Hi all,

Im using a camera-edit LOP to explicitly modify a cameras aperture - but the values seems totally messed up. Regardless if I'm defining H/V aperture or H aperture with aspect ratio - the aperture values seem to be bogus and require numbers in the 1000's.

Using values for example like 36x24 (expected unit of size - mm) completely results in broken focal lengths - and only when I use values closer to a 100 times greater, do I get an fov that seem appropriate.

For example:
If I drop a new node down - the greyed out fields state my camera is set to 20.955 x 15.2908. If I "Set all parameter values from USD primitive" using the "initalise parameters" option - It updates the fields to 3580 x 2013.75. If I enable "Control Aperture" those are the values needed to maintain the orginal fov.

What unit is this in and how do I adjust my filmback size to a known scale.



Followup question. What unit is fstop in, because that also doesn't appear to be a photographic ratio value - as it's producing massively too shallow dof imo for the value i'm specifying?!

There's a scale factor between the units on the camera lop and the units the scene is composed with. If the units are in meters, then the focal length and aperture will also be authored in meters * 0.1. The camera lop makes the conversion for you. However, this causes confusion when editing a camera as it will apply the conversion even though it is reading a raw value.

The raw values on the camera prim in the scene graph details should say something like 0.2 and 0.15 for 20mm and 15mm if it's working correctly.

revilo3d
Followup question. What unit is fstop in, because that also doesn't appear to be a photographic ratio value - as it's producing massively too shallow dof imo for the value i'm specifying?!

F-stop is unitless, however if your image plane is 3000 meters across you're going to have a pretty shallow depth of field.

Edit: was this camera originally created in an old version of Houdini, or another package using an old version of USD? It used to be that the aperture was recorded in mm.

Thanks Jsmack

That makes some sense. In my scene currently the prim value is reading for a horizontal aperture "36", and in the edit camera LOP "3600". Sounds like the prim is capturing the value as mm correctly.

The camera was actually created from Maya with a base scale of decimeters, which prompted me to check my asset scale so I think that explains why the Dof seemed off

revilo3d
Thanks Jsmack

That makes some sense. In my scene currently the prim value is reading for a horizontal aperture "36", and in the edit camera LOP "3600". Sounds like the prim is capturing the value as mm correctly.

The camera was actually created from Maya with a base scale of decimeters, which prompted me to check my asset scale so I think that explains why the Dof seemed off

okay, sounds like a scene scale discrepancy. If maya was decimeters, then the value of 36mm should be recorded as 3.6 on the prim, however if the scale on the layer is set to cm then it would be 36. With Houdini working in meters, a value of 36 on the prim would indeed read in as 3600mm on the edit camera node. It should respect the scale on the layer, but this could become obfuscated easily by different composition arcs.

I just discovered my focal length was also set to 2400 ha!

Setting that explicitly to a 24 (as in mm) my aperture now also works as expected (36mm H Apt).

Totally correlates with the scene scale behaviour you describe. I do think Maya write did not catch the scale correctly it seems it indeed used cm's instead of dm's.

Many thanks!