Faking Reflections on set.

It’s all smoke and mirrors.

This came up recently because there was no glass in the window of the set. All the reflections were to be added in post from photos…. but when the actor stood at the window the Director wanted to see his reflection.

This is a simple trick for a locked off shot. The action is filmed with two cameras one for the main image and a 2nd camera for the refection.

You can find the position of the main camera simply by reflecting the main cameras position in the mirror plain.

  • Both cameras are equidistant from the mirror and targeted at the same point on the mirror plain.
  • A line connecting the nodal point ( focal center ) of each camera is perpendicular to the mirror ( at 90° )
  • both cameras should have the same FOV.



Max Tip: Check for scale transforms on your cameras

If a Camera or it’s parent node has a scale transform it will effect the way SSS looks in Mental Ray.


It’s common practice to rescale the root node to place camera motion from tracking software like pfTrack.

BUT … This will give you inconsistent lighting result with materials that use ‘real world’ scale.

til I figured this this out … I Lost a few hours today scratching my head. ..

Converting between Gain and f-Stops

So cameras work in Stops and composite packages use Gain. To convert between the two we need to do a bit of log maths. Heres how.

To convert from Stops (f) to Gain

Gain = 2f

To convert from Gain to stops (f)

f = logGain

This is equivalent to:

f = ( log10 Gain ) / ( log10 2 )

If you want to see why then we’ll need to look a little deeper at log maths
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Camera Data

It’s worth checking out arenafilm who have a detailed database of filmbacks and lens combinations for various cameras.

Also dpreview have a review of sensor sizes for DSLRs.

I’ll try and keep a useful summary here.

Company Model Film Back aspect Pixels
Arri Alexa 26.136 x 14.702 mm / 1.0290 x 0.5788” (Monitoring/EVF) 16:9 3168 x 1782
Arri Alexa 23.760 x 13.365 mm / 0.9354 x 0.5262” (ProRes) 16:9 1920 x 1080
Arri Alexa 23.760 x 13.365 mm / 0.9354 x 0.5262” (RAW) 16:9 2880 x 1620
Red Epic 5120 x 2700