Photography: Few people really understand front curtain and rear curtain synchronization. It will be more fun once you understand it!

In photography, when it comes to flash, there are often terms front-curtain synchronization and rear-curtain synchronization. What do they mean? Today I will take you to learn more about it.

1. Understand the camera shutter structure

The shutter used by most SLR cameras (Nikon, Canon, etc.) is called an electronically controlled vertical travel focal plane shutter. The name is a bit professional, but it is not Need to understand deeply. As long as you know that it is located in front of the camera's focus plane, it consists of two layers of curtains, an electromagnetic release device and a shock-absorbing device. The front and rear curtains are called the front curtain and the rear curtain respectively.

2. The working principle of the shutter

It is best to understand it in conjunction with the GIF picture below. Let’s take a look at the schematic diagram of how the shutter works. To complete a shot, the shutter goes from fully closed to open, and finally fully closed again. When the shutter is triggered according to the set shutter parameters, the rear curtain opens upward first, and then the front curtain opens downward. At this time, the exposure begins. When the shutter opens to the maximum, the rear curtain begins to close until it is completely closed, and the exposure ends. Finally the front curtain also closes. In fact, this process is very fast. Even for long exposures, the front and rear curtains are in the maximum open state.

3. What is flash sync?

Now that we know how the shutter works, let’s understand the concept of flash synchronization. Because during our shooting, sometimes the ambient light is very dark, and a flash must be used. So how does the flash participate in the exposure?

Flash synchronization means that the flash flashes exactly at the moment when the shutter is fully opened, so that the entire frame can feel the flash. As long as it is lower than the camera's maximum flash speed, general shooting can ensure that the subject is exposed. The maximum flash sync speed of today's good cameras can reach 1/320. Its advantage is that it will not blur when shooting in dark environments. And if it is higher than this maximum flash sync speed, such as 1/500, the rear curtain will begin to expand before the front curtain is retracted in place, resulting in uneven light reception in parts of the picture.

Beyond the flash sync speed, some high-end cameras have a high-speed flash sync function (auto FP), and the shutter can even reach 1/8000. Flash shooting can be used as fill light. Of course, this is not the case here. included in the discussion.

High-speed flash synchronization as calendar light

4. About front curtain synchronization and rear curtain synchronization

Front curtain flash refers to a flash after the front curtain is fully opened. Instantly, the flash starts firing, which is suitable for correctly exposing the subject in dark lighting conditions.

Rear-curtain flash means that there is no flash when the shutter is fully open. It flashes just before the rear curtain closes. At this time, the subject is clear and there is a certain amount of ambient light. If there is The movement of the bright spot will form a trail. In fact, the well-known slow sync is a type of rear curtain flash.

Rear-curtain sync slow flash

Regardless of front-curtain sync or rear-curtain sync, the shutter is fully open at the moment.

5. Practical application of front curtain and rear curtain

Generally, cameras default to front curtain synchronization, which means first ensuring that the subject’s flash exposure is normal and ambient light does not participate. If you are shooting still life, there is actually no difference in the effect of front curtain synchronization and rear curtain synchronization. But when shooting moving objects, you can only use rear curtain synchronization, otherwise the shadow will appear in front of the moving object. Therefore, front-curtain sync is usually used to shoot still life or still life portraits, and rear-curtain sync is used to shoot moving objects or moving portraits.