This should be set to
M for htoa (when assuming 1 houdini unit is 1 meter),
CM for mtoa & equal to the global export scale in c4dtoa.
Default is the width of a full frame sensor
[36 mm]. Adjustments to the sensor size result in different effective focal lengths of the lens.
Distance between the lens and the point at which the light converges in millimeters
The f-stop is a dimensionless number that is a quantitative measure of lens speed. Practically, it is the ratio of the distance from the lens to the point where the focused light converges (the focal length) over the diameter of the aperture.
Distance between the lens and the focal plane in centimeter.
Sometimes referred to as the cat-eye effect, this is a lens imperfection that is not caused by the glass but by the outer edge of the lens housing, also known as the exit aperture. When the main aperture is fully open, light coming into the camera from steep angles will be blocked partially by the edge of the lens. This creates cat-eye like bokeh shapes near the edges of an image. It should be noted that this lens effect is purely mechanical and can not be confused with lens distortion.
The param: optical_vignetting_distance is the distance from the main aperture to the end of the lens housing. The param: optical_vignetting_radius is the radius of the lens housing.
When using a param: optical_vignetting_radius != 1.0, you get assymetrical optical vignetting.
Each time a ray is vignetted (stopped by the lens housing), a new one gets created with a different direction. Then this ray is tried. The amount of such allowed tries is the parameter param: vignetting_retries.
Due to the spherical nature of lenses, light cannot be focused onto exactly a singular point.
Emperical distortion aberration. Note that lenses with a short focal length will generally exhibit greater amounts of distortion. We chose to not automatically model this, but rather keep it up to the user for greater artistic control.
Emperical coma aberration. There may be slight discrepancies between forward and bidirectional results.
Amount of aperture blades. Lower than 3 means perfectly circular.
Sometimes for artistic purposes you might want to make the bokeh a little more square. This can sometimes happen with anamorphic lenses.
This scales the bokeh along the x-axis.
This option replaces the circular/polygonal aperture with a texture-based one. Sometimes you might want to use a bokeh kernel that looks a little bit more organic, such as e.g:
Most image formats are supported.
The minimum luminance a sample needs to have to trigger bidirectional redistribution. The luminance is equal to
r*0.21 + g*0.71 + b*0.072. The default of
1.0 is fine in most cases, although you might want to increase it when dealing with larger, high energy diffuse surfaces which won't contribute a whole lot to the bokeh.
A multiplier on the bidirectional sample count. The actual sample count is adaptive over the image (modulated by screenspace area of the bokeh, etc).
Sometimes for artistic reasons, you might want to boost the intensity of the bokeh (all pixels with energy above param: bidir_min_luminance) a bit.
When boosting the bokeh, a transition zone is needed or you'll get annoying flickering. This transition zone is measured in linear energy.
See: Optical Vignetting
Enabling this triples the camera computations, but provides the shaders with high quality derivatives for use in texture filtering, etc. This should be enabled unless you specifically know what you're doing.