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One-point Perspective Construction
For canonical cubes aligned parallel to the picture plane, all receding edges project to a single vanishing point. Importantly, that point must fall on the horizon (gray line). This simple construction dominated the Renaissance.
In his evocative Annunciation (Ghent alterpiece) from 1430, van Eyck employs an eerie use of soft shadows to capture the depth of the space integrated behind the four panels. The direction of the light matches the location of the windows in the chapel for which this panel sequence was designed.
The vanishing points of the floor and the architectural elements at left and right are discrepant from the horizon defined by the vanishing point defined by the townscape seen through the window. Even if van Eyck had used local optics, the vanishing points would still have to project to the same horizon.
What would the picture have looked like if there were one unified vanishing point? Would it have looked better? Compare "the painting" with a "what if" version below. Notice the rafters in the top corners, and a subtle change in the floor tiles.
Mantegna's dramatic relief, the Lamentation over the Dead Christ (below), is just the kind of painting that might have been achieved with the use of optical aids. However, it had been noted that the Christ has a curious 'dwarfish' look that indicates some perspective problem.
In fact, the parallels defined by the body do not match those of the adjacent edges of the table. The body is relatively straight, but the table is strongly sloped. Both should project to the same vanishing point if Mantegna had used an optical projection. It follows that Mantegna could not have used optics to define the layout of this painting
Why did Mantegna have "mismatched" vanishing points for Christ and the table? Would the picture look better if they were the same? Compare at right to correct the perspective of the Christ figure to match that of the bed, or the bed to match that of the Christ figure. Does the three-dimensionality of the horizontal figure become more vivid? Is this what Mantegna intended?
In Hans Holbein's Ambassadors (1533), several objects in the scene have different vanishing points. Were they painting by eye? Or, as Hockney and Falco claim, simply created from separate use of optical projection for each object. If this were true, the perspective within each object would be consistant. We find a single vanishing point for the hymn book (black), and the carpenter's square (bookmark).
Upon close examination of the book of arithmetic (red), however, we find that the projection lines do not conform to a single vanishing point. This points to the intuitive nature of Holbein's perspective. Holbein cannot have copied the geometry from an optical projection of each book (or indeed from an accurate knowledge of perspective geometry).