One of the challenges of using a 35mm film camera is many of the older models do not have electronic light meters, and the ones that do have Light-meters require batteries. When the battery for my Fujifilm X-T2 digital camera is exhausted, I can quickly pop in a new one. Changing the battery in some film camera, e.g. the Pentax ES II, is a bit more challenging.

To get around the need for a light meter, some photographers use a rule called “sunny 16” or "sunny f/16 rule". This "rule" recommends that, on a clear and sunny day with distinct shadows, the photographer set the camera exposure (aka shutter speed) to the inverse of the film ISO when the lens aperture is set to f/16. For example, if shooting a roll of Ilford HP5 Plus 400, a good starting point for shutter speed would be 1400 sec; for Kodak Ektar 100 it would be 1100 sec.

But what if you want to shoot at f/8 instead of f/16? Reduce the exposure time by a quarter or $\tfrac{1}{x^2} \$ where x is the change in the aperture, which in this case was two full f-stops. On a clear and sunny day, a roll of Ilford HP5 Plus 400 at f/8 could be exposed at ½2 x 1400 = ¼ x 1400 = 1/1600 sec. But what if the sky is cloudy or I am shooting indoors? It gets complicated. Some photographers are good at mental math or maybe memorised a table of possible values or with experience, have just learned what to do for most situations.

I have no intention of doing the mental math required. Despite a college minor in mathematics (differential equations, algebra and other abstract math are required for engineering students), I'm not good at mental arithmetic. It's my Kryptonite. Fortunately, I don't have to do any math. There is an app for that; the Sunny16 exposure calculator. I installed this app on my iPhone. Whenever I need to use the Sunny f/16 rule, I will consult this app to find the combination of aperture and exposure for the given screen.

I copied the following table from Wikipedia and added a column for exposure at ISO 100.

Aperture Lighting Conditions Shadow Detail Shutter speed at ISO 100
f/22 Snow/sand Dark and sharp edges 150 sec
f/16 Sunny Distinct 1100 sec
f/11 Slight overcast Soft around edges 1200 sec
f/8 Overcast Barely visible 1400 sec
f/5.6 Heavy overcast No shadows 1800