How to use Expositor

There are three scrollable regions, an EV ruler, an ISO & F-Stop roller, and a shutter speed ruler. EV is the exposure value, or brightness of your scene, ISO is the sensitivity of your camera, and F-Stop is your camera's aperture. Dragging either ruler up and down will display additional values, but won't change the exposure being shown. Dragging the ISO & F-Stop roller will display more values and change the exposure, by moving the roller's values relative to both the rulers' values.

The relative position of the ISO and F-Stop values isn't important, think of them as independent. The useful relationships are the pairings of EV with ISO and F-Stop with shutter speed, on the left and right. For any pair on the left side, all the pairs on the right side will produce the same exposure. The reverse is also true. In fact, as long as you don't adjust the ISO & F-Stop roller or set a new exposure compensation, any combination of left and right pairs will result in exactly the same exposure. This lets you easily pick among many settings for the ones that best suit your artistic intent!

The buttons in the exposure compensation panel change the exposure shown by modifying the ISO & F-Stop roller. Use them to compensate for difficult lighting conditions, equipment that consistently exposes incorrectly, or to deliberately underexpose ambient light for flash photography. Try tapping some of the exposure compensation buttons, and notice how the ISO and F-Stop values shift relative to each other. This ensures that the settings shown reflect the adjustment you've selected, by changing the geometry of the calculator like a slide rule.

Click on the reset button (or the exposure compensation value itself on the iPhone) at any time to quickly reset the exposure compensation back to zero.

Simple example, basic settings lookup:

Say you're shooting on a sunny day, and your camera is set to ISO 100. First drag the EV ruler to show 'Bright sun', or EV 15. Next, drag the ISO & F-Stop roller until ISO 100 is lined up next to EV 15. Then, by looking at the pairs on the right hand side, you can see all the combinations of F-Stop and shutter speed that will produce a decent exposure, including F8 at 1/500 second, F16 at 1/125, and F32 at 1/30 second.

Advanced example, a creative approach:

Say you're shooting a waterfall on an overcast day. To get a soft look, you'd like to use a shallow depth of field, F4, and slow the shutter down a bit to blur the water, 1/4 second. Find 1/4 second on the shutter speed ruler, and then drag the roller until F4 is lined up next to it.

Now find 'Heavy overcast day' on the EV ruler. Notice that there's no ISO value listed next to it! That's because the scene is much too bright for your choice of aperture and shutter speed, but all is not lost! You can use the exposure compensation slider to calculate the density of an ND filter you could use to bring the ambient light down to a workable range.

Adjust the exposure compensation until an ISO supported by your camera (for this example, ISO 100) is shown next to 'Heavy overcast day'. This should put the exposure compensation at +6 stops, meaning that at ISO 100, F4, and 1/4 seconds, the scene will be six stops overexposed. Just grab enough filters to block six stops, put them on your camera, and you're ready to shoot a proper exposure with the look you want!

Advanced example, compensating for equipment performance:

Sometimes you may be shooting with a camera that's off-spec and is predictably overexposing or underexposing shots, or with modifiers that change its exposure performace, such as filters and extension tubes. Use the exposure compensation controls to correct for these issues!

If your camera chronically overexposes by 1/3 stop, set the exposure compensation to -1/3 stop. If you know a stack of extension tubes makes your camera underexpose by 2/3 stop, set the value to +2/3 stops. If you put a 3 stop ND filter on your camera, set the value to +3 stops. Just tell Expositor how off your equipment is, and it adjusts the calculations to produce the correct expsosure settings, taking your camera's actual performance into account. Once set, use Expositor as you normally would, there's no need for any extra mental math!