Camera basics :: Aperture, Shutter Speed, ISO

Not everyone will need to know all these photographic terms. Point-n-shoot photographers will not bother because their cameras have scene presets or fully auto modes that give them adequate results. So who really wants to know this stuff?

Actually it's written for my own reference, just in case I do fumble with the terms in the near future. Here goes on my explanation of these photographic terms.

Aperture, Shutter Speed, and ISO can be considered as the building blocks of a camera's imaging system (be it the film or the digital sensor). Let's try to explain these terms with an analogy.

If we now imagine that there is a bucket which is filled with a fixed amount of sand, and a water tap that can be used to fill the bucket. Now, we can vary the opening of the water tap by controlling how much we turn the spigot. We can also control the duration of the water tap being open. Finally, we can also choose a smaller bucket to store the water (with the same amount of sand in it).

If we open the water tap larger, it will take less time to fill up the bucket. If we open the water tap smaller, it will take more time to fill up the bucket. If we choose a smaller bucket, it will also take less time to fill up the bucket but the water will be more murky. A combination of these 3 parameters will result in the same thing: a bucket full of water.

So what is what?

Opening size of the water tap is analogous to the Aperture, i.e. how big is the opening.

Duration of the water tap being open is analogous to the Shutter Speed, i.e. how long is the shutter kept open.

Bucket size is analogous to the ISO, i.e. how easy it is to fill the bucket (with the expense of getting murky water, or noisy pictures).

The bucket refers to the film or digital sensor.

And of course, water in this case is analogous to light, which is what the camera is trying to capture anyway.

OK, let's see what happens now.

Aperture is represented with F numbers. The larger the F number, the smaller the Aperture, and vice versa. Normally they are written as f/2, f/4, f/5.6, f/16, f/22, etc.

Shutter Speed is represented with numbers or fractions, with units in seconds. Typically it's written as 1/30, 1/250, 0.3, etc.

ISO is represented in whole numbers. The larger the ISO, the more sensitive the digital sensor or film (or the smaller the bucket size). Whole ISO units are 100, 200, 400, 800, etc.

So we can vary the combinations of these 3 parameters to get the perfect exposure: that's it, really. Of course, different apertures/shutter speeds will give different creative results (e.g. background blur, smoothened waterfalls).

Let's say the camera meters the following exposure: f/4.0, 1/200s, ISO200

Suppose that we want to make the background blur even more outstanding, so we need a larger aperture, say f/2.8. Remember our analogy: the water tap opening is now very big. This means we either need to increase the shutter speed (shorten the duration that the water tap is open) or reduce the ISO (get a larger bucket) to maintain the same exposure (the bucket is properly filled to the brim). So we can either do:

f/2.8, 1/400s, ISO200
or
f/2.8, 1/200s, ISO100

Then let's say we want to stop action (maybe in sports photography) and the shutter speed is too low. So we need to increase the shutter speed to say 1/800s (shorten the duration that the water tap is open). Thus we either need a larger aperture (larger water tap opening) or a higher ISO (smaller bucket), or a combination of both:

f/2.0, 1/800s, ISO200
or
f/4.0, 1/800s, ISO800
or
f/2.8, 1/800s, ISO400

Or maybe we want to have everything in focus from foreground to background (small aperture, e.g. f/8.0):

f/8.0, 1/50s, ISO200
or
f/8.0, 1/200s, ISO800
or
f/8.0, 1/100s, ISO400

With today's camera's through-the-lens metering and auto-program modes, the perfect exposure is calculated automatically. What we need to do is to be aware of the metering characteristics, creative exposure and adjust the compensation accordingly.

Anyway, all the above is fine for ambient light photography; but in Flash photography, choosing a higher ISO will give different results than choosing a lower shutter speed or larger aperture. But that's for another day.


P/S I hope I got all that right!