Bokeh is a photography term that’s a bit difficult to define but is basically soft, aesthetically pleasing soft background blur, often used to make a subject stand out. Also called “background separation” or “subject isolation”, achieving it optically requires a fast lens with an aperture below 2.8 or preferably lower. These lenses can get very expensive, but in the video after the break [Matt] from [DIY Perks] blows all the commercially available options out of the water. Using an old episcope projector, he built a photography rig with background separation equivalent to that of a non-existent 35mm f0.4 lens.
Unlike most conventional projectors used to project a prerecorded image, episcopes were used to project an image of physical objects, like books. To use this lens directly in a camera is impossible, due to the size of the imaging circle projected out the back of the lens. At a diameter of 500mm, there is simply no imaging sensor available to capture it. Instead, [Matt] built a projection screen for the image and photographed it from the opposite side with a normal camera.
The projection screen was made by sandwiching a sheet of diffuser film between two sheets of clear acrylic held in a frame of aluminum extrusions. To block out all other light, [Matt] added aluminum shrouds on either side of the screen, which also serves to mount the lens and a camera. The shroud on the lens’ side is mounted on a separate aluminum frame, enabling the image to be focused by adjusting the distance between the screen and lens. Linear rods and bearings on 3D printed mounts allow smooth motion, while a motor-driven lead screw connected to a wired remote does the actual adjustment. The gap between the two halves was covered with bellows made from black paper.
Initially, the projected image was very dark around the periphery, so [Matt] added fresnel lenses on either side of the screen. This straightens out the light rays coming from the episcope lens before it hits the screen, and focuses it back into the lens of the camera, almost eliminating the vignetting. He used a very similar solution in his DIY 4K projector project.
The result looks almost dream-like, with softly blurred background and foreground while keeping the subject sharply in focus. Even though [Matt] is photographing a projected image inside the rig, it’s impossible to tell. This has the added advantage of allowing the use any camera, including a smartphone.