Understanding gain settings

[Craig Irving]

I can't confirm this, because I haven't read the instruction manual for the HVR-Z7U, but Digital Content Producer says that the gain settings on the Z7U start from -6db up to +21db. I noticed also that the EX1 has a -3db option too.

Coming from an HVR-V1U where gain is not measured below 0db, can someone explain having a negative-integer gain value? I figured you were either adding gain...or not. But taking away gain?

I've never shot on a "Professional" camcorder, I've only used "Prosumer" equipment...so this is an aspect I've never ran into.

I suppose it's safe to assume that (as long as you have plenty of light) the -6db gain setting will have a sharper PQ over the 0db gain setting? Or is it just an easier way to change your exposure without changing the aperture value on the camera, or engaging the ND filters.

[David Heath]

Originally Posted by Craig Irving

....... can someone explain having a negative-integer gain value? I figured you were either adding gain...or not. But taking away gain?

I think you've really got to ask the question just what is meant by the 0dB setting, and that's not such an obvious question as it may at first seem, especially nowadays. It's tied in to the oft asked question of what ASA rating a paticular video camera is - and in practice the answer could be "whatever you set it to".

Seem silly? What's the ASA rating of a Canon D20 DSLR? It's whatever I dial in via the menu - 100, 200, 400, 800 or 1600ASA, and it's no more correct to say that the D20s sensitivity is 100ASA than 200, 400 or whatever. In practice the higher limit will be defined by the max acceptable level of noise, and the lower limit by the max signal that can physically be got off the chip. Add more light to the sensor and a bigger signal comes off - up to a point where a limit is reached. Set the gain for that to be peak white and that could represent the absolute minimum ISO setting for that camera.

In practice that would never be done, as it's desirable to have headroom to allow for highlight handling and to have a "knee" characteristic etc

[Ron Evans]

That's why comparing different cameras from different manufactures at 0 db gain is pointless. 0 is what the camera designers decide is 0. To me the better indicator is how dark a scene can the camera shoot( with whatever gain settings) before grain/noise is unacceptable. On some cameras that may actual be 0 !!! on others may be 12db!! An example is my view of DVX100 is getting close to unacceptable at 6db and my FX1 at 12db. In the end the scene is much the same but settings very different by the numbers.
As far as negative gain goes it could mean that one could keep the lens in its sweet spot longer.
Ron Evans

[David Heath]

Originally Posted by Ron Evans

That's why comparing different cameras from different manufactures at 0 db gain is pointless. 0 is what the camera designers decide is 0. To me the better indicator is how dark a scene can the camera shoot( with whatever gain settings) before grain/noise is unacceptable.

Exactly right. Sony could have marketed this camera with the -6dB to +21dB settings simply relabelled as 0dB to +27dB. At which point, if you define sensitivity as ASA rating at 0dB, it would immediately become a stop less sensitive !!

Maybe they've missed a marketing trick - they could have numbered the same 27dB range -12dB to +15dB and as if by magic make it appear to have double the current sensitivity! :-)

[Eric Stemen]

from my understanding when the rating goes above 0db is when noise is becoming noticeable.

I think manufactures should really go to ISO ratings.

[David Heath]

Originally Posted by Eric Stemen

from my understanding when the rating goes above 0db is when noise is becoming noticeable.

That's not my experience. Some cameras are still acceptable with 12dB switched in, others certainly aren't.

I think manufactures should really go to ISO ratings.

In principle I agree, but a difference between video and still cameras is the ability to alter the camera line-up to create a "look". A consequence is that for a given gain setting, the ISO figure will vary a little dependent on camera line-up.

[Ian Campbell]

The negative gain setting will be great at keeping the shutter, for example, at 1/60th on outdoor shoots in sun. I wish the V1U had this feature. Even with the ND filters on my V1U's engaged it was hard to not overexpose. The negaitve gain setting will also help achive a shallow depth of field when used to its advange. This feature on the Z7 should allow shooters to get better footage in several different ways.

[Matt Davis]

Originally Posted by Eric Stemen

from my understanding when the rating goes above 0db is when noise is becoming noticeable.

IIRC (going back 23 years, egad!) we were taught that the camera has amplification circuits for situations not considered 'normal', and when these were engaged, one could select how much amplification: +3 dB, etc. Adding gain would introduce noise, so the 0dB setting would be the base noise level - the noise 'floor', from which 'you can't get less noise'.

So negative gain would be seen as a form of attenuation, performing the same duties as an ND filter.

[Eric Stemen]

Pretty much like a DSLR when you can select a different ISO on it.

