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Old 24-11-2006, 10:23 PM
Dennis
Dazzled by the Cosmos.

Dennis is offline
 
Join Date: May 2005
Location: Brisbane
Posts: 11,210
Hi Andrew

In the old days of 35mm cameras, a bulb was literally used to remotely fire camera shutters. It was like a bladder than when squeezed, triggered the shutter mechanism with a pulse of air.

In today’s terms, you usually attach an electrical remote release to a camera, which allows you to hold the shutter open for as long as you press and hold/lock the button on the remote release. A remote release is a small battery operated hand unit (with a switch/button) with a cable that plugs into the camera.

When you start exposing a digital ccd camera for longer than say 3 or 4 seconds, you begin to see thermal noise building up in the image. So, you have the actual photons that you want to record from the image you are taking, plus the (unwanted) heat generated noise in the chip and associated circuitry, arising from the long exposure.

You can “remove” this noise by taking and subtracting a second exposure, at the same temperature and duration but with the lens cap on. You are thus effectively taking an image of the chip’s thermal noise (or dark current) as there is no light coming through the lens cap. Most cameras do this automatically, in-camera. I think that most modern DSLRs have a top manual exposure of 30 seconds. If you want to go longer, you switch to the “Bulb” setting and then you can hold the shutter open for as long as you want, or until the batteries run out, or until the thermal sensor kicks in to shut the system down.

Using the bulb setting, most imagers would take several exposures of between 3 and 10 mins then stack these together. So, if you capture 6 x 5 minute exposures, it is almost equivalent of a single 30 minute exposure.

Cheers

Dennis
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