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ND Filters, 48 FPS, and 250D


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#1 cvaldeslora

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Posted 03 April 2008 - 12:07 PM

Hey guys, I have a question. (16mm)

I'm shooting this Saturday an exterior scene at a park. Very simple stuff. Problem is, I took readings earlier today at about the time I want to shoot, and for the stock I have (250D) the reading is just off the charts (f/32). I figure I'd change the time later in the day, but just to prepare for an especially sunny day with ND filters (at least .3, and maybe .6).

However, I also want to shoot these scenes at 48fps, which will be beneficial to my exposure problem. I'm wondering how I should go about using both an ND filter and a faster FPS and ultimately achieve the correct exposure. Additionally, how will ND affect the quality of the image and the depth of field considering the framerate?

Let's say for example that at 24 FPS and no filter I get a reading of f/11. If I crank to 48FPS and add an ND .3, what should I set the exposure to?

What if I add an ND .6 at 48 FPS in the same example?

Thanks!

Edited by cvaldeslora, 03 April 2008 - 12:11 PM.

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#2 Michael Nash

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Posted 03 April 2008 - 02:30 PM

First, follow the "sunny 16" rule. On a sunny day, film speed over shutter speed = f/16. In other words, 50 ASA at 1/50 sec. = f/16. So obviously any ASA faster than 50 is going to put your exposure higher than your lens will go, so you have to use ND filters to bring it back down.

Second, all your exposure compensation is straight forward math, so don't confuse yourself. You simply compensate your exposure for EACH thing that reduces it. Start with your "base" exposure at 24fps and start adding on everything that reduces light: shoot 48fps, you open up one stop. Put an ND.3 filter on, open up another stop. Use an ND.6 instead of .3, and you would open up two stops instead of one. So 48fps + ND.6 filter = three stops compensation.

Frame rate has nothing to do with depth of field. Depth of field is controlled by three things: Aperture, focal length, and distance to subject. Wider apertures give you shallower depth of field. A Neutral Density filter is simply a tool used to control your shooting aperture. Select the density that sets correct exposure at the aperture you want to use.
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