Up until recently the projection of transparencies brought about a problem which had been considered unavoidable: In order to enable every member of the audience to see the full size of the picture, it had to be projected in an extreme upward position, leading to a trapezium-like distortion of the overall dimensions. The special overhead projectors with 13 ° keystone correction in upward or sideward projection exclude such a distortion up to a projection angle of 13 °. Both models are equipped with a vario-focal lens f = 300 - 340 mm.
