Depth of Field calculator
ExpoDev has three Depth of Field calculators to choose from. You select the DOF calculator mode by choosing from None, Check, Distance, or Focus. The DOF calculator can be setting to use either metric or Imperial units of measurement (as set in the app settings). If any of the Depth of Field modes are selected, then any aperture either set or calculated will automatically be set on the Exposure screen as well if you are using Aperture exposure mode. If you then modify that selected exposure aperture in a way that negatively impacts DOF, ExpoDev will show you a warning (on the Exposure screen).
Several of the DOF modes calculate both a minimum and optimal aperture. The optimal aperture is the concept that print sharpness is really a balance between perceived sharpness and diffraction. There is both a minimal aperture to get the perceived print sharpness you want but there is also a point at which using a smaller aperture starts to degrade the sharpness due to diffraction. The optimal aperture is the aperture which balances these two effects. It is the midpoint where you get the greatest DOF with the least amount of diffraction. You can use either aperture when making your exposure to get the DOF you need, but for optimal print sharpness you sometimes might want to use the optimal aperture vs. the minimum aperture. You can choose which one, minimum or optimal, that will be used to automatically set the Exposure aperture in the ExpoDev's settings.
Note: When measuring subject distances for use with Depth of Field, distances are measured from the film plane to the subject.
When None is selected as the DOF mode, no Depth of Field calculations will be performed. However, you can turn on "Favor DOF" to cause ExpoDev to always adjust exposures to favor DOF when an exposure adjustment is necessary (for instance, when rounding exposures to use a standard shutter speed).
Check mode, is the simplest mode and can be used to simply check the DOF. Given an aperture and the subject distance, ExpoDev will calculate the Total DOF, the Near and Far planes, and the Hyperfocal distance for you.
Distance mode is used to calculate the necessary aperture to ensure that a given subject is within a certain DOF. You enter in both the Near and Far subject distances and ExpoDev will calculate both the minimum and optimal apertures as well as the focal plane and the total DOF.
Focus Spread mode and is probably the most practical one to use as it requires no estimations or measurements of subject distances. Focus Spread mode is a way of easily focusing a view camera by measuring the distance that the focus rail has traveled between focusing on both the nearest and farthest points. If you measure and enter this distance (in mm), ExpoDev can then calculate both the minimum and optimal apertures that will make everything from the nearest point to the farthest point fall within the DOF. This makes it really easy to focus a view camera as it does not require you to stop down the lens at all until you are ready to make an exposure. You do all focusing while the lens is wide-open. The really great thing about this method is that it automatically takes into consideration any view camera movements that you are using. This makes figuring out which movements are improving the DOF as simple as measuring the changes in focus rail travel (i.e. if the distance is getting smaller, the movements are helping). You can find more information about this method here, http://www.largeformatphotography.info/how-to-focus.html in the “Procedure II” section.