# Hemispherical projection vs equisolid projection

I would like to confirm whether the hemispherical projection (`-vth`

) of Radiance in functions like `rpict`

has the following projection equation:

r = f * sin(theta)

, where `theta`

is the zenith angle between the ray and the optical axis, `r`

is the distance of the projection from the optical axis, and `f`

is the focal length?

I have this question because the following document from Radiance-online.org (https://discourse.radiance-online.org...) claims that the hemispherical projection in Radiance is the same as the equisolid projection. However, the equisolid projection has the following projection equation:

r = 2f * sin(theta/2)

, which is obviously not the same as the first projection. Is this document wrong?