Getting to Grips with Light - Sunny F16 Rule

Photo by chuttersnap on Unsplash

The 'Sunny F16' rule is there to help you work out whether the shot you want to take will expose correctly when you don't have a light meter to hand. It's a simple rule that applies in clear, sunny conditions (EV15). It states that if you are taking a shot with the aperture set to F16, the shutter speed should roughly match that of your film. In other words, in high noon sun with no clouds (EV15), when shooting at F16 with an ISO of 100, the shutter speed should be about 1/100th of a second. Or, if the ISO is 25, the shutter speed should be about 1/25th of a second. Simple.

Now, of course, with box cameras we cannot adjust our settings, and most have wider apertures than F16, and more often than not the light conditions are overcast...Bear with me. The rule still has some use.

I've written before about exposure values in my post about film speeds, and the sunny F16 rule is another neat trick to make sure you have the right conditions, or the right camera for the job. There's a good argument for having more than one box camera loaded with different film speeds so you can grab the one set up correctly for the light conditions (particularly as they are so cheap on eBay). I have four Ful-Vue cameras, so in theory I could put a different film speed in each of them (actually that's a good idea, why haven't I done that yet?)

Anyway, back to the point. To illustrate we'll assume we're shooting with a Ful-Vue camera with an aperture of F11 and a shutter speed of 1/30th of a second. 

To keep things simple, we're going to assume that the camera and selected film are the only tools at our disposal (no neutral density filters, we'll get to that in another post), and for the purposes of this example we are not going to push or pull the film in development (again, we get to that later).

If, hypothetically speaking, we were shooting a subject at EV15, and the camera had an aperture of F16, then the correct film speed would be ISO 30 to match the fixed shutter speed. You can't buy ISO 30 film, so you're left with a choice of ISO 25 or 50. To be honest either would work fine. ISO 50 might overexpose a little, but film latitude is such that that shouldn't be an issue, similarly ISO 25 isn't going to be too far out at 1/30th  of a second either.

Let's assume we're loaded with ISO 50 film. The aperture size is F11, which is one stop of light greater than F16. This means that to correctly expose the film we now need to toggle one of the other variables. By increasing the stop to F11, we now technically need to decrease the shutter speed one stop to 1/60th of a second...but of course we can't do that. What we need to do instead is toggle the ISO, which is easy on a digital camera, harder on a film camera. Here you have four options:

1)Take the shot. It may be mildly overexposed. No big deal.
2)Find a different subject in weaker light that will expose correctly.
3)Use a different box camera loaded with ISO 25 film and the same shutter speed and aperture.
4)Use a different box camera with completely different settings to correctly expose the image.

Disclaimer: I am not suggesting you travel with 4 box cameras at all times to be ready for most light eventualities. Watch the forecast or look out the window. Choose the best bet, and take your chances, or, if your camera bag will take two cameras hedge your bets.

Here is an illustration. Let's assume I do load each of the four Ful-Vue cameras with a different film speed. They would then each be optimal for different exposure values:

Ful-Vue Exposure Values
F11 1/30th ISO 200 = EV11 (subjects in deep shade)
F11 1/30th ISO 100 = EV12 (subjects in open shade/heavy overcast)
F11 1/30th ISO 50 = EV13 (subjects in cloudy-bright light - no shadows)
F11 1/30th ISO 25 = EV14 (subjects in weak, hazy sun)

So, as you can see, none of these cameras is actually correctly set up for EV15, or sunny F16 conditions. I do have another box camera though with a bit more versatility, my Balda Baldixette, which has 2 apertures. You can see it's EV options here:


Baldixette Exposure Values
F9 1/60th ISO 200 = EV11 (subjects in deep shade)
F9 1/60th ISO 100 = EV12 (subjects in open shade/heavy overcast)
F9 1/60th ISO 50 = EV13 (subjects in cloudy-bright light - no shadows)
F9 1/60th ISO 25 = EV14 (subjects in weak, hazy sun)

F16 1/60 ISO 200 = EV13 (subjects in cloudy-bright light - no shadows)
F16 1/60 ISO 100 = EV14 (subjects in weak, hazy sun)
F16 1/60 ISO 50 = EV15 (subjects in bright or hazy sun - sunny F16 rule)
F16 1/60 ISO 25 = EV16 (subjects in bright daylight on sand or snow)

So, to get perfect shots at EV15, I should leave my Ful-Vue cameras at home, grab my Baldixette, load it with with ISO 50 film and shoot it at F16.

Disclaimer: I do not own four Baldixettes, I'd still have to pick a film speed, shoot with it, then reload.

Interestingly, if you look at the exposure values available when shooting at F9, they are identical to those offered by the Ful-Vue cameras. This is thanks to the Baldixette's faster shutter speed. So it would make the most sense for me to load the Baldixette with either ISO 50 or 25 to allow it to shoot at exposure values beyond the range of the Ful-Vues. This is handy to know as it means if I want to shoot in the snow in bright sun, I should choose the Baldixette at ISO 25.

What's the point of all this?

Again, this is a way of using knowledge of exposure values and a longstanding film photography rule to make good choices about exposures. Ask two questions:
What are the light conditions?
Is this the right camera for the job?

If you have only one box camera then you need to:

1. Be stricter about maybe not taking the shot.
2. Not get hung up about maybe overexposing a little. 
3. Get good at recognising light conditions, particularly the ones your camera is set up for.

If you want to play around with different camera settings to experiment with resulting exposure values, check out this site.