[Rendering] Light Decay

https://wiki.blender.jp/Doc:JA/2.6/Manual/Lighting/Lights/Light_Attenuation

[ref-Render] About camera setting

The aperture size is a property of the lens only and does not depend on the crop factor. It does depend, however, on the actual focal length of the lens (not the "equivalent" focal length).

So you need to obtain the actual focal length by dividing by the crop factor.

actual-focal-length = equiv-focal-length / crop-factor

You can then calculate the size (diameter) of the aperture (strictly speaking the size of the entrance pupil, which is the image of the aperture) by dividing the focal length by the f-number (which does not need to be divided by the crop factor).

aperture size = actual-focal-length / F-value
              = (equiv-focal-length / crop-factor) / F-value

So in your case the calculation is:

aperture size = (50mm / 1.5) / 2.8
              = 11.9mm

---------------------
- Lighting VFX master trick, from Chris Godfrey By Mike Seymour March 23, 2017 at FXGuide
https://www.fxguide.com/featured/lighting-vfx-master-trick-from-chris-godfrey/?fbclid=IwAR2E_iOCfPAtS9L9-TJMVOfC3FjPgvlNR12kuLYRcaeROBNtKyDKrk-RqYo

---------------------
- Before VFX / Blockbuster movies without visual effects.

https://beforevfx.tumblr.com/

---------------------
- David Esp Video Technical Blog
http://blog.davidesp.com/archives/category/moviemaking 

---------------------
-camera f-stop chart
https://www.phototraces.com/photography-tips/f-stop-chart/

[ref-Render] Vray Render Element for Nuke

https://docs.chaosgroup.com/display/VRAYNUKE/Beauty