Photo Friend is a no-frills exposure/depth-of-field calculator app, for photographers and movie makers that know their way. The app works as a light meter by using the phone camera and the light sensor.
The exposure calculator is easy and intuitive. No typing, no configuration needed. Just slide each gauge to the desired value. The other gauges move to make a good exposure.
The shutter, ISO and aperture gauges move to complement each other. The app remembers the gauges moved recently e.g. if you move aperture, then ISO, then aperture again, the app "learns" that ISO and aperture should be left where you want them, and shutter will move to complete the exposure.
The EV gauge never moves by itself; either you move it directly, or you use the light meters.
You can 'lock' ISO, aperture or shutter by tapping three times in rapid succession. The gauge is not actually locked, it just stays put when other gauges are moved. This is useful for e.g. analog photography. To 'unlock' a gauge, just tap three times again.
The calculator estimates depth-of-field (DoF) too. Just dial the aperture, the focal length and the subject distance.
Version 4.50 introduces an additional dial for the ND (neutral density) filter. If you don't use ND filters, as most photographers don't, you can disable this particular dial in Settings.
There are many "ND number systems", the one adopted by this app is the inverse light transmission e.g. ND64 allows 1/64th of the light to pass, offsetting the exposure by 8 stops. (In case of doubt, just play with the ND dial; its effect on other dials should make it clear.)
Provided your phone has a camera and you give the permission to use it, the screen shows a viewfinder. Point to a subject and touch the viewfinder to take a picture. The EV for the scene is estimated, and the exposure chosen by camera is shown as well.
EV estimation through the camera is roughly equivalent to a reflected-light meter.
Once the picture is successfully taken, three new buttons are made available at the right. From bottom to top: show picture (the default), spot meter, and histogram.
In this mode, you can drag the viewfinder up and down for visual inspection. In this mode, dragging has no further effect on EV; it was already copied to the slider.
In this mode, the picture can be dragged to any direction. Bring the desired spot inside the yellow square, and the EV of this spot is copied to the EV slider.
The message informs how many EVs the spot deviates from the whole-picture exposure. At the example above, the deviation is -3.3EV, meaning the perfect exposure for the shadow area would be 3.3 stops slower.
Note that "perfect exposure" means middle gray or "Zone 5" for a lightmeter. For reference, the outside margins of the draggable area are painted "middle gray", as can be seen in the figure above (thin right margin of the picture).
In this mode, a simple RGB histogram is shown. The graph is divided in eleven zones.
The app can also work as incident-light meter or illumination meter, in phones that have this sensor. Typically the light sensor is in the front face, around the top. Turn the face to the source of light, and touch the "meter" to copy the EV.
Since we are measuring incident light, it is important that the light sensor (i.e. the front face of the phone) is pointed straight to the brightest source of illumination, as if you were taking a picture of the light source using the front camera.
The screenshot button saves a screenshot of the app screen on picture gallery. This is useful for e.g. you take pictures on the field and want to annotate the exposure.
Note: due some internal limitations, some parts of the screen cannot be recorded. The viewfinder image is saved only if it is showing a static picture; the live viewfinder is not. The incident light meter is not recorded as well; this limitation will be resolved in some future release.
You can set the distance unit and DoF parameters. Just touch the gear at bottom left corner.
This section has moved to its own page, with a wealth of technical information.
PhotoFriend for Android uses a modified version of ImageViewTouch, provided under the MIT license by Alessandro Crugnola.
This app does not upload any data, does not share data with other apps, and does not store the pictures taken in the course of light metering.