The Smoky-16 Rule

Discussion in 'Casual Photo Conversations' started by tom_halfhill, Sep 9, 2020.

  1. Many photographers know the Sunny-16 Rule: a sunny mid-day exposure is f/16 at a shutter speed of 1/ISO. For example, f/16 at 1/200 second at ISO 200. Now I propose the Smoky-16 Rule, which estimates the daylight exposure in the vicinity of an unprecedented wildfire, like the kinds we're now experiencing in California. Our fires were ignited during a heat wave that reached 130F (54.4C) and dry thunderstorms that brought 12,000 lightning strikes but no rain.

    I took the photo below at high noon today (September 9, 2020) from my third-floor balcony near San Francisco. The Sunny-16 exposure should be 1/800 second at f/16 at ISO 800. Instead, my metered exposure was 1/42 second at f/2 at ISO 800. So, my Smoky-16 exposure is Sunny-16 plus 10 stops -- a difference of about 1,000 percent.

    My camera's auto white balance doesn't fully capture the weird orange glow of this noontime pseudo-daylight, which a friend describes as "Martian." Cars are driving with their headlights, and indoors we've got all the lights on. I slept until 8:30 this morning because I didn't know it was daytime.

    As I write this at 3 p.m. the same afternoon, it's a few stops darker now. So dark that a sharp handheld exposure at f/2 and ISO 800 isn't possible, even with my camera's motion stabilization. I'd need a tripod to make a time exposure.

    Australia suffered similar wildfires earlier this year, reportedly killing one billion animals. Now it's our turn. Unprecedented fires have also burned above the Arctic Circle in Sweden and Siberia. I'm afraid my Smoky-16 Rule has a bright future.

    DSCF5985_small1.jpg
     
  2. We had that for 2 years in a row here in Vancouver, 2017 and 2018. It is a bizarre thing to live through, and obviously not good for your health. I like your rule - I'll check some shots I made back then to see how well it fits. It does make sense though.
     
  3. Looks like my pre-cataract surgery world. Monterey/Carmel area even gloomier.
     
  4. smoky sunrise 8-23-20.jpg
    Smoky eastern Idaho sunrise 8-23-20, with smoke provided by northern California wildfires. 1/400 s, f:16, ISO640. Bad, but not nearly as bad as your San Francisco smoke.

    Exposure will vary by smoke concentration, so I do not envision a general "smoky 16" rule that will apply to all smoky situations.

    Light will be absorbed approximately exponentially by smoke with uniform concentration, with the exponent proportional to the product of an absorption coefficient times distance (Beer's Law). The absorption coefficient will be proportional to smoke concentration, but will also be a function of light wavelength. Light scattering also needs to be considered. So, it is possible to calculate exposure if you know all the variables, but using an exposure meter is a whole lot easier!
     
  5. Excellent
     
    peter_fowler likes this.
  6. Aren't you all using mirrorless cameras? Just turn on Vivid and spin the dials until it looks the way you want.
     
    steve_gallimore|1 likes this.
  7. A slight underestimate there. 2^10 is 1024, so stick a couple of zeroes on that figure to get the linear percentage increase in exposure. Or take its reciprocal to find the absorption of the smoke.

    Either way, that's one heck of a smog!

    Stay safe Tom.
     
    tom_halfhill likes this.
  8. "
    You are right! In my haste, I added instead of multiplying: one stop = 100%, two stops = 200%, three stops = 300% ... etc. Instead, as you say, it should be 100%, 200%, 400% ... etc. So my ten-stop Smoky-16 exposure compensation should be 51,200% more exposure, not 1,000% more.

    Anyway, I hope everyone realizes my Smoky-16 Rule is dark humor. Exposures will vary widely, depending on the smoke density. Today it's not nearly so dark, and the weird orange glow is gone. Instead, a fine white ash is falling. I spent more than an hour this morning vacuuming the stuff out of our home after we foolishly opened our windows for a while.
     
  9. Sorry for the maths lesson Tom, but that's still not right.

    If you take 1 stop (x2) as a 100% increase, then by the same logic 2 stops (x4) is a 300% increase and 3 stops (x8) is a 700% increase. So by the time you get to 10 stops (x1024) that's a 102,300% increase.
     
  10. I think you are correct. Large percentages can be so confusing that I prefer to use multiplication factors (2x, 3x, etc) instead of percentages when expressing differences greater than 100%. I shouldn't have deviated from my practice. Is the following series your progression?

    1 stop = 2x = 100%
    2 stops = 4x = 300%
    3 stops = 8x = 700%
    4 stops = 16x = 1,500%
    5 stops = 32x = 3,100%
    6 stops = 64x = 6,300%
    7 stops = 128x = 12,700%
    8 stops = 256x = 25,500%
    9 stops = 512x = 51,100%
    10 stops = 1024x = 102,300%

    So my revised percentage calculation (51,200%) was off by one stop.
     
  11. A light meter is only as good as you use it. The raw reading would tend to render the foreground as though it were bright daylight. If you metered the sky, it would be rendered as though it were neutral density, much as shown in the first image. I might even close down another stop as to convey the sense of heavy overcast. With digital, you can achieve practically any effect you wish in post, as long as you don't overexpose the highlights. Matrix metering tales much of the guesswork out of exposure - but not all of it.

    If you join the caravan leaving California, be sure to keep your headlights on until you reach Nevada.
     

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