The New Petzval Lens

Let me introduce you to one of my Christmas presents:

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This is a remake of a lens design from 1840 by Joseph Petzval, then one of the leading physicists working in optics. For its time, this lens was very fast and very sharp at the center. Today, we have faster glass, and good lenses maintain sharpness across the entire frame, so why would one bother? One reason is that the progressive vignetting that occurs towards the boundary of the frame creates a radial blur unlike anything else.

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This effect can be controlled by choosing appropriately sized aperture blades. Yes, this lens is so not automatic that you have to manually insert blades with aperture holes. You see them in the top picture scattered around near the lens. The lens comes with a set that have circular holes as one would expect, but nothing prevents you (or me) to use plates with holes in different shapes.

The effect is simple: A small point-like object (like a light source) that is out of focus is usually rendered as a slightly blurred small disk. This is what makes up the bokeh of the lens, and it is one of the most important characteristics of fast lenses (where you will have a lot of the frame out of focus, usually). If the hole in the aperture blade is not a disk but (say) a square, then the small dot that is out of focus will become a small square. Likewise, you can have star shaped blurs or even multiple blurs if the aperture plates has several holes.

Aperture Plates

Above is my first set of self-designed blades for the Petzval lens. I created this by first scanning in the actual plates for size and shape, vectorizing them in Adobe Illustrator, adding my own design, exporting them as an AutoCAD DXF file, and importing them into the software that drives my Cameo Silhouette die cutter. The result are little pieces of card stock paper.

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Here, for instance, is a neocubistic sculpture (from the Sculpture Trails Outdoor Museum near Solsberry, like all portraits in this post), using an aperture plate with several square shaped holes. Below is an image using a plate with a fractal cross.

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This opens up many possibilities. One can design aperture plates to complement the motive by enhancing the background, or one can distort an otherwise distractive background beyond recognition.

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Imagine a technology where a modern lens contains instead of regular aperture blades an electronically controlled screen that, somewhat like liquid ink, can be used to create aperture holes of any shape. In a film camera this would make it possible to continuously modify the out of focus area. Alfred Hitchcock would have used this to make the famous tower scene in Vertigo even more vertiginous.


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