What are the types and common modes of flash?

TTL systems vary from manufacturer to manufacturer. Common ones are as follows:

Advanced TTL

Canon's first step to improve the flash exposure design is to create A-TTL, also known as' advanced through-the-lens' flash photometry, which was widely used in early EOS series movie cameras.

A-TTL flash (including only 300TL and EZ series flash) will emit a short pre-flash during the metering stage (that is, when the shutter release button is half pressed). This pre-flash is recorded by the external sensor in front of the flash, which is used to determine the appropriate aperture to ensure sufficient depth of field, especially in the case of short distance. Once the shutter is opened, the flash will really illuminate the scene. The start time of the flash depends on whether the camera is set synchronously by the front curtain or the Hou Lian, while the duration of the flash is determined by the standard OTF sensor. The sensor measures the average brightness of the scene. If you shoot in a bright environment and start auto-fade, the flash output can be reduced by 0.5 to 1.5. Once the flash lamp determines that the foreground object has been completely illuminated by measuring the reflected flash in real time, it will cut off the energy of the flash lamp tube and the flash lamp will go out immediately. The shutter remains open for a set time.

E-TTL (evaluation TTL)

1995, Canon released another form of flash technology-e-TTL, that is, flash metering through lens evaluation. E-TTL uses the main flash bulb to emit a beam of low-power pre-flash with known brightness to determine the correct flash exposure. It measures the reflectivity of the scene by pre-flicker, and then calculates the output power of the flash lamp needed to realize the middle tone according to these data. It also uses pre-flash, but it overcomes the shortcomings of A-TTL for the following reasons.

First of all, the E-TTL pre-flash occurs just before the shutter is opened, not when the shutter is half pressed. Therefore, unlike A-TTL, E-TTL pre-flash is actually used to determine the exposure of flash, and it is not excited in the spotlight metering stage. Some users may be surprised that E-TTL sends out a pre-flash before the official flash. Under normal settings, this process happens so fast that the pre-flash is difficult to detect, although you may catch a glimpse of the mirror before it rises.

Secondly, the pre-flash line is analyzed by the evaluation metering system inside the camera, that is to say, it measures light through the lens, which is not as easy to be fooled as an external sensor. Unlike TTL flash exposure meter, E-TTL sensor is not easy to be seen by curious people, and it is hidden in the case of pentaprism.

E-TTL is superior to TTL and A-TTL because it is used to fill flash memory. E-TTL usually performs well by adding subtle and natural fill light flash to daytime photography. E-TTL exposure is also related to the current focus, and it is theoretically easier to obtain excellent exposure.

P-TTL (pre-flash TTL)

P-TTL mode calculates the best flash output by combining the multi-zone metering system of the fuselage and the main distance information from the lens during pre-flash (the lens from Pentax F series autofocus lens and the lens that can provide distance information after it), which is more accurate than the traditional TTL mode. Without the influence of reflectivity, a suitable exposure can be obtained on a black background.

I-TTL & amp； Electronic TTL II

Nikon's i-TTL is partially similar to Canon's E-TTL. Firstly, the amount of flash is estimated by pre-flash, and then the brightness of the subject at the focus position is measured. When the flash is on, the E-TTL will command the flash to stop flashing after the light shining on the main body reaches its due brightness, and Nikon's i-TTL will also substitute the distance information into the operation as a correction, and then command the flash to stop flashing, which is of course more accurate. The acquisition of this distance information is completed after the focusing is completed, so there is no time delay at all. Nikon's i-TTL has only five partitions, but because of the further correction of distance information, these five partitions are accurate enough.

In Canon's newly developed E-TTL II mode, when the shutter is released, the flash will emit a pre-flash to monitor the ambient light and calculate the required amount of flash, and the flash will emit the correct flash to balance the ambient light and artificial flash. In complex situations, such as strong reflection outside the photometric area connected with the focus, it can also be exposed correctly. E-TTL II adds average flash metering on the basis of evaluating flash metering. Because of the new algorithm, the flash results are more accurate and stable. Even if the subject moves, the E-TTL II automatic flash system can effectively avoid underexposure and overexposure.

Slow synchronization is an advanced function realized by combining camera and flash. The slow synchronization of flash means that in the early morning, evening or night with certain light, the shutter speed is appropriately reduced and the flash is used at the same time, which can not only ensure the normal exposure of the subject, but also properly expose the background and enrich the picture effect.

Generally, the shutter of 135 single-lens reflex camera has two shutter curtains, the first shutter curtain and the second shutter curtain. (Also known as front curtain and Hou Lian)

The basic action procedure is: press the shutter->; The curtain in front is open-> Film sensitivity->; Hou Lian is closed.

The difference between front curtain synchronization and Hou Lian synchronization lies in:

Front curtain synchronization: the flash lights up at the moment when the first shutter curtain is opened.

Hou Lian synchronization: The flash lights up before the second shutter curtain closes.

There is no obvious difference between fast or high-speed flash synchronization and shooting relatively static subjects (buildings, stopped vehicles, mountains, the prospect of Yuan Ye or gardens, etc.). ).

However, in slow or long exposure mode, or when shooting activities are relatively prone to moving and moving objects (people, moving vehicles, animals, sports and dance themes, etc.). ), using different flash synchronization modes will get completely different images.

Especially in low illumination environment.

For example, in the evening or in low light, use a standard lens or a wide-angle lens to shoot the oncoming picture of an oblique cyclist. The camera is fixed on a tripod, and the shooting angle and picture remain unchanged. With the shutter speed of 1/8 seconds or 1/4 seconds and the Hou Lian flash synchronization mode, you will get a picture with a tail behind the subject and a clear subject.

The principle is that when the shutter is open, the flash does not emit light, so the subject is in an underexposed state (of course, if the illumination is too low, there will be no image left on the film), but the film can still record the underexposed image, and the subject is in a moving state, so what is recorded on the film is smear.

When the time of 1/4 seconds or 1/8 comes, that is, the moment before the shutter is about to close (I don't have the specific time data), the flash lights up, and then the shutter closes.

At this time, due to the light of the flash, the subject left a clear exposure image on the film, and then the shutter closed immediately, so the last point of this series of smears was a clear subject illuminated by the flash. It looks like a photo with a sense of movement and speed.

Simply put, the function of Hou Lian synchronization is:

You can freeze the last image of the subject on the film with a slow shutter.

Contacts can be understood as sensors, which transmit the information of the fuselage to the flash, or transmit the information of the flash to the fuselage to facilitate the measurement and control of the flash.