Thursday 10 February 2011

Chris's crash course in Exposure

I wrote these notes for a friend some years back, whilst tidying up a few things I found it again and thought it might be best as a blog entry.  So forgive me if this all seems familiar.

Shutter speed

Shutter speeds are shown as fractions.  Until the go over a second, then they are shown with " after them.
So 500 means 1/500th of a second and 2" means 2 seconds.


Aperture is the size of hole you are shooting through, it is recorded as a fraction.
So f2.8 will be a big hole or "wide open" and f/22 is a tiny little hole. Aperture controls depth of field - f2.8 is not much, f/22 is loads.
When you view through a lens normally you are seeing it wide open. The depth of field preview button allows you to see what it looks like at your chose aperture.

Depth of Field (DOF)

Depth of Field is the amount of the picture that is in focus (which is affected by aperture as previously mentioned). Also the longer the lens the less dof you have available, but don't worry about it to much its easier to see than explain.

A Shallow depth of field means the poppy is in focus
but the rest of the field is blurred.

When to use a tripod?

The rule of thumb for hand-holding shots is that the shutter speed should be no slower than 1 over the focal length.
So if you have an 18-55mm lens at the wide end (18mm) you can handhold as slow as 1/20th of a second, but at the long end (55mm) you should chose 1/55 of a second or slower. Image stabilisation allows you to go 1 or 2 stops slower.
With good technique you should be able to shoot at slower speeds than the rule, but how much slower is something to experiment with and learn for yourself.

What’s a stop?

If you put the camera in AV mode you will see that you can change the aperture in stages E.G. f/11 to f/16 to f/22 each of these stages is known as a “stop” the same applies to shutter speed. As shutter speed and aperture are intimately related changing one will change the other.

So what's that mean in practice?

  • A. If you want to take a landscape shot and you want to get everything in focus then you need an aperture of f/8,f/11 or above. So you set the camera to Av (aperture priority) and set it to f/11. Take a look at the shutter speed. Can you handhold it or do you need a tripod? If you want a longer exposure (for blurry water type effects) then set a smaller aperture (higher number).
  • B. If you are shooting a single figure in poor light and you would like a blurry background then pick a low f number (f/4 or similar) then check your shutter speed to see if you can hand hold it.
  • C. If you are using a tripod there is less to worry about, so you just adjust the aperture for the dof you want, or adjust the shutter speed for any blurring you want.

Understanding Exposure - highly recommended as further reading.

What about ISO?

ISO also moves in stops, so 100-200 is one stop 200-400 another. Increasing the ISO will reduce the shutter speed. You don’t get something for nothing though; increasing the ISO adds digital noise to the image (100 is virtually noiseless whereas 1600 has loads of noise). Though many modern cameras are virtually noiseless up to 400 ISO.

Why would I change the ISO?

So you are hand holding, you have the lens wide open but the shutter speed is still showing as 1/25th, which is way too slow for your lens. If you increase the ISO from 100 to 400 you get 2 stops back which changes the shutter speed from 1/25th to 1/100th and now you can hand hold again….that’s magic!

Further reading

A Beginner's Guide to Aperture and Depth of Field

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