Salt, Barry. "Film Style and Technology in the Forties." Film Quarterly 31 (1977): 46-57. JStor. 9 Apr. 2008.
This article discusses the technical side of films in the late 1930s through the end of the 1940s and gives great insight into what was common technique in the era and what was groundbreaking in terms of filmic technique and style. There is a large segment of the article dedicated to the style and technique used in Citizen Kane, with special attention given to Gregg Toland - the cinematographer of Citizen Kane who was largely responsible for the look and feel of the film through his use of special lighting techniques and coated lenses.
Specifically, Gregg Toland was responsible for innovating techniques to increase the range of objects that were in focus in any given shot – a technique called deep-focus. The prevalent use of deep-focus shots throughout the film was rare for its time, and as such Citizen Kane is often credited for starting the trend of heavy use of deep focus with more shallow focus limited to certain close shots in films since. Furthermore, the use of wide angle lenses to give more dynamic framing to the characters was innovative in this film, and is believed to have heavily influenced the look of The Maltese Falcon less than a year later.
Lastly, this article mentions that the average shot length in Citizen Kane is not significantly longer than other films of the period, even though this is not commonly believed to be the case. Citizen Kane has an A.S.L. of about 12 seconds, compared to an average A.S.L. for other films of the period, and Kane only has a few shots that last up near 2 minutes.