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Old February 6th, 2009, 11:31 PM   #1
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-3dB Gain... Why?

So on another thread Bill Ravens suggested that anything other than 0dB gain would add noise. (I hope I didn't misinterpret the comment) I think the conventional wisdom was that
-3dB was the preferred gain setting. Doug from Vortex, and possibly others have said so.

I noticed that on some picture profiles -3dB was causing me to 'brick wall' about 1/16" from the right side of the histogram, maybe 5%-10% of the total latitude, whereas 0dB allow me to to go all the way up to the right edge.

I did a shoot outdoors the other day purposefully with 0dB and I really liked the results. I felt like I had more latitude (no proof with scopes, just my eye on a good monitor). So until somebody convinces me this is a bad idea, I'm going to continue to shoot at 0dB. Let me know what you think!

PS: I still cannot figure out how to get the black left side of the histogram to go all the way to the left, I still get a brick wall there no matter the settings or the black level or black gamma on my picture profiles. If anybody has an answer to this, please chime in.
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Old February 7th, 2009, 12:37 AM   #2
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Hi Keith,

I don't know that the EX3 histogram display is an accurate way to determine true camera black levels. You may need to try scopes as you mentioned in your post.

-3db 'gain' will offer some noise reduction but decrease dynamic range a little. I do use -3db as often as possible as I find it to give me clean pics with a nice duynamic range.

No reason not to use 0db at all, especially if you are comfortable with it, just as I am with -3db.

If you can set the camera up on +3db, it will no doubt perform admirably as well.

The EX3 is a great camera for the cost, so enjoy the camera at whatever setting you and your clients are happy with.

Best wishes
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Old February 7th, 2009, 01:44 AM   #3
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I've only used it when doing certain slo frame shots. Like 32 frame accumulation. Keeps the blacks more black..
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Old February 7th, 2009, 07:30 AM   #4
 
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Hi Keith...

Perhaps a few words of explanation are in order.
Firstly, manufacturers set the voltage to their sensor block on the basis of noise levels. Traditionally, 0 dB is defined as the point of highest signal to noise. Higher voltage levels result in higher idle current, which produces noise. Lower voltage levels reduce the S/N ratio, so, in effect, the noise becomes more predominant relative to the signal.

The EX1 may be different because of the CMOS sensor block. I really don't know. All I can go by is the generic explanation that gains below zero usually result in more noise. Having said that, I shoot at -3dB on my EX1 with regularity, with no increased noise footprint. However, looking at the EX1 on a scope clearly reveals that the highlights roll off at about 90% of the sensor capability at -3dB. I would conclude, from this, that use of -3dB setting would be restricted to scenes of low illumination and dynamic range, which is quite contrary to when one would select -3dB....usually selected for overly bright illumination, rather than the correct addition of ND filters in this case.

May I suggest that you shoot some test shots in low illumination at various settings. Bring the test shots into your NLE and adjust the exposure levels. Make note of which shots have more shadow noise than others. Also, make note of which shots blow out sooner than others.

I hope this helps.
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Old February 7th, 2009, 10:28 AM   #5
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Originally Posted by Bill Ravens View Post
Hi Keith...

Perhaps a few words of explanation are in order.
Firstly, manufacturers set the voltage to their sensor block on the basis of noise levels. Traditionally, 0 dB is defined as the point of highest signal to noise. Higher voltage levels result in higher idle current, which produces noise. Lower voltage levels reduce the S/N ratio, so, in effect, the noise becomes more predominant relative to the signal.

The EX1 may be different because of the CMOS sensor block. I really don't know. All I can go by is the generic explanation that gains below zero usually result in more noise. Having said that, I shoot at -3dB on my EX1 with regularity, with no increased noise footprint. However, looking at the EX1 on a scope clearly reveals that the highlights roll off at about 90% of the sensor capability at -3dB. I would conclude, from this, that use of -3dB setting would be restricted to scenes of low illumination and dynamic range, which is quite contrary to when one would select -3dB....usually selected for overly bright illumination, rather than the correct addition of ND filters in this case.

