Questions & answers

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 1.05 kW per square meter, but only 43% of that is visible light. EV is related to the power and the color of the lighting source.

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 one might be fooled by, for example, a sunny snow scene at EV16.

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

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 have found an egregious error in the DoF calculation, or you would argue for the adoption of different formulae, please send us an email.

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

This goes against the generally accepted mantra that a smaller sensor increases DoF range. That is because a smaller sensor calls for a smaller focal length given the same field of view. Reducing the focal length has a quadratic increasing impact on DoF. The next answer explains this relationship further.

The 35mm-equivalent focal length is not focal length; it is the field of view (FoV) expressed in a different unit. For example, an APS-C camera with a 33mm lens has the same “normal” FoV of 54.4º as a full-frame camera with a 50mm lens.

The DoF calculation is always based on the true focal length. The 35mm-equivalent is useful if you prefer to think in terms of FoV instead of absolute focal length, or if your camera displays 35mm-equivalent focal lengths. Some superzooms cannot show true length at all.

If you activate the 35mm-equivalent length, do not forget to configure the sensor size. The focal length dial cannot go below 10mm. This is enough for most formats, but not for 1/2.3-inch and 1/2.7-inch sensor sizes, typically paired with a 3mm or 5mm lens. In these cases, use the 35mm-equivalent focal length to dial in the correct value.

Circle of confusion (CoC) is the smallest sharp circle on a picture. Any feature smaller than the 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.

If you choose d/1500 in Settings, the CoC will be estimated as 1/1500th of the sensor’s diagonal size. The sensor size also needs to be configured correctly.

Experiment to find the value that best matches the DoF you get with your equipment. Although the classic d/1730 is the default, my own equipment, an APS-C DSLR, is closer to d/1300. Some sources say d/1730 is too optimistic even for full-frame cameras.

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. These factors make the circle of confusion bigger, so 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-inch sensor camera; d/1000 would be a better bet.

Testing is needed to determine the best CoC value for your equipment. This article suggests a method for determining the actual resolution of your camera.

Smaller sensors are more affected by diffraction. I have experimented with this, but 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 cameras do not go below f/16, and phone cameras are fixed at f/2.

If you care about diffraction and its detrimental effect on overall sharpness, keep an eye on the aperture slider: higher apertures are tinted brown instead of black. In this redline range, the diffraction circle is bigger than the configured CoC.

Megapixels are a number people can grasp, and there is a close relationship between CoC size and effective megapixels. If the Zeiss formula, d/1730, is a good CoC estimate for a camera, that 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. The final part of this article explains them in more detail.

Note the difference between sensor megapixels and effective megapixels. If the sensor has 100MP but the circle of confusion has a diameter equivalent to four pixels, the effective resolution is only 13.5MP. The extra sensor resolution is not completely useless; it provides better color resolution.

Yes.

It assumes the general exposure set by the phone camera is zone 5, which corresponds to middle gray (sRGB 110).

A pitch-black portion would be −5.5EV and an all-white portion would be +5.5EV. The total EV span is 11 stops, consistent with the 11-zone segmentation of the histogram.

The sRGB encoding is non-linear, and its theoretical dynamic range is 13 stops. In theory, the conversion from sRGB to the zone system is approximately:

sRGB (0–255) → linear (0–1) → ×2¹³ + 1 → log₂

In our tests, the most sensible results came from assuming a dynamic range of 11 stops and using an inverse-gamma mapping instead of a logarithm:

sRGB (0–255) → linear (0–1) → n^1/2.2 → zone (0–1) → ×10