Joined December 2007


In another attempt to make everyone’s head explode with maths, I am going to try to explain the relationship between your Shutter Speed and your Aperture.

One of the things that a lot of photographers enjoy is taking pictures of water so that it looks all smooth and silky.

To do this you need long shutter speeds, especially if you are photographing a waterfall or the ocean.

You need, of course a good solid tripod.

Ok, assuming you have a good tripod …

Take a metered reading of the scene, probably use Average Metering. This reading may for example come out something like:

ISO400 – f1 – 1/500sec


Now clearly 1/500sec is not going to make your water all soft and blurry. You will need a shutter speed much much slower than that. The longer the shutter is open – the more silky smooth the water will look.

Now every decrease in Shutter Speed [also known as a “Stop”] will let in TWICE as much light as the previous shutter speed.

for example -

  1. 1/250sec lets in twice as much light as 1/500sec.
  1. 1/500sec lets in half as much light as 1/250sec.

This also applies to your ISO settings…

  1. ISO400 lets in half as much light [half as sensitive] as ISO800
  1. ISO800 lets in twice as much light [twice as sensitive] as ISO400

… it even applies to your Apertures…

  1. f4 lets in twice as much light as f5.6
  1. f5.6 lets in half as much light as f4

This is the same up and down each of these scales.

Each “Stop” up the scale [up meaning “more” or “bigger”] lets in twice as much light as the previous “Stop”…. each Stop down the scale [down meaning "less or “smaller”] lets in half the light of the previous Stop.

That is all a bit confusing & mathematical… don’t worry about it too much now, it will make sense soon.

Soooooo, your metered reading was

ISO400 – f1 – 1/500sec


But you need a shutter speed of say 4x seconds… what do you need to do?

You need to decrease your shutter speed by 11x Stops.

Have a look at this “EV Chart” :


Count the number of steps from 1/500sec to 4sec … its 11 steps or correctly named – 11 Stops

1/500th – 1/250th – 1/125th – 1/60th – 1/30th – 1/15th – 1/8th – 1/4th – 1/2 – 1sec – 2sec – 4sec

But if you only alter the shutter speed then you will get a very over exposed image – you need to compensate… you need to let in the same amount of light that you would with ISO400 – f1 – 1/500sec

So… you need to adjust your aperture and or your ISO

Let’s take a look at this “Shutter Speed Chart”, each diagonal row of colours will give you the SAME exposure, but with different visual effects…


  1. As you move diagonally up the chart you get more blur because of the slower shutter speeds and more depth of field because of the smaller apertures.
  1. As you move diagonally down the chart you get sharper images because of the faster shutter speeds , and less depth of field because of the wider apertures.

According to the EV Chart, you would need to use f45 @ 4sec … BUT you probably wont be able to get that f-stop with your standard every-day lenses [for the same exposure, you read the EV chart from left to right, f1 @ 1/500th = f45 @ 4sec]. So what are you gonna do if you probably wont get smaller than f22 from your standard lenses?!

According to the Shutter Speed Chart – for a 1 second exposure you will need to use f22 [if your lens will let you] but that is only 9x Stops… You still need to reduce the amount of light coming in by another 2x whole Stops! … what to do?

Reduce your ISO down by 2x Stops, from 400 to 100.

So your new setting will be :

ISO100 – f22 – 1sec


and this will give you the same amount/volume of light on your sensor as ISO400 – f1 – 1/500sec

Now, unless you have some very nice lenses – you wont be able to use f1 … f4 may be the largest aperture you can use, so you may need to choose a slower shutter speed like 1x second…. or you can use a Neutral Density Filter to reduce the amount of light coming in. Polarising filters will reduce the amount of light coming in by a couple of Stops but its dependant on how you adjust the filter – so its not very accurate.

I bet you have a headache now? Its all a bit technical, but important that you learn it so that you can take the kinds of shots where water is all smooth and silky!

Remember it this way….

If you adjust your shutter speed up [slower] you must adjust your Aperture DOWN [smaller] by the same amount.




The Ultimate guide to Neutral Density Filters


Apertures, f-Stops, & why ‘BIG’ is actually ‘small’


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