As a rule, the Copyright Act provides the author with the exclusive right to dispose of the work in either the original or an altered form.  This means that the author of an adaptation must first obtain permission from the copyright holder of the original work. For example, translating a novel requires obtaining permission from the original work's author.
According to the Copyright Act, permission from the copyright holder is not required if the person has drawn freely on a work to create a new and independent work . This provision is based on the separation of an idea and its expression. Copyright does not protect ideas or principles, but only their independent and original expressions. As a consequence, an idea or theme can be realized in an independent and creative manner without infringing on the copyright of the author of the original work. Whether or not an adaptation is independent and creative is ultimately decided on the basis of a case-to-case assessment of similarities. According to legal literature, the assessment of similarities is based on the evaluation of similitude between the works' essential parts . The decisive factor is whether the works are similar to the extent that they evoke a feeling of likeness.
Supreme Court decision KKO 1979 II 64 discussed the similitude of a photograph and a painting based on the photograph. Art student Pekka Jylhä had created a painting from a photograph taken by Kalervo Ojutkangas and exhibited it to the public with the intent to sell it. Comparing the works, the Supreme Court considered it evident that the painting could not have been created independent of the photograph. The posture and clothing of the children on the painting and on the photograph and the background displayed substantial similarities. On the other hand, the Supreme Court took notice of the works' differences. Minor differences that were due to the different production methods of a painting and a photograph substantially influenced the works' overall effect. Therefore, the Supreme Court did not consider the painting to constitute a reproduction of the photograph, but an independent work created on the basis of the photograph. 
Notes and References
 Copyright Act
 Copyright Act
 Oesch, R. 1993. Oikeus valokuvaan : tutkimus valokuvan suojan kehityksestä ja sisällöstä.