How does long-exposure photography work?


How does long-exposure photography work?

In: Technology

Long exposure photography is done by holding the camera’s shutter open for a long. To prevent the camera from gathering too much light due to the slow shutter speed, you use a very small aperture, or make the scene really dark, or use a neutral density filter (basically a dark filter in front of the lens to block out light).

When the cameras shutter is open for a long time, two things happen. More light hits the sensor and motion is blurred since the sensor records all tho movement it seen in that time.

Exactly the same way as short exposure photography does. Seriously.

Cameras have a sensor and a shutter. You always have to open the shutter for a while to let the light register on the sensor. There’s no fixed definition of what “long exposure” means exactly. It’s just a fuzzy term for durations that feel “long-ish” to humans.

Usually you use a tripod, and sometimes a tracking mechanism to track the thing that you don’t want to be really blurry in the frame. But that really applies at all timescales. It’s just a question of how much things change while the shutter is open. If something moves really fast, then even at normal photography speeds things can come out blurry.

Imagine you have a grid of buckets, covered by a retractable roof. If you retract the roof while it’s raining, the buckets will fill up with water.

Suppose it’s only raining over a very small region. Only the buckets under the rain will fill up, while the other buckets won’t.

Now, consider, what if that small region of rain is moving. If you open and close the roof very quickly, the rain won’t have much time to move and only a few buckets will be filled.

But if you open it, and leave it open, you’ll see all the buckets it passes over will get filled. You’ll end up with a trail of full buckets, even though the source was smaller than that area.

This is more or less how cameras work, but with light instead of water, and sensors (or film) instead of buckets.

You either set a long shutter time, or you set the camera to open the shutter on the first press and close the shutter on the second press.

Canon DSLRs had/have a 30 seconds limit to the programmable shutter timer.
They call the other method “bulb” mode.