In the post apocalyptic genre there are many conventions seen throughout that appear in almost all films of this genre. When typed into google, these images (see right) are the first to show up and depict a barren, dying world with little to no life left.
These are a few of the codes and conventions:
Location

Area of abandonment; e.g. sandy, wiped out setting, derelict, barren forest.
In 'I am legend' a citywide abandonment was shown, where New York had become overgrown.
These settings show an almost unrecognisable world and emphasise that an apocalypse has occurred.
Danger may also be evident throughout.
Lighting
Low, dark lighting
Sound
Tense scores are common, for the fast, action filled moments of the film.
Slower, sadder songs are used to emphasise the loneliness of the main character, or when a tragic event occurs.
Sometimes, a tense build up is used to create suspense. This is used in the opening of 'Book of Eli' when Eli is about to shoot the cat.
Main Character
The main character is usually either the 'hero' trying to stop the crisis of the Post apocalyptic world, or just a regular human in the thick of it.
Usually male, the audience needs to see the character as dominant otherwise surviving in this world seems unrealistic.
Main themes however are that the character is usually alone physically or mentally and almost always a male.
Costume
Dirty, ragged clothes are usually worn to emphasise the lack of resources; including water.
In 'The Road' for example dirty clothes with holes, usually cast offs and in layers is the norm.
Women
Women are few and far between in this genre, probably due to the fact they are seen as the weaker sex by many people. In 'Book of Eli' Solara is made out to be sexual, wearing dresses and neatly brushed hair, both of which are usually uncommon in post apocalyptic worlds.
Challenges/Survival
Majority, if not all of the characters no longer have their old lives, many loosing loved ones to maximise the death and dangerous world.
Most have the challenge of returning somewhere, finding someone, fighting someone etc.
Death is a major theme throughout all of this genre. 'The Road' is a great example of this; images of families killing themselves, cannibalism or being brutally killed for supplies is featured.
Items of significance are also featured. Eg suncream, or sunglasses for a desert dystopia, boots and lots of clothes for a cold and wet climate.
Plot
Flashbacks of the old world, usually the main character remembering something from his past.
Zombies or people representing loss of hope are a common theme. Although zombies have lost all sense of humanity, people with a lack of hope could be considered to fall under this category of lost humanity with them being changed after the apocalypse.
Interestingly, the protagonist is usually presented with hope for the future whereas the antagonist isnt.