Exposing with False Color

D

David Evans

Well-known member
Hey guys,

I'm really interested in controlling light ratios using just False Color via LCD of my Ursa Mini 4.6k. I have a couple of questions:

1 - How do you transpose IRE values to contrast ratios? When exposing with light meters, we know the ratios because we get the f.stop reading, so we know how to expose. But let's say I want a 3:1 on the talents face. I'll aim for pink on one side (cause I know that's the color reproduced by the Ursa Mini, where caucasian skintones should fall in to), and which color should I aim on the other? The same on K:B ratios. I don't think false color on my Ursa Mini shows IRE values, so it's just colors. Should work, though, just wanted to know the technique behind this.

2 - Should I expose for 18% gray card or skin tones? If the theory is correct, shouldn't matter, right? If expose to 18% gray, skin tones should automatically be correctly exposed, and vice versa?

Thanks
 
There are many ways to approach it.

One thing is to shoot a chart of stops so that not only you can use a waveform to see how stops are distributed across IRE values in your camera, but so that you can also determine how many usable stops you are able to accept from your camera...as in signal to noise ratios. In a less scientific, and not recommended method, you can divide the 109 or 100 IRE by the number of stops your camera manufacturer claims your camera should deliver.

Exposing all skin to 18% is a very tricky thing. First off, not all skintones are the same, nor are all places in your stage or scene supposed to read as the same luminance value. From a linear signal standpoint, 18% gray is roughly 50IRE. Different gamma curves will shift that middle gray, and so can ISO changes in RAW.
 
I suggest you light a gray card for pink, then stop down three stops. Note the color. You can do that if you wish another way, and that would be to start with pink, adjust your f stop (or shutter, or iso) until the color changes, and make yourself a handy table for reference. You have all the tools then to answer your own question.
 
I don't really do stuff like this where I'd have the time or ability to control the lighting that way, but I've been interested in knowing how the contrast ratios/stops relate to the PERCENTAGE values as well (I believe IRE is not the right term 'cause IRE is an analogue thing). Up until relatively recently I figured if you were at 70% and took it down a stop you'd be at 35% but apparently that's not at all how it works. Then I looked it up and apparently the answer is not simple at all.

I figured the way to do determine your look/ratio might be to do tests. . .if you know you're doing a particular project test out various ratios/percentages on a person sitting in a chair or whatever, look at them later on a monitor you trust and make notes as to which look you find most pleasing (key side of face reads 50%, fill side at 30%, or whatever), and use that as your bible for the project.
 
unless Im mistaken you have to buy that content from Hurlblog is for purchase only (except the one about setting up a flanders scientific with false color).
 
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