May I suggest that you shoot some test shots in low illumination at various settings. Bring the test shots into your NLE and adjust the exposure levels. Make note of which shots have more shadow noise than others. Also, make note of which shots blow out sooner than others.

I hope this helps.

Pardon my ignorance ...but I thought that the purpose of the -3db setting was to give us just one more tool (especially in bright light), to help narrow the focal length. In -3db, you're going to tend to open the iris just a bit more, thus, creating a slightly better/more narrow isolated focus. I thought this was everyone's understanding of the purpose of it? Maybe not...

But if you're saying, more noise & less dynamic range, it may not be worth the trade-off.
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Old February 7th, 2009, 12:07 PM   #6
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Should be less noise but less dynamic range. Look through the VF with high peaking on and you'll see less noise at minus db settings.
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Old February 7th, 2009, 01:22 PM   #7
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Quote:
Originally Posted by Bill Ravens View Post

The EX1 may be different because of the CMOS sensor block. I really don't know. All I can go by is the generic explanation that gains below zero usually result in more noise. Having said that, I shoot at -3dB on my EX1 with regularity, with no increased noise footprint. However, looking at the EX1 on a scope clearly reveals that the highlights roll off at about 90% of the sensor capability at -3dB. I would conclude, from this, that use of -3dB setting would be restricted to scenes of low illumination and dynamic range, which is quite contrary to when one would select -3dB....usually selected for overly bright illumination, rather than the correct addition of ND filters in this case.
Great point.. it is very contrary. You would think that -3 dB would be great to reduce exposure on a bright day. But the fact that the camera rolls off the highlights earlier (90%) totally takes away from any advantages of doing this. In the EX cameras 0 db is the ideal S/N ratio.

The whole -3 db thing seemed to start as a marketing term. Where manufactures could rate their cameras as being faster by marking +3 Db as 0 and 0Db as -3. So -3 in some cameras would be the ideal S/N ratio (the Varicam for instance). Other cameras (includiing CCD base cameras) are like the EX where the -3 db setting is actually intended to help reduce exposure but has a negative effect on the image highlights. This is easy to identify when looking at your video feed on a scope.
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Old February 7th, 2009, 01:49 PM   #8
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Keeping the previous posts in mind then, I think the biggest advantage to using the -3 gain setting would be when trying to achieve a shallow DOF. Having another exposure adjustment at your disposal (such as gain settings) is beneficial to keep the iris open as much as possible.

But it certainly sounds like 0db picture gain is the way to go to get the most exposure latitude for every day use.

- Don
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Old February 9th, 2009, 11:38 PM   #9
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Great article on just this issue. Gain and Negative Gain.

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Old February 10th, 2009, 12:12 AM   #10
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Very good article. I tend to shoot my EX1 on -3db gain most of the time. My lighting is nearly always controlled though, and I tend to like what I get. I also have the time and take the effort to balance the exposure in post, so the loss of dynamic range isn't so crucial to my shooting. But I don't know if I'd do that if I were running and gunning, and needed to turn the footage around quickly.

I also tend to shoot a bit flat. I know others shoot with much more saturation than I do but I generally find that I can do what I need to in post without much fuss and noise.

I guess everyone needs to experiment and find out what they like doing and what works for them.
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Old February 10th, 2009, 10:17 AM   #11
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I thought Alistar's explanation of -3DB gain in this post good enough to link to.

http://www.dvinfo.net/conf/1009697-post8.html

Basically with a CineGamma in which whites can hit 109 the loss of lattitude when using
-3db gain still gives you more than enough for video.
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Old February 10th, 2009, 11:00 AM   #12
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Great stuff Craig, the article pretty much puts it in a nutshell. Just going to change my settings now - have been shooting -6db on the PDW700, but it's a such a quiet camera anyway that if it's going to have a downside then there's no point.
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