This picture was taken with the aperture on 4, which is wide open, giving a shallow depth of field.

A couple of years ago I had no idea what that sentence meant.  And it really, really bothered me that some bastard made up the rule that big equals small.  And small equals big.  It made my head hurt.

Now I get it.

But I had to get my head around the whole big aperture small number shallow thing.  This helped:

This is a tall glass of water with a straw in it.  Or, it’s a tall vase with a chopped-off flower stem.  The thing to notice about this vessel is that it is deep, and it is narrow.

Let’s say, for argument’s sake, that this vessel is 20cm tall.  Imagine there are little lines all the way up the side, showing 5cm and 10cm and 15cm and 20cm.

Now look at this one:

This is the same amount of water, poured into a wide, shallow dish.  This dish is only 2cm deep.  So it’s shallow, and wide.

Are you starting to get it?

Narrow opening = deep glass

Wide opening = shallow dish

Think of the depth of the water as ‘depth of field’.  If you look down into that glass, you can see all the way to the bottom, and it’s a long way.

If you look into the dish, you can see all the way to the bottom, but it’s not very far.

Shallow depth of field means you can’t see very far, or there’s not a lot to see.  It’s limited.  So, only some of the image will be in focus.

Greater depth of field means you can see a long way.  It’s not limited.  More of the image will be in focus.

On your camera, the smaller the number (2, not 20), the wider the aperture  and the shallower the depth of field.

The bigger the number, the narrower the aperture, and the greater the depth of field.

This picture was taken with a narrow/small aperture.  Big number.  Greater depth of field.  Everything from the leaf to the waves is prettty much in focus. You can even make out the boat and the horizon.

This picture was taken with a tall glass of water.  Deep.  Narrow.  Big number.

I’m going to keep repeating myself until it makes sense to you, too.

OK, here’s another one:

Cottesloe Beach.  I wanted to get everything.  So, I needed to be able to see a long way.  I needed a tall glass of water.  Narrow opening, deep glass.  20cm. Big number. Deep depth of field.

All the birdcages are visible.  The building behind is in focus.  Greater depth of field.  Tall glass.  Big number.  Small aperture.

Now look at this one:

A little tumbleweed on the beach.  The tumbleweed is in focus, but the water is blurry.  You can only see a little way.  It’s shallow.  It’s a wide dish with a little number up the side.  Big wide open aperture, small number, shallow depth of field.

One more:

This flower was about the size of my hand, with a 5cm long stamen that reached out to be photographed.  The aperture was 4, which means it was wide open.  Small number, shallow dish, wide vessel, shallow depth of field, just one small part of the image is in focus, there’s no depth to this picture, it’s a very shallow depth of field picture. Wide aperture, shallow depth of field.  Small number.

And now, a drawing:

The number is expressed like this:

f/4

f/2.8

f/16

The number might be a whole number, or a fraction of a number.  I have two good lenses, the Canon 24-105mm f/4 and the Canon 70-200mm f/2.8

The f/2.8 gives me shalllower depth of field, because it goes down to 2.8.  But f/4 is still pretty awesome.

Although my life will not be complete until I’ve got the 50mm f/1.2

I hope this little tutorial has been helpful.  Next lesson? ISO.

OMG.  WTF?

Tags: aperture, depth of field, f/2.8, f/4, focus, photography