Thinlens Sensor: Scene units and aperture radius

[maxfrei750]

I'm trying to export a camera with depth-of-field from Blender to Mitsuba (if everything goes well, then this might even be added to the Mitsuba Blender Add-on).

To get DOF in Mitsuba, I need to use the thinslens sensor, which (among other parameters) expects a focal_length and an aperture_radius. While Blender specifies the focal length (in mm, either directly or via the field of view), the aperture radius is defined via the f-stop property. According to Wikipedia, I can calculate the aperture radius from the focal length and the f-number.

Naturally, in the real-world, the aperture radius will be in units of length. However, the Mitsuba documentation of the aperture_radius states that it has to be specified in scene units:

aperture_radius | float | Denotes the radius of the camera’s aperture in scene units.

What is the best way to tackle this problem?

Thanks!

[njroussel]

Hi @maxfrei750

The focal_length parameter of the thinlens plugin is also only used to compute the diagonal field of view. Here's the relevant source code: https://github.com/mitsuba-renderer/mitsuba3/blob/master/src/render/sensor.cpp#L167-L181
It's given in mm because it's relative to the 35 mm film. In reality, it doesn't really matter, as it's only used to compute the field of view. Note that this also explains why only one of fov and focal_length should be provided.

You can arbitrarily set the scale of your scene relative to scene units (1 mm == 1 scene unit for example). Hence, the aperture radius being defined in scene units. The f-stop measure is a relative measure, you only need the focal length and aperture diameter to be in the same units.

Does this make sense?