Without knowing the technical "innards", the gain setting on cameras increases the brightness to help cope with low light situations. and increasing the gain can introduce video noise, depending on how well the camera's firmware is designed.

On my EX1 there are 3 gain settings, which I can set up to suit the circumstances. I generally have them set to -3db, 0db and +6db and mostly I use -3db.to minimise noise.

Thank you Peter. Any idea why it's expressed in "db" which rather sounds like decibels?

[I]Thank you Peter. Any idea why it's expressed in "db" which rather sounds like decibels?[/I]

A decibel, while normally associated with audio, is perfectly valid for video as well. Every 3 db of gain doubles the amplitude of the video signal, but can also increase the noise level of the signal as well. It is therefore designed to be used sparingly.

It probably harks back to Sony's pedigree in producing broadcast cameras. The gain control has been a feature of broadcast cameras going back decades, traditionally offering +3db or +6db of gain to obtain acceptable pictures in very low light conditions. However you should remember it does not actually increase the sensitivity of the camera, it merely increase the video amplitude.

Geoff

"Every 3 db of gain doubles the amplitude of the video signal"

No, 3dB is double the power ratio (10 x log 2). Double the amplitude is 6 dB (20 x log 2)

The "dB" is decibels and decibels are used for any signal strength not just audio.
It makes perfect sense to use dB because the eye's sensitivity to light is similar to the ear's sensitivity to sound i.e. logarithmic.

Bob.

[I]"No, 3dB is double the power ratio (10 x log 2). Double the amplitude is 6 dB (20 x log 2)"[/I]

True except in some cases and very confusingly it refers to the voltage ratio because the power is irrelevant and meaningless, so 3dB of gain in a video camera does produce double the signal voltage which is about 1 stop.

http://en.wikipedia.org/wiki/Decibel#Voltage

[I]In connection with video and digital image sensors, decibels generally represent ratios of video voltages or digitized light levels, using 20 log of the ratio, even when the represented optical power is directly proportional to the voltage or level, not to its square, as in a CCD imager where response voltage is linear in intensity.[38] Thus, a camera signal-to-noise ratio or dynamic range of 40 dB represents a power ratio of 100:1 between signal power and noise power, not 10,000:1.[39] Sometimes the 20 log ratio definition is applied to electron counts or photon counts directly, which are proportional to intensity without the need to consider whether the voltage response is linear.[40]"[/I]

Bob.

Doesn't it work similar to the ISO setting on still cameras? It's just a different means of measurement.

@Kimberly
Your camera should have an "AGC Limit" setting. That is a very important setting I usually don't leave it in OFF. I would make some test shots at low light with the gain set to manual and see how much you can turn up the gain before the noise becomes unacceptable and then set the AGC Limit to that value.

I didn't wish to know that just before going to bed!

What right has the optical world got to bastardise a term coined for the telephone world? :)

(The decibel (dB) is one tenth of a bel (B) named in honour of the telephone pioneer, Alexander Graham Bell).

It reminds me of when audio power amplifier manufacturers coined the term RMS watts, to describe the average power the amp could deliver for a sine wave.

By the way, in what way is the ear logarithmic? A doubling in loudness corresponds to about 10 dB increase in sound pressure if well above threshold but not too loud, according to some experts.

Decibels were not invented to mimic loudness but to simplify calculations of a chain of lossy telephone lines, attenuator pads and amplifiers. The arithmetic involved sums and differences rather than multiplies and divides. Technicians could do it in their heads rather than have to fiddle with a slide rule.

Now I am going to bed.

And do NOT get me started on Kelvins and White Balance!

Grazie

Anyway, all you need to know is that 6db is the equivalent of opening the lens by one more stop. If your camcorder is in Auto mode it adjusts the iris and shutter speed by itself in order to get a proper video signal. As you dive deeper and the light gets weaker, you can goose the video gain. My FX-1 camera has three settings: 6, 12, and 18db, so the last one is equivalent to being able to open up the lens (or slow down the shutter speed) by three stops. But, as mentioned, the background noise is also amplified. At 18db the video noise on a dark background can easily be seen. Not terrible, but it's there.

Thanks everyone, that makes more sense. Yes, my camera does have the AGC button. I have never fussed with it much because I never understood what it does. Will play around with it on next week's dives.

Regards,

Kimberly

OldSmoke said, "I would make some test shots at low light with the gain set to manual and see how much you can turn up the gain before the noise becomes unacceptable and then set the AGC Limit to that value."

The maximum setting will depend on shutter speed along with if you are shooting in progressive or interlaced.

There are those times when light is extremely low and you would need an extra stop of light by setting the shutter speed to 30 and have the absolute max gain you can have without too much noise. Also, you might want to use 60i instead of 60p to effectively get another stop by reducing the noise level. That will of course reduce the resolution.

So you need certain settings for progressive and certain settings for interlaced.

I have the PJ-650, PJ-710 and the CX900 Sony cameras and all 3 of them show more noise with 60p than 60i. Of course all 3 of them will have an extra stop to work with when using a shutter speed of 30.

So you have to make a number of extremely low light tests with various settings to find what AGC limit best meets your needs depending on different camera settings.

White balance can also make a BIG difference! On my CX-900, if I use auto white balance with an outdoor night shot, it can occasionally jump to a setting that makes the noise much more apparent. So locking the white balance will prevent that.

You have to consider a number of combinations of camera settings not just one.

