What is a safety shutter? How to calculate the safe shutter speed?

Safety shutter means that the shutter speed should be set at least to avoid hand shaking.

The faster the shutter speed, the faster things can solidify. Generally speaking, if there is a moving subject in the picture, the sooner the shutter is, the better. But the high-speed shutter also shortens the exposure time and reduces the brightness of the picture, so it needs a well-lit environment or a large aperture lens to support it.

In the case of unsatisfactory lighting conditions, even if it is dark, it is impossible to increase the shutter speed to ensure the brightness of the picture. When the shutter is slow to a certain extent, the slight jitter of the hand will lead to the dynamic blur of the picture. In order to ensure hand-held shooting, it is necessary to set a speed value that can ensure hand-held shooting and lower the shutter as much as possible. This is the safe shutter value.

There is no so-called formula for calculating the safety shutter value, but people have found through long-term experience that it has a certain relationship with the focal length of the lens. The longer the focal length, the easier it is for the picture to shake and the higher the safety shutter setting. On the contrary, the shorter the focal length, the less likely the picture is to shake, and the safety shutter can be set lower.

Generally speaking, when using a full-frame camera, the safety shutter can be the same as the focal length, for example, the focal length of 50mm, and it is relatively safe to set the shutter above 1/50 seconds, and the focal length of 200mm, and the safety shutter should be set above 1/200 seconds. On APS cameras, the safety shutter is 1.5 times larger than that of Quan Huafu (this is because the equivalent focal length of APS frames is generally 1.5- 1.6 times that of Quan Huafu), for example, the focal length is 50mm, and the safety shutter should be set to 1/80 seconds or more with a focal length of 200 mm

Of course, some photographers who practice the "iron hand" can guarantee a certain film output rate below the safety shutter, which requires long-term practice.