ExpoDev has two Exposure modes to choose from, Aperture and Speed. In Aperture mode, you select the aperture to use and ExpoDev will calculate the shutter speed. In Speed mode, you select the shutter time and ExpoDev will calculate the Aperture to use. You can switch between modes and ExpoDev will try and preserve the exposure as closely as possible when changing modes. This isn't always possible though because of the effects of reciprocity failure adjustments.
When ExpoDev calculates an exposure, it will always be rounded to 1/6th of a stop. How it is rounded depends on the DOF mode. When Favor DOF is On or DOF is being calculated, ExpoDev will always round exposures in a way that preserves DOF. If Favor DOF is Off or no DOF calculation is being performed, ExpoDev will then round exposures to the nearest aperture or speed setting which may or may not preserve DOF.
If the calculated or selected shutter time is greater than 1/2 of a second, reciprocity adjustments will be automatically applied (if your selected film profile specifies that reciprocity adjustment should be applied). The Use Timer button will also appear.
When Aperture is selected as the Exposure mode, you can select apertures from f/1.0 to f/512 in 1/6th stop increments and ExpoDev will calculate the shutter time to use for that exposure. If the exposure time is less than 1/2 of a second, ExpoDev may need to adjust the aperture to use a standard shutter time. When an exposure needs to be adjusted, ExpoDev will show both a Use and an Ideal exposure (ideal will be shown as informational only, as using non-standard and not-settable shutter speeds).
Note: If a DOF calculation is also being performed and you modify the aperture in a way that decreases the DOF, a warning will be shown.
When Speed is selected as the Exposure mode, you can select from either standard shutter speeds (1/2 to 1/8000) or a shutter time in minutes/seconds/tenths and ExpoDev will calculate the aperture to use for that exposure.