Canon Patent Application: Celestial AF "Moon shooting mode"

For those that commonly take photos of the moon or stars, Canon is aware of the difficulties that you may have getting precise focus, especially of the brighter objects in the sky such as the moon.  Problems arise when for instance, the lens isn't calibrated for infinity and since celestial objects are at infinity for the sake of taking pictures of them, small variations in focus can occur.  This could even happen on a Canon mirrorless camera with Dual Pixel auto focus because the nature of the focus is phase detection AF, and not contrast AF.

Canon proposes in this patent application to switch to contrast AF instead of using phase detect AF.  In Figure 2, they show dual pixel auto focus, showing that this is most likely a mirrorless or powershot camera application.

In recent years, the number of pixels in image capturing apparatuses such as cameras and videos has increased, a slight defocus state of an image has become conspicuous and, more precise focusing is desired. This is the same in shooting an image of celestial bodies (stars, moon, and so forth) in the night sky. 

Focusing on celestial bodies is performed by calculating a focus position at which an area represented by high brightness signals is strictly minimized when regarding each celestial body as a point light source. As stars and the moon which are subjects in astrophotography at night are located substantially at infinity and there are specific exposure settings for astrophotography, there is an independent shooting mode for astrophotography different from other scene modes. Hereinafter, a mode for shooting the moon as the main subject is referred to as a "moon shooting mode". 

Normally, the focus position at which an object located substantially at infinity is in focus is uniquely determined by performing infinite focus adjustment in individual image capturing apparatus. However, due to a difference between temperature at a time of the infinite focus adjustment and temperature of the image capturing apparatus at a time of actually shooting a celestial body, a difference in posture, and so forth, the focus may shift during shooting. For this reason, it is necessary to often adjust focus even during shooting a celestial body whose distance from the image capturing apparatus does not substantially change during shooting. 

US Patent Application 20490158760

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