Camera lenses

Shallow Depth-of-Field Adapter

Let’s get a little technical about video cameras for a bit. Specifically about how the optics and the size of the image sensor affect depth-of-field.

Depth-of-field describes the part of the picture which is in focus. With a shallow depth-of-field only a narrow plane, a very particular distance from the camera is in focus. Conversely, a deep depth-of-field means that most of the picture is in focus, whether it’s up close or far away. If you create a small image of a scene with a small lens, you get deep depth-of-field. If you create a large image of the same scene with a large lens, you get shallow depth-of-field. That’s a basic nature of optics.

Since the 1970s, video cameras had image sensors 2/3 of an inch when measured diagonally. This is about the size of a 16mm film frame, so the natural depth-of-field of video cameras used to resemble that of documentary-standard 16mm film and nobody gave it much thought. Our new HD camera has a 1/2″ sensor. Since it’s smaller than the previous sensors we were used to, the depth-of-field is deeper; everything is in focus most of the time.

It’s not always good to have things in focus. A cinematographer wants to guide the audience’s eye with selective focus. And 35mm film, with its shallow depth-of-field set the standard of what “good cinema” looks like. So we wanted to turn our small-sensor camera into something that looks more like a large-sensor, shallow-depth-of-field Hollywood-style movie camera.

RedRock Micro 35mm lens adapter on our Sony EX3 camera.

The answer is an adapter. We just bought one made by RedRock Micro. It is essentially a spinning disk of ground glass upon which a lens made for 35mm still photography projects an image the size of 35mm film. The video camera’s own lens then shoots this projected image off the spinning disk of glass. We have a whole set of prime (not zoom) lenses to cover most situations.

Our Nikon lenses were originally used for 35mm still photography.

The results are beautiful. Not only do we get shallow depth-of-field, but since the first image is projected onto the ground glass, we get very subtle diffusion reminiscent of movie film projected onto the silver screen. It’s very cinematic and extremely flattering for all interviews.

Will we use it all the time? No. It’s an awkward rig and it eats up a little bit of light. It prevents us from using auto-focus and image stabilization. So it’s not for run-and-gun situations. But I don’t think I’ll ever shoot another interview without it.

RedRock Micro Adapter in use.