Basic Daylight Exposure (BDE), also known as the Sunny 16 Rule, is a method of exposure based on using the brightness level of the sun as a reference or standard. As long as the sun is 20 degrees or more above the horizon, on a clear day, the brightness is consistent and does not vary throughout the day until the sun drops below 20 degrees on the other horizon. This allows photographers to know and trust exposures at this time of day, on any day, in any city in the world without testing or taking meter readings.
The Basic Daylight Exposure formula is actually quite simple:
- 1/ISO @ f16 for any front lit subject or object.
Translated, this simply means if you use the ISO number as a shutter speed number, your exposures will be correct and true at f16 on a front lit subject.
- For Example - ISO 100
/100th @ f16 (or any equivalent or reciprocal exposure)
Using ISO 100, the first chart will show the reciprocal or equal exposures. This means that you can shoot at 1/100 sec at f/16 or 1/1600 sec at f/4 and get the same exposure.
Be careful though. This doesn't mean that the photo will look the same. Changing the aperture will change the depth of field and changing the shutter speed may affect how motion is recorded in your image.
The newer digital cameras and lenses work with apertures with numbers that might not be familiar to many and often do not match the aperture numbers available in many of the modern digital light meters. The equivalent numbers are listed in the table in both 1/2 and 1/3 stop increments (full-stop increments are in bold).
Light Meter Aperture Reading
Most light meters will give you aperture readings in 1/10 increments. To help you translate that to a working aperture you can find the Meter Aperture Readings in the table in the far right column.