Video Cameras Minimun Sensitivity??ISO?

Manu Anand · July 25, 2005

Ok

This has kind of been bugging me for a while but here goes

most Video Cameras in their specifications tell you what there Sensitivity is

for example

DSR 570WSP = f11 @3200 K ,2000 lux (185 foot candles)

which equals ISO 800.... but thats for minimum sensitivity .....and not the cameras ISO

now heres my question .

What is this sensitivity in terms of the video signal.

Is it a Barely visible image....which is ? = what ? IRE 8,9,10,12--------35------50

and my research shows a reading without defining IRE level is actually useless.

While lighting for a 570 Ive never rated my meter @ iso 800.

But the information provided will let you to beleive that the camera is unbeleivably sensitive.

While typing the question i did some research

Apparently the EIA has new standards in place.

Heres an interesting article i fouind in my research and i finally understand what the defined IRE level should be for minimum sensitivity.

http://videoexpert.home.att.net/artic1/250sens.htm

In the DSR 570 manual before me for the camera "sensitivity"it mentions no gain up and does not define what results the f11 3200,k 2000 lux gives in a quantifiable manner.

In another recent thread recently someone had rated an f11 2000 lux camera @ ISO 800. I think its more around ISO 320

I dont think one can apply minimum senstivity measurements of cameras and directly interchange them to their ISO setting unless one knows the quantitative definition of minimum sensitivity.

Anyway i think im repeating myself now

Hope to hear your thoughts and insights on the above

Manu Anand

New Delhi(currently in Bombay)

DP

Laurent Andrieux · July 26, 2005

Hi

>"DSR 570WSP = f11 @3200 K ,2000 lux (185 foot candles)

which equals ISO 800.... but thats for minimum sensitivity .....and not the cameras ISO"

This is not for "minimum sensitivity". This is for default/standard sensitivity. (25 or 30 fps, filter 1 (3200 K), gain = 0 etc.)

The ISO standard DOES NOT APPLY to video cameras, as well as it DOES NOT APPLY to motion feature film stock.

ISO is a still picture standard. Motion picture film stock manufacturers give an EI = Exposure Index, that one should confirm with keylight tests.

Video camera manufacturers give a couple illumination/f-stop.

The thing is that one who is using a light meter has to compute a sensitivity - and meters are metered for the still picture ISO norm (that once was the ASA one) - so that it gives an acceptable reading/value at a given exposure time under a certain illumination.

The "minimum sensitivity" that usually appears in the video camera specifications assumes a maximum electronic gain, sometimes a lower shutter speed etc. It basically is a purely commercial spec for non specialists. That therefore assumes a maximum noise rate, a poor Signal/noise ratio, a poor dynamic resolution (due to the slower shutter speed) etc. BUT it only is a commercial spec. Amateurs are sensitive to such arguments that are actually meanless like "sensitivity : 0 lux".

You can guess that the sensitivity obtained on a low cost camera shouldn't seriously be that bigger than a professional one, can't you ?

"What is this sensitivity in terms of the video signal.

Is it a Barely visible image....which is ? = what ? IRE 8,9,10,12--------35------50

and my research shows a reading without defining IRE level is actually useless.

"

Sensitivity is not seeingable by the IRE, just like an amount of light is seeingable by a light meter.

Keep thinking in terms of (as manufacturers tell you...) "this is the required illumination for setting this iris aperture". The more sensitive the camera is, the closer the aperture should be, as to get a proper setting".

When the setting is fine a black object has to look black (low IRE) a bright object has to look bright (high IRE).

Supposingly, a white object is to be about 100 IRE.

"pparently the EIA has new standards in place.

Heres an interesting article i fouind in my research and i finally understand what the defined IRE level should be for minimum sensitivity.

"

Sorry if I sound a bit harsh, but, honnestly, reading this made me sort of angry... This so-called "new standards" sounds like total b*****t to me. It's an ugly load of blah-blah there, and I picked up a few things that one should think about :

"First you need an efficient lens. A lens with a low f-number

(such as f1.4) focuses much of the light from your scene onto the

camera's sensitive CCD chip. A lens with twice as high an f-number

(f2.8) will pass 1/4 as much light through it, making the camera 1/4

as sensitive."

This is wrong the camera's sensitivity doesn't depend on the lens. It's a diffrent object.

