTL flash control. Readers may have the experience of shooting on a cloudy day with less ambient light. If using TTL flash control, although the subject exposure is accurate, but the background exposure is seriously inadequate, during the day shooting night effect. For example Nikon F4 matrix balanced filling type flash control mode in the program automatic exposure, aperture priority automatic exposure mode, the shutter speed at 1/60 ~ 1/250 seconds between automatic adjustment; in shutter priority automatic exposure mode, the aperture value is automatically adjusted, adjusting range depending on the lens, the largest the range of f/1. 4 ~ f16. There is a good use of the fill flash method, and when you take portraits during the day, you can add fascinating "eye light" to the characters". We know that not all natural light can reflect on the human eye, and only when light enters the eye at a certain angle can the light be reflected. When the flash is inserted in the socket of the camera accessories, always can produce flash in the eye on the reflection, so that the subject is full of vitality, full of charm, especially suitable for shooting innocent children and youth nutmeg girl. 3D five segment photometry TTL balanced fill flash We know that the use of TTL flash control in the backlight type contrast large occasions, often make the subject overexposed or underexposed; if you use flash control filling, can solve the above problems, but in some special occasions, such as in the open shot is black, because the brightness of the environment almost no filling flash control will make flash will release all the energy, so the subject may or excessive exposure; in other cases, the photographic picture contains highly reflective objects, such as mirrors, metering system can not measure the brightness of lighting flash of that moment, which is still not up to fill flash the purpose of. So it still can't solve the problem completely. The reason for this problem is that all of the metering devices used to implement TTL flash control in previous SLR machines are single components, The method of measuring the light reflection of the film plane is somewhat similar to the average metering or central weighted average metering method mentioned in the third chapter, thus having the defects of the two metering methods. Nikon's F90, launched in August 1992, solved the problem perfectly. We know from the third chapter, in order to solve the existing problems of the center weighted average metering, Nikon pioneered the partition type metering (matrix metering), effectively solve the problem of light in the backlight and contrast the occasion. Well, since there is a similar problem with single metering unit and TTL flash control, is it possible to solve the problem in the same way? Nikon F90 pioneered the 3D five segment metering system, TTL flash control system to give a positive answer. The metering element is arranged on the bottom of the F90 mirror box is divided into five sections, through a condenser lens array partition back from the spotlight film plane reflection light, and five projection photometric elements in the back, so the flash exposure measurement monitoring process is no longer on average, but according to several parts of the picture the monitor. With the five segment metering element, the 3D five metering unit can be balanced and packed with flash control. In this way, it works with a dedicated SB-25 flash and a D AF lens. 3D five metering unit balanced fill flash control workflow for: The 1, D AF lenses transmit focus distance information to the F90; 2, SB-25 turn on the mirror, and the shutter is not open, this period of time, send a series of weak flash, this flash is called "monitoring pre flash""; At the same time measured back from the reflection of the light gray shutter curtain 3, five segment TTL metering element; 4, the actual computer camera brightness five TTL photometric elements measured and according to the lens focusing distance data, monitor the pre flash flash index and the aperture of the calculated theoretical values are compared; 5. The computer is analyzed and decided: I according to the relative reading of the reflected light, determine which segment of the five section of the metering element is subject to TTL flash control; II determines the amount of light required to balance the ambient light based on the metering method (whether 3D, central weighted average metering, or spot metering); 6. Shutter open, main flash, flash amount determined by (5). This method of control ensures proper exposure to very strange lighting conditions. Picturesque bread contains reflective objects such as mirrors and white walls, and objects (non subject) are very close to the camera. It can also deal with the sunrise and sunset, such as the sun in the picture, but the background is very far and the illumination is even. If you use the general TTL flash control method, it is easy to make the subject of the body pale or lack of exposure. From the above description, 3D five TTL filling type flash metering element is very advantageous, it is equivalent to the fuselage into a flash meter, by monitoring the pre flash function in formal flash before measuring the amount of flash, then officially flash. This is the best TTL flash control mode ever seen, but unfortunately, it comes with the SB-25 special flash and the D AF lens. But as long as the use of Nikon's AF special flash and AF lens, its control effect is still better than the fourth and fifth section mentioned the way to control fine.