One of the fundamental issues of capturing a photo is how much light to allow into the camera, how long to “expose” the film (or sensor) to light.

Most modern cameras now come with internal mini-computers and light meters that make many of the exposure decisions automatically based on what they think you want. It assumes you are in average conditions and as a result gives an “average” photo. But sometimes that’s just not good enough and we need to guide the camera’s computer to take account of our creative preferences.

Dave’s talk explained the background to the “Exposure” process, what the camera tries to do automatically and more importantly how and when to override it’s “average” algorithms.

So fundamental is the exposure to the photo capture process that any talk that tries to explain it will also divert into side-subjects of ISO, aperature and then focussing.

David covered all of these admirably and also left time for questions afterwards.

I can’t repeat all of David’s talk here just to mention some of the rules of thumb he included.

(1) If you have no light meter, guess !!  Also called the sunny sixteen rule. On a bright sunny day, use a shutter speed of 1/ISO speed (i.e.) with ISO set at 200, then the correct shutter speed with f16 is 1/200sec.

(2) To avoid camera shake use a shutter speed faster than 1/focal length of the lens. (i.e) when using a 200mm lens, use a shutter speed faster than 1/200sec.

(3) When shooting stars, the longest exposure before stars start to trail is 500/focal length of the lens.

And some more friendly advice, when photographing stars, most cameras might allow you to focus beyond infinity, but don’t do it. You don’t know what will happen …

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