Shoot without a lens
A pinhole camera is a light-tight box with a hole instead of glass. There is nothing to focus, so every point in the scene projects as a small disc rather than a point, and the whole frame carries the same softness. Depth of field is effectively infinite and resolution is low everywhere.
The blur here runs across the entire image before any shading, which is what separates the result from a defocus filter. Nothing stays sharp.
Light falloff and image circle
Illumination drops off toward the corners for two reasons: the path from the aperture is longer, and the light strikes the film at an increasing angle. The combined effect follows roughly a cos⁴ law, far steeper than a lens vignette.
Light falloff sets the depth of that shading. Image circle sets where it reaches full strength, measured against half the image diagonal. At 82% the corners are dark well before the frame edge, which is the look of a small aperture projecting onto a format it does not quite cover. Push it past 100% for even coverage with darkening only at the extreme corners.
Optical softness
The projected disc grows with aperture size and with the distance from hole to film. A 0.3mm hole at 50mm gives noticeably crisper results than a 0.5mm hole at the same distance.
Values here between 0.6% and 1.5% read as a pinhole. Above 3% the image stops being a photograph and becomes a shape study, which some subjects can carry and most cannot.
Grain and shadow fade
Grain simulates the noise of a multi-second exposure. Shadow fade raises the black point, matching how flare inside a home-made box camera and the base fog of an old sheet of film both prevent true black.
Both are additive on top of the falloff, so heavy falloff plus heavy fade produces grey corners rather than black ones.
Example: architecture at midday
Start at 78% falloff, 82% image circle, 1.1% softness, 30% grain, 22% fade. Strong sun gives the deep tonal range the falloff needs. If the corners crush to solid black, raise image circle toward 100% or drop falloff to around 60%. Subjects with large flat areas of sky show grain most, so lower it there before touching anything else.