Steve Cushing Impresionist Fine Art Photography

Embracing imperfection, recording emotions, one impression at a time…

Projector lenses on Digital Cameras


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Camera and projector lenses work on essentially the same principle, just in reverse. They both collect light from one side and project it out of the other. For camera lenses, they collect light from the world and projected it onto your small sensor. For projector lenses, they collect a concentrated amount of light from a film or sensor and project it out into the world. People have been projecting images on walls and screen like surfaces for centuries. References go back to the 5th century in China for pinhole image formation, and the Greeks were aware of the phenomenon by the 4th century.

While the primary functions are opposite, both types of lens can theoretically work in either direction.

The only major differences is that camera lenses offer more control over features like aperture and focus distance. Projector lenses are focused on the projector screen by moving the lens toward or away from the light source.

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Depth of field is not as important with projection as it is with photography, which is why most projection lenses have low f-numbers (wide apertures). Brightness is key with projection.

Most projection lenses have no iris, so they are corrected for wide open work. The predominant lens design for projectors is thus a flat-field type, where the focal length to all areas of the imaging medium from the rear lens element is constant and in the same plane. This design ensures that when we focus our projector’s lens once, the entire image comes into and stays in focus. However, given that the sharpest setting for a lens usually is two or three f-stops down from fully open, focus uniformity also is important.

Increased depth of focus is necessary when projecting onto uneven surfaces, such as a curved wall or screen. There will be some sacrifice of image brightness, but focus will remain constant across the horizontal and vertical dimensions, but this depth of field is still quite shallow.

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It is possible to purchase an iris to add to a projection lens. I use one mounted on an m42 adapter.

See Aperture
HERE







But this is not essential as most creative photography is all about using lenses wide open to create
bokeh, a shallow depth of field suits the technique and in a projector lens the plane of focus for a projected image is very shallow (perhaps less than an a couple of centimetres) so as to be essentially two-dimensional. As a result, projection lenses typically have low f-stop numbers, providing a larger aperture and, consequently, brighter images on the screen.

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So why use projection lenses?

  • Projection lenses are cheaper than a lens with same data as normal photo lens.
  • Some projection lenses are faster than photo lenses.
  • Some projection lenses show special rendering (bokeh, swirl).


So the only real difference from a practical point of view between projector lenses and camera lenses is that the lens built for a camera has many functions not needed for a projector. Of course optically making a large plane image on a screen from a plane image o is a little different than working in the real life. However, to get a sharp projection the lighting still needs parallel beams that same as a camera.

I used a lot of cheap slide, cine, DLP projection lenses from Leitz, Isco, Scheider Kreuznach, Zeiss, ROW Rathenow, Emil Busch, Rollei, Meopta and others. Some of those lenses work well on medium format too.

The Leitz Wezlar Colorplan 90mm f2.5 (probably the closest I'll ever get to owning a Leica.


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To have some controller the depth of filed Move closer of further away from the object. Often as the background bokeh is important to me I will explore the focal distance needed for the bokeh and set this before positioning myself the correct distance from the object to get focus on the object.


The shallow depth of field can create interesting backgrounds.This image was created simply by having a distant dark background, in this instance the road.
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How to Convert a Projector lens


You need to attach the lens to a helicoid in some way. Cheap helicoids are easily available. My own favourite method is to glue a 42mm or larger adapter step up ring to the lens but if you want a "factory look", including aperture adjustment, you can also housed the lens block into a body of a dead 135mm vintage lens. Just pick up a damaged old lens and remove the glass elements.

See
Pentacon AV80, Pentacon AV100, Pentacon AV140, Pretzval 130, Will-Wetzlar AV90, Rolli-Projar 85 and Anamorphic Lomo 130


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