Need Help- F stop equivalency

[Phil Warren]

For digital effects purposes, I need to match exposure- I don't claim to be a DP, but I've figured out the necessary equivalents for focal length and depth of field, but how do I match F-stops? I assume that even after matching the lens equivalency (say, figured out my 12mm DVX100 lens = 80mm 35mm cine lens), the F-stop will not be proportionate- IE knowing depth of field and doing the math for the equivalent lens will not yield the equivalent F-stop. Help? I fear I've thought myself in circles and now I'm retarded...

[Charles Papert]

Actually Phil, the conversion factor to cine should be around 4.4x, netting you an equivalent 53mm to your 12mm. Exposure is exposure; a f2.8 is the same amount of light delivery regardless of the format of the lens (barring certain factors such as light loss due to adaptors etc). And depth of field also remains constant. The big variable is field-of-view; your 12mm focal length on DV delivers a 53mm-equivalent image in the cine format while a 12mm cine is obviously much wider. However, regardless of that factor, the 12mm cine lens will have the same depth of field as the 12mm DV lens, and will be similarly affected by exposure.

So in a nutshell: f-stops, focal lengths and the resulting depth of field are absolute; field-of-view is relative, based on the format.

[Phil Warren]

Wow, that was exactly what I was looking for. Thank you very much!

[Charles Papert]

We aim to please at DVInfo.net! And welcome!