I think what you are seeing is diffracted stray light from from the pinhole, due to it being directly illuminated by the sun. There may be some small contribution caused by sunlight coming through the pinhole and illuminating a spot inside the camera, but I think it is a secondary effect. The best solution is to use a sunshade, or to aim the camera away from the sun.
This question connects my professional life (optical engineer) to my avocation, so I couldn't resist proving this to myself and sharing it with all of you.
A typical photographic scene has luminance Lscene of about 5 kcd/m^2 - see
https://en.wikipedia.org/wiki/Orders...de_(luminance).
The sun illuminates the pinhole with illuminance Esun of about 100 klux - see
https://en.wikipedia.org/wiki/Lux.
The intensity of light diffracted by the pinhole is (lambda/D) * 1/(pi^3 sin^3(theta) - see
http://proceedings.spiedigitallibrar...ticleid=947646
where lambda = wavelength of light, D = diameter of pinhole, theta = angle of the sun to a point on the image, pi = 3.14159.... The diffracted intensity has units of 1/sr (diffracted luminance/incident illuminance).
The scene illuminance at the film = pi/(4F^2) * Lscene
where F = f/number of the pinhole.
The sun illuminance at the film = Esun * (diffracted intensity) * pi/(4F^2).
The ratio of the two illuminance values (sun to scene) is = Esun * (diffracted intensity) / Lscene.
The wavelength of visible light, mid-spectrum is 0.0005mm. Assuming D = 0.05mm and theta=30 degrees, we put this all together to find the ratio is about 1/19, or about 4 1/3 stops. That is, the diffracted stray light from the sun is about 4 1/3 stops down from the average scene brightness, which corresponds to a background fog.
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