The lens that opens up to 2.8 will require the same light to have a correctly exposed image at this iris aperture that the one that opens up to 1.4...

The amount of required light at a specific iris opening is the sensitivity... it remains the same...

"On a professional camera, "minimum

illumination" indicates the minimum amount of light falling on a white

surface that will produce .5 volts (70 IRE) of picture information

with maximum lens opening and maximum gain boost. "

Where do they get that from ? A white surface, correctly exposed will look white, and produce 0.7 V (100 IRE).

"The chart is illuminated with 3100 degree Kelvin

(3100° K) video lights adjusted to spread the light evenly over the

chart."

Usually, one is supposed to work at 3200 Kelvin - Kelvin are not a degree scale...

"Eventually, the trace that

represented the white bar only reaches 50 IRE, the cutoff point. Put

another way, when white things look 50% white (50 IRE), experts

consider the picture to have minimum acceptable brightness. The

technician, fumbling in the dark, locates his light meter and takes

another lux reading.

*Test 2 Black Level

The darkest bar on the chart is 2% white and appears as a black

bar on the TV monitor. On the waveform monitor the signal makes a

trace at approximately 7 IRE on the scale. Cameras measuring below 4

or above 10 on the scale for black level do not qualify for further

testing (these cameras, since they are not making correct black,

cannot have whites that are trustworthy). "

The bars are generated by a specific electronic circuitry and have nothing to do with the sensor parts of the camera...

"If the S/N ratio is 17 dB

or greater, the picture is considered acceptable."

By who ? a 17 dB S/N ratio is pure s**t, as far as I'm concerned.

"(...)Remember earlier how

professional TV cameras sported S/N ratios around 63 dB? That's one

reason why professional video looks so good compared to what the rest

of us make at home. "

Here we are : those people are not talking about any professional way of considering things, since they oppose the professionnal way to theirs...

Therefore a "0 lux minimum illumination" should be enough for them...

Manu Anand · July 26, 2005

Namaste( standard indian greeting),

>This is not for "minimum sensitivity". This is for default/standard sensitivity. (25 or 30 fps, filter 1 (3200 K), gain = 0 etc.)

ok my fault there for taking it as minimum senstivity.its standard sensitivity

The ISO standard DOES NOT APPLY to video cameras, as well as it DOES NOT APPLY to motion feature film stock.

yup i agree with you ..the reason i brought this up was recently reading a thread in here where the ISO equivalent for a f11 2000 lux was given as ISO 800.

and people do talk in terms of ISO equivalents for Video Cameras.

>You can guess that the sensitivity obtained on a low cost camera shouldn't seriously be that bigger than a professional one, can't you ?

Ok of course i can guess that...my question is what is the standard default sensitivity???? what are the parameters for calculating that.

>Sensitivity is not seeingable by the IRE, just like an amount of light is seeingable by a light meter.

im not saying its seeable but they sure are measuring it all im asking for is how? question is what is the parameter in the test that helps them reach this standard sensitivity reading? In an experiment to measure senstivity they arrive at a certain value ...my question is how...? fstop and illumination arent enough to define it.

>Keep thinking in terms of (as manufacturers tell you...) "this is the required illumination for setting this iris aperture". The more sensitive the camera is, the closer the aperture should be, as to get a proper setting".

I am argueing against this way of thinking not for it .

>The amount of required light at a specific iris opening is the sensitivity... it remains the same...

Again my question.......To expose it how???? Your missing the point of my argument. what is the amount if light you speak of....and does it expose something perfectly....that determines senstivity information they provide us with.

are you saying at standard sensitivity settings a white object will reproduce at 100IRE

>"On a professional camera, "minimum

illumination" indicates the minimum amount of light falling on a white

surface that will produce .5 volts (70 IRE) of picture information

with maximum lens opening and maximum gain boost. "

>Where do they get that from ? A white surface, correctly exposed will look white, and produce 0.7 V (100 IRE).

Hes not saying it will be correctly exposed hes talking about minimum illumination.

at which they give the senstivity data and for minimum illumination they measure at 70 IRE for a white surface.

