[6-24] "HSS"

Yesterday I described the first feeling of taking pictures with flash, wirelessly, in what felt like a technological miracle.

High-speed sync (HSS) allows a multiple flashes on a single image.

This is kind of complicated, but it adds to joy of modern photography:

I used to think a shutter opens and closes instantly (all open, all closed). This is not true; if you use your hand to block light, the hand has to move, or "reveal" the light. As you move your hand away, some part of the film sees light first. The hand does not magically disappear; if you move your hand down, the top of the lens see light first.

Does this defeat the purpose of constant exposure?

Well, no. You may block the lens (same hand) in the same direction, with the same speed, so that every part of the film gets equal exposure.

This is how shutters work. An image is composed of multiple time elements. IOW, the bottom an image occurs after the top.

Below 1/200, the flash occurs as the entire sensor is exposed. Bam! all that light energy is captured by the sensor.

>1/200, the whole sensor is never visible at any moment; a thin rectangular segment of shutter slides. The flash will only illuminate a sliver of the image. You will get a black lines wherever the shutter is blocking the sensor (if no ambient light).

HSS fires >1 flash so unexposed areas also see flash. HSS knows the speed the shutter is moving, and fires many equal flashes cover the whole sensor.