[David Heath]

Going back 23 years (and I can go back further, but we won't talk about that.....!) I'd suspect we're talking tubes - the first pro solid state camera I saw must have been 1987. (And nobody could believe the lack of registration errors.) Years ago, the problem was simply getting an acceptable noise level at all, and gain was seen very much as an "emergency" measure - noise at 0dB could be bad enough.

In which case, it made sense to use as much as possible of the dynamic range of the sensor as possible. See at what point it would limit - no more output no matter how much the incident light was increased, and make "peak white" and 0.7v correspond to somewhat below that. In which case, negative gain could never work - you'd never be able to get a signal off the sensor big enough to now make 0.7v and peak white. A situation which using an ND wouldn't give.

Roll forward, and S/N figures of the sensors improved, and now it became acceptable for peak white to correspond to a sensor output much lower than it's maximum, and now that headroom could be used to improve highlight handling, enable the use of "knee" characteristics, and improved flare compensation. Negative gain now becomes possible, but at the expense of highlight handling - it reduces the headroom.

It's hard to generalise, but now matters have improved to the extent where the limiting factor for camera noise can be the quantisation level of the codec, rather than the front end, at least for 8 bit systems. Hence the RECORDED noise level for the lowest few gain settings may quite conceivably be independent of that gain up to a certain point.

So how do you now label the scale? Good question. The suggestion earlier on about gaining a marketing advantage by going from -12 to +15 (and doubling the measured ISO rating at 0dB) was intended tongue in cheek, but.......? It should at least make users careful about drawing too hard conclusions from "headline" numbers.

Adam Wilts review of the EX measures that cameras ISO rating to be 400 in 1080p mode, and 800 in 1080i, in both cases at the 0dB setting of the gain switch. Going back to the earlier comments about digital still cameras, that illustrates (unfortunately) the problems of adopting a similar scheme for video cameras - should that switch setting be labelled "400", "800", or "400 in 1080p, 800 in 1080i"? And that's before other line up aspects are even thought about.

[Andy Nickless]

Thanks for all the info in this thread.
Now at last I think I'm beginning to understand Gain . . .
(or am I)?

[Barry Wilkinson]

A thing that I dont think has been mentioned in this thread is that if you dont have negative gain when the light is too much you have to put in a filter so that the chips can cope. This filter is made of glass and any extra glass between lens and sensor will deteriorate very slightly the image quality. Any passage of light through glass impairs image quality, the more glass the worse it gets. This is one reason the movie industry sometimes uses "prime lenses" ie not zooms, as they are made up of fewer pieces of glass than a zoom and have better image quality.
Modern glass coatings minimise this effect but as Scottie on Star Trek would say..." you cant change the laws of physics" in a scottish accent of course!

[David Heath]

Originally Posted by Barry Wilkinson

......... if you dont have negative gain when the light is too much you have to put in a filter so that the chips can cope. This filter is made of glass and any extra glass between lens and sensor will deteriorate very slightly the image quality.

Generally, the filter wheels in a typical camera will have a tungsten-daylight filter, and a couple of NDs, (which are generally correction as well), and a clear setting. But the latter isn't just an open hole, it's a clear piece of glass, so optically has the same impairment quality as you mention.

It has to be done this way to keep the optical path length constant no matter which filter is in position, if Filter 1 did correspond to an open hole instead of clear glass, it would need a big lens retrack every time it was selected/deselected. For the same reason, all the filters have to have the same thickness.

Obviously this doesn't apply to filters placed in front of the lens, just those in the filter wheel in the camera body.

[Andy Nickless]

Originally Posted by David Heath

But the latter isn't just an open hole, it's a clear piece of glass, so optically has the same impairment quality as you mention.

It has to be done this way to keep the optical path length constant no matter which filter is in position.

Thanks David, that's cleared up yet another mystery that's been puzzling me. As an ex Pro Stills Photographer, I've often wondered about this.

[Rick L. Allen]

Originally Posted by Craig Irving

I suppose it's safe to assume that (as long as you have plenty of light) the -6db gain setting will have a sharper PQ over the 0db gain setting? Or is it just an easier way to change your exposure without changing the aperture value on the camera, or engaging the ND filters.

Craig, Simple answer is that negative gain settings (-3 db) etc. are a simple way to add the equivalent of a neutral density filter. Most consumer cameras tend to shoot a little hot so using -3 db helps saturate colors and allows plenty of room for the iris to open open up without noise.

FYI - 6 db is equal to 1 F-stop.