What is the fine-grained topic of the following text: The two series of works presented in the exhibition – Fotoformas, initiated in 1946, and Sobras, made in the 90's was the artist's last production– are undoubtedly the most representative sample of Geraldo de Barros' distinct abstract approach to photography and were produced in periods of the artist's career when photography was his main concern. Only return the topic.
Geraldo de Barros' abstract approach to photography in his series Fotoformas and Sobras.