>Usually, one is supposed to work at 3200 Kelvin - Kelvin are not a degree scale...

kelvin was a degree scale till 1968 after which it became "the kelvin"

"Eventually, the trace that

represented the white bar only reaches 50 IRE, the cutoff point. Put

another way, when white things look 50% white (50 IRE), experts

consider the picture to have minimum acceptable brightness. The

technician, fumbling in the dark, locates his light meter and takes

another lux reading.

*Test 2 Black Level

The darkest bar on the chart is 2% white and appears as a black

bar on the TV monitor. On the waveform monitor the signal makes a

trace at approximately 7 IRE on the scale. Cameras measuring below 4

or above 10 on the scale for black level do not qualify for further

testing (these cameras, since they are not making correct black,

cannot have whites that are trustworthy). "

>The bars are generated by a specific electronic circuitry and have nothing to do with the sensor parts of the camera...

Test 1 : Here even i get confused above he said 70 IRE now he says 50 IRE.

He is talking about shooting a gray chart .. so the black on that is 2%white .. not about the colour bars in the camera...im well aware that colour bars are produced by the electronic circuitry.

Ok i think your missing the point of my argument...or query

all i wanna know is what are the parameters of the sensitivity of a camera.

Acc to you its simple...

>This is not for "minimum sensitivity". This is for default/standard sensitivity. (25 or 30 fps, filter 1 (3200 K), gain = 0 etc.)

OK but in this default sensitivity what are the parameters...what does it mean?

Does it mean a white surface will reproduce 100 IRE @ f11 2000 lux?

f11 2000 lux @ 1/60 would be about an ISO equivalent of 800 (keyword being equivalent)

but in my experience the 570 whose standard sensitivity is given as that is closer to an equivalence of ISO 320

and no need to apologise it wasnt my article.

but you did get worked up a bit even though i think im on your side.

Just been hearing about the video camera ISO equivalents a bit too much.

Laurent im argueing against manufactureres giving out information like say for the "XL2 minimum illumination is 2 lux" which they did. And the XL2 is a so called bridge between the consumer and professional world.

Manu Anand

New Delhi (currently in Bombay)

DP

Edited July 26, 2005 by Manu Anand

Daniel Sheehy · July 26, 2005

OK but in this default sensitivity what are the parameters...what does it mean?
Does it mean a white surface will reproduce 100 IRE @ f11 2000 lux?

yes, pure, detail-less white will reproduce 100 IRE @f11, 2000 lux, on a camera of that sensitivity unless you change the sensitivity by boosting the gain.

(though you don't always want white to look that way...)

i asked about ISO equivalents initially to allow me to use a photographic light meter for video work, very convenient!

incidentally laurent, the website you refered me to (http://betacam.palsite.com/uvw100feat.shtml) rated the UVW 100 @ f11 for 2000 lux, but my manual rates it @ f8 for 2000 lux...

And, would you be able to expand on that rule you quoted, particularly where you get the constant [250] from?

(sorry, i know this is from a different thread, but its kinda related..)

Joshua Provost · July 26, 2005

There is a procedure for determining the ISO rating of a video camera that has been discussed here or perhaps at dvinfo.net/conf. I believe it goes like this:

1. Light an 18% gray card.

2. Allow the video camera to auto-expose and read the F stop

3. Take an incident reading in front of the card and adjust the ISO sensitivity of the reader until the F stop it gives you corresponds to the F stop the camera adjusted to

4. You now have the ISO rating of the camera

I think that's right.

Laurent Andrieux · July 26, 2005

Ok,

would you be able to expand on that rule you quoted, particularly where you get the constant [250] from?
<the rule is : N²/t = E.S/250, where N is the F stop, t the exposure time, E the required lumination>

I had different sources, from different teachers at Louis Lumière school who gave me this formula. Some using 245, others 250, others 270.

I actually don't know where it comes from, nor how to evaluate this constant, sorry. I found 250 gave me the result that comes closer to my experience.

But I found a good way to estimate a camera sensitivity, from the manufacturer's specification, that actually confirm the formula :

Set a 2000 lux illumination, with a light meter. Some of them allow to work in lux/fc as well as in f stop (spectra IV, Minota 5...). Assuming you compute the proper shutter speed, search the ASA/ISO setting that give you the indicated f-stop. If you calculate from the formula, you'll get the same value.

ncidentally laurent, the website you refered me to (http://betacam.palsite.com/uvw100feat.shtml) rated the UVW 100 @ f11 for 2000 lux, but my manual rates it @ f8 for 2000 lux...

