(film) ISO's relation to Sunny 16 rule?

Question

So I documented myself on Sunny 16 to shoot on 35mm film, mostly Kodak Portra 400.

All the videos or articles I read mention that the Sunny 16 is basically setting the camera's settings at 1/250 if you have 200 ISO film (so 1/film_speed) and f16 if it's very sunny with sharp shadow edges, f11 with sunny with softer shadow edges etc.

However and here comes my question, it seems they explain it the same way with all ISO film speeds and just change the 1/film_speed formula to the film speed. So in the formula above, everything would be the same except 1/500 if you have 400 ISO film in the camera etc.

Therefore, how can I take into account and visualise in my sunny 16 evaluations with different ISOs, for example if I have 400 ISO film speed?

Thanks !

Answer 1

Therefore, how can I take into account and visualise in my sunny 16 evaluations with different ISOs, for example if I have 400 ISO film speed?

A film's ISO rating indicates its sensitivity. ISO 400 means that the film is twice as sensitive as ISO 200, and four times as sensitive as ISO 100. The sunny 16 rule says that on a sunny day, shooting at f/16 and a shutter speed that's the inverse of the ISO will give about the right exposure. So, if you're using ISO 400 film, f/16 and 1/400s will give the right exposure. If you have ISO 1000 film, f/16 and 1/1000s gives the right exposure. The shutter speed is adjusted according to the ISO value because film sensitivity and exposure duration vary inversely: if your film is twice as sensitive, you should expose it for only half as long.

Sunny 16 doesn't mean you have to always shoot at f/16, though -- it's just a guide to get the right exposure value. Once you know the right shutter speed at f/16, you can easily determine the shutter speed at other apertures because aperture and shutter speed also vary inversely: if you use an aperture that's twice as large, you're letting in twice as much light, so you should expose for half as long to get the same exposure level. Conversely, if you decrease the size of the aperture (remember: that means the f-number goes up), then you have to increase the exposure time. For example, if you want to shoot at f/8 instead of f/16, you'd compensate for the larger aperture by reducing the exposure time by two stops because f/8 is two stops faster than f/16. Or, if you wanted to shoot at f/22, you'd increase the exposure time by one stop because f/22 is one stop slower than f/16.

Answer 2

You use the ISO rating to determine Sunny 16 shutter speed. Sunny 16 is a guide for bright sun exposure without a light meter, based on shutter speed of 1/ISO seconds being appropriate for f/16. All equivalent exposures are included.

Example, if ISO 100, then 1/100 second at f/16,
or 1/200 second at f/11,
or 1/400 second at f/8, etc. (Equivalent Exposures)

And of course, Sunny 16 includes approximations for cloudy days, judged by the shadows being cast.

However in its day, there were different methods to specify film speed ratings. ASA standards started in 1946, and ISO adopted same in 1974. Sunny 16 was used before that. In the 1930s and 1940s, Weston light meters used their Weston film speed ratings, which was typically 1/3 stop different than ASA or ISO today.

Bright sun typically meters EV 15 at ISO 100 (depends on sky clarity). This is also 1/3 stop different than Sunny 16, which can explain the 1/250 vs 1/200 second variations. We cannot judge shadows closer than 1/3 stop.

http://www.scantips.com/lights/sunny16.html has some of this history.

Answer 3

The 1/shutter speed @ f/16 is the basis for the “Sunny 16 Rule”. This rule of thumb predates modern cameras. For the better part of camera history, most adjustable cameras sported a between-the-lens shutter. This is a mechanical device consisting of thin metal leaves that were snapped open and then snapped closed by springs.

The timing of the duration of the shutter operation is analogous to a spring-operated stop-watch. We are talking about a main-spring, a gear train, and an escape mechanism. Primitive stuff as compared to today’s electronic mechanisms. To make life simple and to make the shutter speed sequence “elegant”, the typical number set was: 1 – 1/2 – 1/4 – 1/8 – 1/15 – 1/30 – 1/60 – 1/125 – 1/250 – 1/500 of a second. Note the smooth progression was despoiled at 1/15 which should be 1/16 and again at 1/125 which should be 1/120. Again the sequence actually used is “elegant”. Now the rest of the story: The between-the-lens shutter design starts the exposure as a tiny opening that expands to full open. At the ends of the opening cycle, the blades must stop their motion for a preset time. As the shutter closes, the opening decreases until the shutter fully shuts off the light. In other words, the shutter’s open and close cycle runs up and down the f/numbers. The efficiency of this mechanism is about 66%. That, plus errors in the escape mechanism, prevents accuracy better than 1/3 of an f-stop.

So you are worried about not being able to set the shutter at 1/400 of a second! Instead you are forced to use 1/500. The bottom line is: We can’t set the exposure closer than 1/3 of a stop -- why worry?