The impressive paintings of this great individualist, the inimitable style of which is a reflection of his impressions and experiences, have strongly influenced the development of modern painting in Austria. Yet his paintings and his graphic arts created just after the war ( in the forties and fifties) make evident that his works are original documents which are easily distinguishable from the works of other painters.
Lettner work was strongly influenced by literature, especially by works of those poets and writers who - like himself - looked into the profound questions of human existence , e.g. Dante, Baudelaire, Cervantes, E.T.A. Hoffmann : He practically illustrated their words in the form of art works which emotionally corresponded to the poets.
His works of his first great creative period Beginning and Centre of my Painting until 1959 still reflect his experiences of the war which he represents in a powerful expressionist style often showing pessimistic characteristics and the feeling of a silent resignation. However, this changes after his study trip to North Africa where he intensifies his perception of colours when he gets known to oriental areas of life. All of a sudden hi is capable of creating great and powerful colours, portraying subjects in simplified terms and reducing his works to the essential part. Thus it was that the outlines of his pictures made more room for colours as means of expression.
His second great creative period ( 1960 - 1965 ) is called the Orphic World. In this period he works without associative suggestion and objective relation and the image of man entirely gives way to an illusionary room. With vital and tempestuous enthusiasm Lettner reveals wishes, desires and fears. Compared to his earlier works he also uses newly created techniques.
After his journeys to Israel and Greece Lettner starts his third creative period called Human Pictures. The human character turns up again, the subjects become true to life and his paintings are more colourful than ever before. By the use of new colours and an excellent varnishing technique his works become light, clear and diaphanous. Lettner seems to have regained his zest for life. He devotes himself again to his central subject : to man and to another important aspect : to religion and to cultic elements.
In 1980 his heart condition forces him to give up the oil painting and he retires from the world with resignation producing subtle watercolour paintings with very personal confessions. Nevertheless, he comes back 1984 with undiminished power of expression , will and creation. Despite all the restrictions due to his illness he demonstrates the whole power of his artistic vitality and ability: he creates expressions, a real world translated into art, paintings of youthful freshness and directness. The alert eye will become apparent that these paintings are of outstanding artistic quality and come from deep in his heart. Moreover, we can feel that Lettner has given away his abilities, his imagination and the abundance of his feelings.
On the highest point of his creativity Lettner dies in 1998 in Innsbruck.
In the years 1971, 1977 and 1983 films about Lettner work were produced and shown on TV. Moreover, he got the title of a professor and received many prizes, honours and medals. The commercial success and his great breakthrough, however, was only after 1991 when he worked together with Hermann Schafferer, the owner of a gallery in Innsbruck. Schafferer is well known as an expert of Lettner works.