View Full Version : Negative Gain (ie -6db)


Steve Phillipps
September 13th, 2008, 08:02 AM
The PDW700 has a -6db gain setting, and most pro camcorders have a -3. I think I read something about this elsewhere but can't recall, just wondering about the pros and cons of negative gain. Do I remember rightly that you get less noise (obviously) but also less dynamic range? I kind of always assume the less gain the better, so would tend to want to shoot -6db on the PDW700 (as it has such good light-gathering you still get plenty of exposure even at -6), but if it's going to lose out in DR or something else then it's probably not worth it as the 700 is so noise-free anyway.
Steve

Rick L. Allen
September 14th, 2008, 07:01 AM
Steve, I've never heard anything about the lower gain settings creating less dynamic range. Higher gain settings don't create more dynamic range just more noise. My $.02 worth. I generally like to run my cameras at -3 anyway. I really can't wait to get my hands on a 700!

Brian Drysdale
September 14th, 2008, 01:09 PM
Steve, I've never heard anything about the lower gain settings creating less dynamic range. Higher gain settings don't create more dynamic range just more noise. My $.02 worth. I generally like to run my cameras at -3 anyway. I really can't wait to get my hands on a 700!

Not well known, but it does reduce the dynamic range by reducing the headroom available for overexposure.

Understanding gain settings [Archive] - The Digital Video Information Network (http://www.dvinfo.net/conf/archive/index.php?t-113738.html)

However, many people do happily use -3 as a ND and for reduced noise.

Steve Phillipps
September 14th, 2008, 01:58 PM
Thanks Brian, was starting to think I'd imagined it!
It's above my head though, and I'm still no clearer as to whether it's a good idea or not.
Steve

David Heath
September 14th, 2008, 04:54 PM
Steve, I've never heard anything about the lower gain settings creating less dynamic range. Higher gain settings don't create more dynamic range just more noise. My $.02 worth.
Think of a chip as being a device for converting light into an electrical voltge. The more light you put in, the higher the voltage - up to a point. At a certain level the output of the chip will limit, no matter how brighter it gets, the voltage is constant.

That may be (say) 600% larger than the amount of light needed to produce a nominal white at the 0dB setting - but the chip output between 100% and 600% gets processed to put detail into the highlights, and stop them abruptly burning out.

Put negative gain in and you now need (say) 200% to produce the nominal white, hence the headroom necessary for processing highlights is reduced.

Those are the underlying principles anyway, in practice it's dangerous to generalise. Practically, a valuable use for negative gain can be to control depth of field, the NDs may give coarse ranges of iris, which gain settings can fine tune.