Tuesday, August 21, 2012

GBTimelapse - AutoRamp with 3 ND Filters

Walk-through using GBTimelapse AutoRamp with 3 ND filters

Last fall during a visit to San Francisco, I stayed at a hotel in Emeryville across the bay. I always ask for a room with a view and was lucky to get one on the eleventh floor facing the SF bay. This was a great vantage point for a time-lapse. I used my 5DMkII with 16-35mm lens on a Manfrotto tripod and ball head, with the camera tethered to my Lenovo laptop. Since I was in a hotel room I had AC power for both the camera and computer. I was able to get both the sunset and the sunrise. The window wasn't very clean, but with the camera focused on infinity it looked right through the window grime. Below is the result, with some of the night footage removed to make it shorter.

Technical setup
After achieving a good focus, I set the lens to manual focus and attached three ND8 filters. At 3 f-stops each that gave me a total of eight stops light reduction. But the sky was so bright, 8 f-stops was not enough to set the aperture at f/2.8; so I had to start the time-lapse at f/5.6 and rely on AutoRamp to reduce the Av down to f/2.8 as the sky darkened.

Watch the video below to see how I setup the AutoRamp parameters. It shows the process of removing ND filters at sunset and adding ND filters at sunrise.

There is some bright flashing in the video as the sun peeks out from the clouds. A couple of times that bright sun caught the dirt on the window and gave a bright flare over the screen.

The night sky wasn't very dark because of all the bright lights. The yellow color of the city lights bothered me too. Since making this video I've added the ability to ramp the white balance color temperature with the sunset and sunrise. It's very easy to setup and I'll show this new feature in a blog post some time soon.

Facing into a bright sun like this is very difficult. With three ND.8 filters I started at f/5.6 and bulb Tv of 0.275". It would have been better to start at f/11 and a bulb Tv closer to 1 second. Longer bulb times are more accurate. When you use a short bulb time (near a camera's minimum) the bulb timing error produces some flicker. I could also have done better by using stronger ND filters. If I had used there ND1.2 filters for a total of 12 f-stops, I could have used longer bulb times.

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