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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, or the light sensor when available.

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

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 meter.

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, but it stays put when other gauges are moved. This is useful for e.g. analog films where ISO is not easily changed, so it makes sense to keep ISO locked for the entire roll. To 'unlock' a gauge, just tap three times again.

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

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 the phone's camera is shown as well.

EV estimation through the camera is roughly equivalent to a reflected-light meter.

Figure 4: Screenshot of Photo Friend for Android

In Android, if your phone has a light sensor, the app can also work as an incident-light meter or illumination meter. Typically the light sensor is in the front face. Turn the face of the phone to the source of light, and touch the viewfinder to copy the EV to the calculator gauge.

Since it is an incident-light measurement, it is important the light sensor is directly pointed to the brightest source of illumination for the scene.

Figure 5: Photo Friend for Android - incident light sensor

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

Figure 6: Screenshot FIXME detalhe Dof

You can set the distance unit, as well as some DoF parameters, at Settings. Just touch the gear at bottom left corner.

Figure 7: Screenshot of Photo Friend Settings - Android
Figure 8: Screenshot of Photo Friend Settings - iOS

Questions and Answers

Q. What EV or Exposure Value stands for?

A. Exposure value is an absolute unit of measurement for light, suitable for photography. It is also known as LV or EV100. An EV of 15 corresponds to a 'perfect' shiny day. The Sun delivers 1.05kW per square meter, but only 43% of that is visible light. EV is related to the power and the color of the lighting source.


Q. What is better for light metering: the camera or the light sensor?

A. It depends a lot on the phone, namely the quality of the components. For scenes with illumination within the ranges of both sensors, EV estimation should be very similar for reflected light and incident light (give or take one stop). It is possible that one sensor is more capable than the other at very low light (below EV2); try them out to see which one goes further down. Light sensors may also have a maximum reading of EV14, so it might be fooled by e.g. a sunny snow scene (EV16).


Q. How is depth-of-field (DoF) estimated?

A. The app uses the classic "exact" formula that can be found in many places, including Wikipedia.


Q. Why the DoF values in your app are not equal to this XYZ DoF table that I have here?

A. Every DoF calculator and DoF table may use a different formula. Some use "approximation" formulas that are simplified versions of the "exact" formula. Some take diffraction into consideration; this app does not. If you think you've found an egregious error in DoF calculation, or you'd argue for the adoption of different formulae, please send an e-mail or post a comment on this page.


Q. How the sensor size configured at Settings can affect DoF calculation?

A. In theory, a smaller sensor crams more megapixels in a smaller space, so the circle of confusion becomes smaller, and the DoF range is actually decreased when a smaller sensor is used.

I know, I know, this goes against the generally-accepted mantra that smaller sensor increases DoF range. This is because a smaller sensor asks for a smaller focal length given the same field-of-view. Reducing the focal length has a quadratic increasing impact on DoF. Also, read the next question as well.


Q. What is the "circle of confusion" in Settings? Why d/number?

A. Circle of confusion (CoC) is the smallest sharp circle on a picture. Any feature smaller than CoC will be blurred, even with perfect focus. Since even perfectly focused objects are slightly blurred due to CoC, objects slightly out-of-focus look as sharp as perfectly focused ones. This creates the range of depth-of-field. A perfect optical system would have a CoC of zero and no DoF range.

The CoC is commonly estimated by dividing the diagonal sensor size (d) by an arbitrary number. The well-known "Zeiss formula" is d/1730. Other typical values for APS-C and full-frame sensors are d/1000, d/1300 and d/1500.

So, if you choose d/1500 in Settings, the CoC will be estimated as 1/1500th of the sensor's diagonal size. Of course, you need to choose the sensor size correctly as well.


Q. Which divider should I choose for the circle of confusion?

A. You should experiment to find the value that best matches the DoF you get with your equipment. Even though I adopted the classic d/1730 as default, my own equipment (APS-C DSLR) is more like d/1300, and some sources say that d/1730 is too optimistic even for full-frame.


Q. How do sensor size and circle of confusion relate?

A. Generally speaking, smaller sensors have more trouble producing sharp images. Optical limitations of the lens become relatively more important, there is more noise, and so on. All these factors make the circle of confusion bigger, therefore the divider should be smaller. For example, d/1730 might be reasonable for a full-frame DSLR, but it is too optimistic for a 1" sensor camera; d/1000 would be a better bet.

Of course, testing is needed to determine the best CoC value for your equipment. This article of mine suggests a method to determine the actual resolution of your camera.


Q. Why don't you take diffraction into account for DoF calculation? Smaller sensors are more affected by diffraction.

A. I have experimented with this. Diffraction only overtakes CoC at very high apertures. I feel that manufacturers already avoid bad combinations of sensors and apertures. For example, new prime lenses for full-frame don't go below f/16, and phone cameras are fixed at f/2.


Q. Why do you mention effective megapixels (MP) along with the CoC dividers?

A. Because MP is a number that people can grasp. There is a close relationship between CoC size and effective megapixels.

For example, if the Zeiss formula (d/1730) is a good CoC estimate for my camera, it means that my camera can resolve 1730 distinct "pixels" diagonally. Using the Pythagorean theorem, we can convert this to vertical and horizontal pixel counts, and then find the total megapixels. There are additional considerations; in case you are curious, check the final part of this article.

Note the difference betweeen sensor megapixels and effective megapixels. If the sensor has 100MP but the circle of confusion has a diameter equivalent to 4 pixels, the effective resolution is just 13.5MP. (The extra sensor resolution is not completely useless; it gives us better color resolution.)

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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