Everything above is uniformly in focus. It all looks sharp but flat.
You might not use the depth of field tool very often but it's another neat toy to play with. Takes
bit of time. It is really useful (vital?) if you are going to print your photograph.
With depth of field applied by photo editing, the background is out of focus but the fish and right-hand
coral are in focus.
This draws attention to the fish and right-hand coral and the picture is more 3-D and interesting.
When snorkeling we are moving towards and/ or away from our subject, due to current, surge or just because
we are floating. Often our fish is moving towards and away from us as well.
So, the distance to focus on keeps changing.
To lessen this movement causing blur, as we shoot, the shutter speed is set reasonably fast.
Also, we tend to have the biggest aperture (small f stop number) the light allows, for an extended depth
of field. This gives the best chance of the fish being in the sharp focus range, even if the distance
to it has changed since we focussed.
But, some photographs look better if the subject only is in focus, and some or all of the
surroundings are slightly out of focus.
The subject really stands out. The photo looks far more 3-dimensional and is much more
interesting. Photo-editing software to the rescue!
Best to read the 'help' instructions relating to your particular photo editing
software to see how to do this as they all vary a bit.
One way is to draw around your object, (fish or coral) with your cursor. Then dial in some blur
outside it. Job done quite quickly.
Another way, which takes more time but gives a better result, is to use a selection tool (in PaintShop Pro
it's called a Magic Wand).
Add a bit of gradient or 'feathering', (perhaps 2 - 4 pixels or so). You don't want an obvious line between
the very sharp and slightly blurred areas.
Then put you selected subject on a new 'layer'.
Sharpen the subject layer and perhaps blur
the original layer. Don't overdo it.
Merge the two layers with the sharp fish or coral on top of the slighlty blurred background layer.