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For UI surfaces that you display on the device but don't directly control, make sure that any views you use reflect the host app’s theme. Two examples are notifications and launcher widgets.

For the rays entering the lens to be essentially parallel the source must be some distance away... that distance is described as being infinity, but it obviously is not. And that distance is greater for lenses of larger diameter; so that all rays leaving the source at a significant angle diverge farther and miss the lens entrance (in reality the rays are not entirely parallel).

The focus distances (subject, and image) are the distances from the lens to the subject, and from the lens to the image plane, respectively.

Focal length is measured from the point where parallel rays from the source begin to converge to a focused point on the image plane. This point is the optical center (H' in the diagrams).

This ties the app's main theme to the system-controlled night mode flags and gives the app a default dark theme when it is enabled.

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For instructions on applying a dark theme to web-based content using a WebView component, see Darken web content in WebView.

If your app uses a dark theme, such as Theme.Material), Force Dark isn't applied. Similarly, if your app's theme inherits from a DayNight theme, Force Dark isn't applied due to the automatic theme switching.

Force Dark can be controlled on specific views with the android:forceDarkAllowed layout attribute or with setForceDarkAllowed().

The focal length of a lens is a measurement made when the lens is imaging a far distant object such as a star. Such an object is said to be at an infinite distance. We focus the image of this target object on a screen. If the lens is a simple symmetrical single piece of glass, we measure from the center of the lens to the focused image. Likely this measurement will be made indoors on an optical bench. The target will be an artificial star. This is a tiny lamp that emits a parallel beam of light.

I came across many articles where the focal length of a camera lens is defined as the distance between the optical centre of the lens and the sensor. One such article is here:

Realistically the difference between subjects at infinity and pretty damn close is insignificant when you consider the sensor plane / subject distance ratios as applied to the lens law equation you quoted.

For launcher widgets, or if your app uses custom notification content views, test the content on both the light and dark themes.

Camera lenses are rarely simple, they are a complex array of lenses conceived to mitigate image marring aberrations. This makes the measuring point difficult to locate. A special optical bench known as a nodal slide is needed. The complex lens has two cardinal measuring points. The focal length is measured from the rear nodal, the object distance is measured from the front nodal.

So, for instance, if an object is quite close to the lens and we change the distance between the lens and the sensor such that the object is focused on the sensor, the said distance does not remain same as f in the lens equation (since u is not large to the extent that 1/u tends to infinity). In that case, the distance between the lens and the film has to be uf/(u-f), which is different from f.

For information on using dark themes in web-based content, see Darken web content in WebView. For an example of dark theme applied to a WebView, see the WebView demo on GitHub .

When the subject you are photographing is reproduced 1:1 on the image sensor (if your lens is able to focus that close; i.e., at that magnification level), then the subject focus distance is 2ƒ, and also the image focus distance is 2ƒ (this comes from setting u = v in the thin lens equation and solving for v).

Apps must opt in to Force Dark by setting android:forceDarkAllowed="true" in the activity's theme. This attribute is set on all of the system- and AndroidX-provided light themes, such as Theme.Material.Light. When you use Force Dark, test your app thoroughly and exclude views as needed.

This difference becomes more significant with very close or Macro Photography. To quote from Effective Aperture and Macro

And for a telephoto lens you extend the diameter of the objective element opening forward until it intercepts the image cone. The focal length is greater than the lens itself.

Use the system-provided notification templates, such as MessagingStyle. This means the system is responsible for applying the correct view styling.

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Avoid using hardcoded colors or icons intended for use under a light theme. Use theme attributes or night-qualified resources instead.

The thin lens equation you mention, relates the subject focus distance u, image focus distance v, and the fixed focal length ƒ. The only time the lens is positioned at an image focus distance v equal to ƒ is when the subject is at infinity. When the subject in focus is closer than infinity, the lens is further from the lens than the distance ƒ.

The locations of the front and rear nodal are difficult to find without proper tools. The lens maker chooses their location based on need. They can be inverted, and they can fall in air forward or rearward of the glass in the lens barrel. These accommodations are made to shorten the barrel or make room for reflex mirror or allow a short focus lens to focus on the film / sensor.

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Fortunately for photographers, assuming a constant effective focal length equal to the defined infinite distance definition approximates well within necessary parameters through a very large range of practical use.

An app can handle the implementation of dark theme by declaring that each Activity can handle the uiMode configuration change:

For a wide angle retrofocus lens you extend the diameter of the front opening (defines the limits of parallel rays) backwards. The focal length is less than the physical length of the lens.

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On API level 31 and above, use UiModeManager#setApplicationNightMode to let the system know what theme your app runs. This lets the system match the theme during the splash screen.

When the app’s theme changes, either through the system setting or AppCompat, it triggers a uiMode configuration change. This means activities are automatically recreated.

Remove any hardcoded colors such as background colors set programmatically to white. Use the ?android:attr/colorBackground theme attribute instead.

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In some cases, you might want an app to handle the configuration change. For example, you might want to delay a configuration change because a video is playing.

But, according to the lens law (1/v + 1/u = 1/f), when an object is at infinite distance (very far away), the distance from the lens where the image of that object forms is the focal length.

We recommend using Material Design Components, since its color theming system, such as the theme attributes ?attr/colorSurface and ?attr/colorOnSurface, provides easy access to suitable colors. You can customize these attributes in your theme.

Force Dark analyzes each view of your light-themed app and applies a dark theme automatically before it is drawn to the screen. You can use a mix of Force Dark and native implementation to cut down on the time needed to implement dark theme.

The terminology becomes fractious when you are talking about the imaging distance of a subject at close range. When the image is in-focus is that the focal length? It's the length required for focus but the definition of focal length is for subject at infinity.

If the object being imaged is closer than infinity, the projected image cast by the lens becomes elongated. This happens because the lens has limited ability to refract (bend) light waves. As the camera approaches the object, its image becomes blurred, now we must adjust focus to retain a sharp image. This is accomplished by increasing the lens to film / image sensor distance. This elongated projection distance changes names from focal length to back focus distance.

Focal length is a fixed property (i.e., constant) of a lens element, lens group, prime lens, or for a given zoom setting of a zoom lens. It has nothing to do, directly, with where the lens is located with respect to the subject, or image/sensor plane of the camera.

When an Activity declares that it handles configuration changes, its onConfigurationChanged() method is called when there is a theme change.

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The confusion comes about from terminology and an omission of rigor that would be distracting for most purposes. When the articles you mentioned, and many like them, say "focal length measures 'the distance, in millimetres, between the optical centre of the lens and the camera’s sensor (or film plane),'" they leave out the implied subject at infinity portion.

Put simply, when working at greater magnifications, roughly 1:2 or more, the displayed aperture on your lens or in your camera will be slightly different from what the true f-stop is, and this number will continue to change as the magnification of your shot increases. This is due to the focal length of the lens beginning to change as focus extension changes; since the lens is physically further away from the sensor or film, there is a change in exposure and f-stop.

Try the Compose way Jetpack Compose is the recommended UI toolkit for Android. Learn how to work with theming in Compose. Color Scheme - Material Design 3 →

Quoting from the first page, focal length measures 'the distance, in millimetres, between the optical centre of the lens and the camera’s sensor (or film plane).'

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Android 10 provides Force Dark, a feature for developers to quickly implement a dark theme without explicitly setting a DayNight theme.