Sorry about that. I'll tell you what, it also astonished me to seee that sensitivity at the first place. The thing is, the UVW 100 has certainly increased in sensitivity along the years. Usually, with hyper HAD, you get a f8, with power had a f10 or f11.

I guess your camera is fitted with hyper HAD (This camera existed before power HAD were on the market) and I guess that this site refers to nowadays one, fitted with power HAD sensors.

There is a procedure for determining the ISO rating of a video camera that has been discussed here or perhaps at dvinfo.net/conf. I believe it goes like this:

1. Light an 18% gray card.

2. Allow the video camera to auto-expose and read the F stop

3. Take an incident reading in front of the card and adjust the ISO sensitivity of the reader until the F stop it gives you corresponds to the F stop the camera adjusted to

4. You now have the ISO rating of the camera

I think that's right.

I'm sorry about that, Joshua, but, unfortunatly, this method doesn't work because you won't get a proper auto iris setting with a 18% grey scale.

I guess video camera manufacturers take account of the fact that these cameras are used outdoor, for ENG use and, therefore, the 18 % refernce is not the best one. This value is the geometrical mean beetween the used albedo values in reflectance, something like 2.5 % for blacks and 85 % for white. Outdoors, with sky, clouds etc, this value is not high enough. Remeber that "historically", video engineers, in general, read the white value more commonly as a reference (100 IRE) and are afraid to go over that .

This is why video camera manufacturers have a setable value called " IRIS SETUP", that can be set to "peak" - so that the auto iris manages that highlights don't go over 100 iRE, or to "average", so that the 18 % value would work. The default factoryu setting is halfway beetween thes two extremes. That's why setting the iris in auto on a 18 % grey card doesn't work, with default settings.

Now back to sensitivity, IRE etc.

Since the manufacturer give you a couple f-stop/required illumination (usually specifying a 89.9 % reflecatnce) it is of common approach to have a white board lit at 2000 lux, then open the iris so that you get 100 IRE on the waveform monitor.

If you don't have a waveform monitor nor a luxmeter, the approach I have, in the Joshua's path, that I experimented, is , better than a 18 % chart, to have a scene framed so that no highlights nor large black areas are in the frame. A talent with a 6 grey chart or a clor + b/w one suits fine. The average illumination should be hitting most of the field. Make as it is 2000 lux, see what aperture is given by the auto iris. It works. Never expect more than a third of stop as precision...

Back to Manu's post and to the site he refered to. I consider that if you see the minimum required illumination so that it gives you 50 IRE on a white card, it means you consider that being one stop underexposed is acceptable... Why not ? (I don't) The only thing is that one has to assume and know that, and see if he considers being one stop underexposed is acceptable...

Edited July 26, 2005 by laurent.a

Daniel Sheehy · July 27, 2005

I guess your camera is fitted with hyper HAD (This camera existed before power HAD were on the market) and I guess that this site refers to nowadays one, fitted with power HAD sensors.

yup, the camera described on the site has a power HAD while the UVW 100 I work with uses a hyper HAD.

this gives it the same sensitivity as the DXC 637, even though the UVW 100 has a much noisier picture.

Manu Anand · July 28, 2005

Thank you Laurent for your patience.....and for explaining various aspects of my question.

The article I referred to ...even I had problems with but I was trying to do my research to see what i can find before i post my question ...and that was the only article i came up with after various permutations and combinations in google.

Next time I get my hands on the camera to test... I'll test this and let you guys know.

Sometimes while lighting with a meter i just find ISO 800 is way of the mark for an f11 2000 lux sensitivity camera..and i get closer results with an ISO 320..but maybe im doing something wrong.

Will test this... and let you know.

Manu"Its just rained a 1000mm in a day" Anand

Bombay

Edited July 28, 2005 by Manu Anand

Laurent Andrieux · July 28, 2005

You're welcome.

Let us know !

(don't forget that position 3 of the filter wheel is just an 85 in front of the sensor, therefore cutting 2/3 of a stop).

The different tests I've made on different cameras along the years have always confirm the specifications and the formula would give me a coherent value. If you find a value that is 1/3 of a stop different, it shouldn't matter.

I had a camera once that didn't give the good value. I compared it to another of the same model, it's the camera that had a problem...