Exposure Explained…. via Beer?

F stops, shutter speeds and ISO can be confusing.

I like to think in terms of things I understand when trying to explain difficult concepts, and it happens that beer can be roughly equated to exposure…


So the F stop relates to how fast a beer flows from the pump:-

  • Guinness takes ages, that’s got a tiny flow – so that’s a small aperture, like F16, F22 or even F35
  • Hand pulled bitter comes out really fast – that’s like a fast aperture, say F1.4 or F2.8
  • Lagers and cider are somewhere in the middle, you’re looking at F5.6-F11 there

Shutter Speed

Shutter speed relates to how long it takes to fill your glass

  • Guinness takes ages so that has a long shutter speed – maybe 30 seconds
  • Hand pulled bitter comes out really fast – so we’re talking fast shutter speeds, 1/500th second is pretty fast, though most cameras go to 1/8000th these days
  • Lagers and cider are somewhere in the middle, 1 second maybe

The link…..?

Can you see how the 2 link… a slow pouring beer like Guinness takes a long time to fill the glass, whereas a fast hand pulled beer fills the same glass in a fraction of the time. In both cases, you get a full pint, but you just need to vary the pouring time because of the different flow of the beer.

What is ISO?

What about ISO then….

Well this relates to the size of the glass!

It’s actually the “speed” of the film, the sensitivity of your camera’s sensor, but glass size is easier to think about. It’s basically how much beer you need to fill the glass.

Lets say ISO 400 = 1 Pint

  • ISO 800 is like a half pint glass – i.e. it takes HALF AS MUCH BEER to fill it – it’s twice as “fast”
  • ISO 200 is like a 2 pint glass (or a litre stein you see in bavaria) – it takes TWICE AS MUCH BEER to fill it.
  • ISO 100 would be a 4 pint glass, it takes 4 times as much to fill!
  • ISO 1600 would be a 1/4 pint glass – so it takes a quarter of the beer to fill.

Keeping up….?

So lets gather all this together in camera speak.

  • We have a “flow of beer” = Aperture
  • We have a “time the flow runs for” = Shutter speed
  • Volume of the glass we’re pouring in into = ISO

STOPS – What Are They?

All these things are related closely by a unit called a “stop” – a weird name really.

Aperture, ISO and Shutter can all varied by “stops”, but they each have different names and “steps” between them. The Stops are named as follows:-

  • Aperture Stops – F1 F1.4 F2 F2.8 F4 F5.6 F8 F11 F16 F22 etc.
  • Shutter speed Stops – (units of time, measured in seconds) 1/1000th 1/500th 1/250th 1/125th 1/60th 1/30th etc.
  • ISO Stops –  50 100 200 400 800 1600 3200

All Doubles and Halves

The Shutter speed and ISO stops double or halve when you move along the list. This is because each “stop” is “double or half” of the neighbour.

  • 1/500th has neighbours 1/250th (double the time) and 1/1000th (half the time)
  • ISO 400 has ISO 200 or ISO 800 as neighbours.

Aperture Stops Are Strange

So why does aperture look so odd?

Well its a bit more complex, but each stop is about 1.4 from the neighbours – so there is a relation ship there. (it’s to do with the square root of 2 = 1.41).

So from

  • F1 to F1.4 is 1.4 times –
  • next is F2 (that’s 1.4 * 1.4)
  • next is F2.8 – (that’s 1.4 * 2.0)
  • and so on

So what does all this mean?

You have 3 variables which control the exposure of a picture – get them wrong and you’ll get a badly exposed shot. i.e.

  • if you don’t let enough light in you get a dark shot  (or a glass that’s not full if we talk about beer)
  • if you let too much light in you get a completely useless white shot  (or a load of beer flowing over the top of the glass on to the floor)


Here’s the critical thing you need to know – if you change one of the settings by a “stop”, you double (or half) the brightness of the shot.

Yes – remember, it’s all doubles and halves

  • If you increase the shutter speed by 1 stop, the shutter stays open for twice as long and you get exactly twice as much light on the sensor (or twice as much beer in the glass)
  • If you increase the Aperture by 1 stop, the flow of light is doubled and you get exactly twice as much light on the sensor (or twice as much beer in the glass)
  • If you increase the ISO by 1 stop, the amount of light needed to get an exposure is halved and you get an images exactly twice as bright (or you reduce the size of the glass by half – but the flow of beer is the same, you just may end up with some spillage if you’re not careful!)

Practice and Learn…

Exposure is best learned by using your camera, getting curious and trying various settings – I’d recommend putting your camera in Manual exposure mode “M”, and taking lots of shots and seeing what happens – go through each Stop in turn and learn that way.

There is a lot more to it than this, but next time you’re in the pub, watch the beers being poured and think “exposure”!

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