If you’re still unsure about how to determine exposures from scratch, use a trick called High ISO Testing as your guide. Here’s how it works. For each successive increase of your ISO dial and full stop in opening the aperture notch of your lens, your subsequent exposure time will be cut in half. Let’s say you boosted your ISO to 6400—a 6x difference from ISO 100—and fully opened your aperture to f/2.0—increasing the amount of light from a mid-range setting of f/8.0. While these settings will potentially yield an image with unappealing contrast, increased grain and limited depth of field, you can save valuable time by shooting an exposure bracket to identify a well exposed histogram at these settings. Let’s say the ideal histogram for this scene corresponds with a shutter speed of 4 seconds. You can then do the math to calculate the required exposure time for the same scene captured at ISO 100 and f/8, which would be a total of 32 minutes.