I have made many, many tests getting to know what my cameras can and cannot do. This gives me the best results for whatever situation comes up.

One best leaves the 'auto' settings for run and gun shooting or another one of those 'You Tube' videos...

Know your cameras!

... and the CX900 Sony cameras and all 3 of them show more noise with 60p than 60i

Hmm... my AX100 doesn't show that behavior.

"Hmm... my AX100 doesn't show that behavior."

Maybe it is when you are shooting 4k. I noticed the specs on the 4k cameras where the minimum lux is higher at 4k VS. other settings.

Otherwise, I can only guess as to what the reason is. It would be interesting to verify as to why this is.

You might check multiple modes to see if this holds true on all of them.

In tested it in the AVCHD mode because this is the only mode that allows for both, 60p and 60i.

Well, I have had a sleep and able to face another day.

Bob quoted "In connection with video and digital image sensors, decibels generally represent ratios of video voltages or digitized light levels, using 20 log of the ratio, even when the represented optical power is directly proportional to the voltage or level, not to its square, as in a CCD imager where response voltage is linear in intensity.[38] "

I looked at reference 38, and it talks about photomultiplers, a vacuum tube device still used for detecting very weak light. It has a square law detection, so the output voltage is proportional to the light intensity (power per unit area). When looking at the output signal and comparing it to the residual noise at the output, decibels are calculated as 20 log voltage ratio as is done elsewhere in the universe.

I presume that the mere mortals who frequent this forum do not use video cameras that incorporate photomultipliers or any other square law detectors, so Kimberly, 3 dB on your camera represents a power or intensity ratio of 2 or a light amplitude ratio of 1.414. (60 dB represents about 10 f stops, so 3 dB is about 1/2 a stop.)

@ OldSmoke

When I tested, I did so with 100% auto. No AGC limit, no exposure offset and no manual settings of any kind. I did so using very, very low light as in barely enough light to be useful at all.

I tested at night using 60p to start with and the light dimmer set to slowly bring up the light until the camera just changed from the maximum exposure gain to one notch less. I then slowly backed down on the dimmer until the gain just went back up to the max.

There was X amount of noise in the image. It is a bit hard to see in the little viewer so I recorded the video.

I then changed to 60i and recorded that.

Put both videos on the time-line with 60p first and look at both paused, not playing, and I saw that the noise was somewhat greater with 60p than with 60i.

I get the same kind-of results getting a video of my house with just the street light. The 60i is much cleaner than 60p.

If doing all of that and you still get the same results both ways, then maybe the AX-100 has an equally softer or sharper settings when using AVCHD for both 60i and 60p. If sharper with the same lower noise then chalk up an extra positive for the AX-100.

Meanwhile, I do know what the CX-900 and my other cameras can do. The most important thing to know is what your camera can and cannot do.

Oh, by the way, I yap a lot of about knowing ones cameras but it should be mentioned as to how important audio is as well. YouTube is just plain awful when it comes to audio! Not because of YouTube itself but those who upload videos to it.

Yes Peter, correct. +6 db is double the video level, my apologies. However it should not be thought of as having anything to do with power or light, rather it is a relative gain change in the video amplifiers. Much the same way that the modulation depth or the noise level is measured in db, relative to a full amplitude video signal.

The reason I suggest that it should not be thought of in terms of light or f stops is because the side effect of increasing video gain is to increase the noise by the same amount as the signal, so there is a significant penalty to increasing gain, as against increasing the light reaching the sensor, which has no noise penalty.

Geoff

"The reason I suggest that it should not be thought of in terms of light or f stops is because the side effect of increasing video gain is to increase the noise by the same amount as the signal, so there is a significant penalty to increasing gain, as against increasing the light reaching the sensor, which has no noise penalty."

When thinking about gain, it is about how much of a penalty one can tolerate. Which is why the tests with the AGC limit. Which is also why one considers aperture, shudder speed and the size of the sensor.

Zoom in with the camera and watch the light go down. I used a good fixed 2x lens on front and got better low light results when zoomed a specific amount. Simply because I did not have to zoom as much with the camera itself and thus kept a larger aperture. Note: I said a GOOD 2x fixed lens. There are other limitations to zoom flexibility though. So there are trade-offs...

How many ways can one get more light WITHOUT using more gain. Too many cameras try to go with more gain than with other settings when using auto settings. And Sony consumer/pro-sumer cameras go for more brightness than needed and thus more gain than necessary.

Many Sony cameras I have/had tend to overexpose.

"shudder speed"

Sorry, but I had to smile.

You must have an American accent to make that typo. They tend to voice un-voiced consonants, particularly "t'", after a stressed syllable. So "butter" is pronounced "budder", etc. (Grazie probably would pronounce it "bu'er" where ' is a glottal stop.)

"Many Sony cameras I have/had tend to overexpose."

I run my Nikon DSLR with -1EV for quick snaps and correct the photo later if the camera got it right after all.

@ PeterDuke

Noting my edited poste, at least I caught a number of other stupeed typose.

Just for fun, I will leave it as is.

I am going to get ready for Church tomorrow before I screw that up as well.

I am so tired, I had a BAD week! My blood pressure severely dropped early Monday morning then my blood sugar went severely low as well. My ears waxed up and I could not hear anything and I had a spell when I could barely eat.

Next week can only get better?

Crazy older age! Grrrrrrr...