Blackmagic Forum

Hey all,
Finally got around to shooting a dumbed down version of an exposure test for the blackmagic cinema camera. (Lighting grey card to t stop on lens then adding 1,2,3,4,5,6 stops more light and less light) to see where the camera holds exposure and where it hits the noise floor/clip.

Anyways, I noticed that once brought into resolve and applied the BMCC FILM setting in the raw tab, then added the BMCC to REC 709 middle grey went from being at 50IRE to around 30IRE. To get it back to middle I had to add exactly 1.5 stops of gain in the RAW tab. HOWEVER if I did not apply the BMCC FILM setting and let it add the regular REC 709 the image was exposed properly, came out sharper and had more contrast.

Soooo I know that when shooting in LOG the waveform reads middle grey at about 30IRE so is it just not applying the gain setting in the REC 709? Or is this camera Really rated at 320 ASA?!!?.......which would mean around 6 1/2 to 7 stops overexposure and 5 stops underexposure......

I found the same thing in my tests, and posted about it here: http://forum.blackmagicdesign.com/viewtopic.php?f=2&t=11862

Although I pinned it more at 400asa (my concern was to get flesh tones to 50%, not an 18% grey card) but obviously from the tests you have done and those I have done a fair amount of overexposure is ideal with this camera- which to me is a refreshing change from most video cameras where you are almost sacrificing skin tones to protect highlights with underexposure.

To me it is a much better tradeoff. If you need it, you can expose for 800 if that's all the light you got. Most of those shots will be high contrast/low-key anyway. Consider a nighttime shot where your key will be under-exposed in the final grade anyway. The goal there, as it has always been, is to get a few points of reference at or above key to sell the darkness (a backlight, a hot point in the BG, etc) So your not really giving up too much if you find yourself in a situation where you need the extra exposure.

But if your end goal is a normally exposed image in low-light for 709 of about 5-6 stops (or if your getting heavy on contrast or grade, maybe 8-10), you can expose for 800 and find yourself not pushing values up in the grade too much. Maybe expanding a bit in the upper end, but still acceptable. If you have the light or the stop, give it all you got. A fast lens isn't the same tradeoff it would be in s35-DOF at a 1.4 it is still within the range of a good AC, compared to a 2.2 or 2.8 in S35. But it all depends on the type of shots you typically work with. If your run and gun, stuck with slower lenses and working in natural light, your options are what they are.

Just don't get too hung up on demanding a 1-1.5 stop overexposure-if your pushing an 800 iso shot to final grade, yes you'll have to expand the tones, but your going from 12 bit to at best 10 bit, so quantization isn't an issue, the only tradeoff is the noise floor will be the native chips capabilities. Overexposure would only reduce that slightly, and allow you to bite into the shadows a bit more than you otherwise could.

Why would you add the BMCC REC.709 LUT to the the RAW data processed with BMC Film Color Space?

If you are decoding the RAW and than Rec.709 just select that in the Camera Raw Color Space.

Thanks mike! Very interesting. Adam i've seen a few posts where they recommend doing that. So if I didn't want to just spit out a rec 709 file would I just set it to bmcc color space and grade that, no lut?

Just noticed that it only changes the exposure like that in resolve 10... resolve 9 maintains 800asa exposure