Combo Iris Rack / Vari ND Rack

David William Harris · November 20, 2013

I'm curious if anyone knows of an example of this technique used in a film. I've read that it's used in the movie Speed RAcer, but I've yet to see that movie. I know it's kind of a lame reason to want to watch a movie, but now I want to see it just to see how this effect plays on screen.

For those who aren't familiar with the concept; Let us imagine a shot in which a character is rather close to the lens in about a MCU. The focus is on him, we're at a F/1.4 or maybe a F/2 with an Variable ND 2.1 in front of the lens. Two shadowy figures walk up behind our foreground character and stand to his left and right, also facing the camera. As one of the shadowy figures begins to speak, we seamlessly iris rack to F/22 or F/16 while simultaneously opening up the variable ND to as clear as it will go (in a perfect world it would be totally clear, but maybe the best we'll get is a ND 0.3) This would increase the depth-of field so that now the shadowy figures are in focus, as well as our main character in the foreground.

So, if you know of any example of this type of effect, please let me know. Also, if you know of some apparatus that would make this a smooth transition, let me know. I'm imagining some gear or rod that would connect the iris to the variable ND so both can be pulled simultaneously.

Cheers, and thanks.

Tim Tyler · November 20, 2013

I think you're talking about varying the depth of field during a shot without affecting exposure, right?

Although I haven't seen Speed Racer I'm pretty sure 99% of it was shot on a green screen. A shot like the one you describe would be easy to create and highly adjustable in post if the footage had a keyed background.

Cinema lenses all have geared T-stop rings so that they can be controlled remotely using silent motors typically mounted to the camera's support rods. Steadicam and jib operators use these everyday. An additional motor and gear ring connected to a variable ND could allow the ND to be remotely controlled manually or with some kind of automation for more precision.

And somewhat related, FYI - Some of the better film cameras have adjustable shutters that can be automatically ramped in conjunction with FPS speed changes so that exposure / T-stop does not change during a speed change (i.e. normal speed changing to slow motion).