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Photo Friend: exposure calculator & light meter

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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.

Get it on Google Play

Get it on App Store

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.

Figure 1: Screenshot of Photo Friend for Android

As a calculator

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 use the light meters.

Figure 2: Screenshot of Photo Friend for iOS

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.

Figure 3: Screenshot of Photo Friend for iOS - locked dial

The calculator estimates depth-of-field (DoF) too. Just dial the aperture, the focal length and the subject distance. Note that "hyp" stands for "hyperfocal distance".

Figure 4: Screenshot of depth-of-field slider

Reflected light meter

Provided your phone has a camera and you give the permission to use it, the screen shows a small 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.

Figure 5: Picture taken. Note the three new buttons at the right

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.

Show picture mode

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.

Figure 6: The picture can be dragged up and down in viewfinder.

Spot metering mode

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.

Figure 7: Drag the picture until the desired spot is within the yellow square.

The message informs how many EVs the spot deviates from the whole-picture exposure. In the example above, the deviation is +3.3EV, meaning the perfect exposure for the white curtain would be 3.3 stops faster.

(Note that, for a light meter, "perfect" means middle gray or "zone 5". Subjectively, the whole-picture exposure that overexposed the bright curtain is probably right because we normally want white objects to look white, not gray.)

For reference, the outside margins of the draggable area are painted "middle gray", as can be seen in the figure above (left margin of the picture).

Histogram mode

In this mode, a simple RGB histogram is shown. The graph is divided in eleven zones.

Figure 8: RGB histogram with eleven zones (0 to X)

Incident light meter (Android only)

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.

Figure 9: Photo Friend for Android - incident light sensor

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.


You can set the distance unit and DoF parameters. Just touch the gear at bottom left corner.

Figure 10: Screenshot of Photo Friend Settings - Android
Figure 11: Screenshot of Photo Friend Settings - iOS

Questions and Answers

This section has moved to its own page, with a wealth of technical information.

Third-party licenses

PhotoFriend for Android uses a modified version of ImageViewTouch, provided under the MIT license by Alessandro Crugnola.

Privacy policy

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.

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