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Sunday, August 24, 2014

••◊ Tips for Outdoor Lighting, part 2: Reading a Light Meter

This week's entry falls into the category of "great-big-DUH!"  I was shooting the outdoor lighting tutorial last week and facing a few production delays while I figured out the difference between the light meter reading and the five to seven stops of ND filtering I was using to control exposure in mid-afternoon sunshine.  While flipping through the Sekonic manual later that day looking for a solution I found that the "Cine" version of my meter allows you to program in .3-.9 ND's.  I would have expected this to be programmed into my meter as well given the price, but it really doesn't matter.  Only three stops of ND?...Completely useless in Southern California sunshine.  Just look at the meter reading below.  This was taken at about 4pm and it's still reading f/64-ish at ISO 800!  My lenses max out at f/22 and nobody runs in that range anyway because of diffraction.

The other kicker to this is that cameras like the C100 have their full highlight range starting at ISO 850.  The FS-700 is typically ISO 640 with Cine-gamma 4 or ISO 500 with Cine-gamma 3.  Using less than these ISO's can sometimes not be possible or actually hurt your highlight range.  So the only good answer is to use neutral density filters (ND filters) to control the harsh sunlight.

One simple method I thought of the next day is to simply use an ISO level which corresponds to the amount of ND you've applied.  I didn't even know that my meter could read at ISO 3, but sure enough it does.  So for instance if you have a camera like the Canon C100 and want to run at ISO 800 (I'll talk about ISO 850 a bit later), then you can apply the following table.

ISO                        ND                         Stops
800                        0                             0
400                        .3                            -1
200                        .6                           -2
100                        .9                           -3
50                          1.2                         -4
25                          1.5                         -5
12                          1.8                         -6
6                            2.1                         -7
3                            2.4                         -8

So for instance if we have the camera set to ISO 800 and the meter reads f/64 as the above pictures shows, then I scroll the ISO setting down to ISO 3 and it reads f/4, I know I need 8 stops of ND to get down to a more typical cinema aperture.  Likewise, if I just go ahead and install 7 stops of ND I can scroll my meter's ISO setting down to ISO 6 and take proper readings, which should read in the neighborhood of f/5.6 in this example.  This is much simpler and quicker than figuring out the ND calculation in my head, which can be a real mental challenge when you're trying to write, direct, produce, shoot, and otherwise run a production.  Shortcuts that help the production work faster are always welcome.

Now for the pesky ISO 850 setting that Canon provides as the "optimal" sensitivity...That's only about 0.1 stop more sensitive than ISO 800.  So for practical purposes you can just use the ISO 800 reading or subtract 0.1 stop from the meter reading if you feel you have a very accurate reading.  Sure enough, if you look Canon's charts in the article I linked to above you'll find that there's only 0.1 stop of difference in highlight retention between ISO 800 and ISO 850.

Hope this helps some folks.  I certainly need to do some work on understanding how to properly do meter readings in different situations.  That's a whole advanced DP topic unto itself.

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