Originally posted by sterretje ISO is easy; it's the size of the bucket (higher ISO is smaller bucket)
sterretje, I LIKE that - the "size of the bucket" analogy. Then you might illustrate it all with a tap running with light rather than water:
To take a photo, you must fill the bucket with light. You do this by picking a bucket and a tap, and then opening your tap long enough to fill the bucket.
Shutter speed would be the amount of time you keep the tap open.
Aperture would be the pipe size (pipes ranging from 1/1.2mm down to 1/22mm)
ISO would be the size of the bucket you fill (high ISo = small bucket)
With a big bucket (low iso) you need to keep the tap open long to fill the bucket. You can reduce this amount of time (use a faster shutter speed) if you pick a tap with a big pipe (small f-value).
So, in poor light conditions, you pick a small bucket to fill (high iso), because there isn't much light to tap. You also need to keep your tap open longer (slower shutter speed), and you need to pick the biggest pipe available. I think I might use that, next time I need to explain these concepts to a newbie.
(EDIT: I just realized you can use this to explain noise at high ISO settings too. Noise can be "splashing" - so if you pick a small bucket, there is likely to be more "splashing" than with a big bucket. Unless you are lucky enough to have a k5, the new "splash-free bucket" :P If you have good noise reduction software, you basically have a nice cloth with which to wipe off the splashes afterwards!
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Randy, I like your toilet paper explanation too, it's perfect for illustrating why you get movement blur with slow shutter speeds. Great when people take the time to explain basic concepts well.