Conventions of editing

The conventions of editing: 

Continuity

Using different scenes to show the same scene or sequence continued

Here edits are used from different angles to show that it is all part of the same fight scene

jump cuts, 

An edit which cuts drastically from one scene to another without continuity, showing different angles, different close ups  or zoom outs, even different locations.

Here jump cuts are used to great effect in the shower murder scene from Alfred Hitchcock's Psycho, were cuts to different angles match the frenzy of the knife attack, even though no scenes show the knife penetrating the skin


the 180 degree rule

Cutting round from one scene to the other persons perspective

Here is an example of a 180 degree cut in an opening scene from the Avengers: Age of Ultron

Dissolves

Used to fade from one scene to another - often shows change in scene or movement through time

This has been used to great effect in this music video for the song, "Cry" by Godley and Creme.


,shot-reverse-shot, 

One scene facing forward cut to next scene facing opposite direction - cut back to first scene facing forward. Used often in conversation or to show quick movement between two people.

Here is an example from Shot/reverse shot in Spider-Man

cutting to a sound-track 

Editing down a sequence to match to the best in a sound track or piece of music

Here is an example from who will save you now set to images from Team Fortress 2

Purposes of Editing

Why is editing needed?


Creating pace
Generally speaking, shorter scenes are read and understood more quickly by the audience and make for a faster moving sequence, such as action scenes. Note the length of each scene in this example of the fight scene from Avengers: Age of Ultron:

Continuity

Editing is sometimes used to keep continuity in a sequence. This is to make sure actors are wearing same cloths, in same position etc

Removing Mistakes

This a bloopers or scenes showing mistakes taking from the second series of Mrs Brown's Boys.

Selecting the best take
Editing often gives directors and actors several chances to choose the best version of a scene

Censorship
Sometimes editing is needed to remove a scene that would  have the movie blocked by the BBFC - a famous example is 11 seconds from the Oscar winning Movie Last Tango in Paris had to be taken out at the last moment

Reducing a film or sequence to a preferred length
Sometimes a film has to be edited down to a shorter version to be taken into distribution. Here is an unused scene from Snow White and the 7 Dwarfs: