grouping by time interval

I'm writing a photo database for my family photos. I've got a massive
back log so I'd like to some large scale annotation and break things up
into manageable chunks.

I think one useful assumption would be tha any set of photos taken within
of a few seconds of each other (say 5 or less) are /probably/ about the
same subject...

How would I go about detecting and marking events where the time interval
between two photos is less than 5 seconds

photo_id | photo_time | photo_group
1 | 2005-08-20 19:05:15 |
2 | 2005-08-20 19:12:59 |
3 | 2005-08-20 19:13:03 |
4 | 2005-08-21 19:13:10 |
6 | 2005-08-21 13:02:48 |
7 | 2005-08-21 13:04:58 |
8 | 2005-08-21 13:06:06 |
9 | 2005-08-21 13:06:10 |
10 | 2005-08-21 13:09:19 |

would become something like

photo_id | photo_time | photo_group
1 | 2005-08-20 19:05:15 |
2 | 2005-08-20 19:12:59 | 1
3 | 2005-08-20 19:13:03 | 1
4 | 2005-08-21 19:13:07 | 1
6 | 2005-08-21 13:02:48 |
7 | 2005-08-21 13:04:58 |
8 | 2005-08-21 13:06:06 | 2
9 | 2005-08-21 13:06:10 | 2
10 | 2005-08-21 13:06:19 |

My first thought was to reach for perl :-) but I was wondering if there
was an 'easy' way to do it in postgresSQL ...

--
Michael
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Michael John Lush PhD Tel:44-1223 492626
Bioinformatician
HUGO Gene Nomenclature Committee Email: hgnc(at)genenames(dot)org
European Bioinformatics Institute
Hinxton, Cambridge
URL: http://www.genenames.org
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~