all about exposure, please help.

C

c3kings

Well-known member
Dear All,

I am trained to use film cameras in school, and what I learn from school is ASA and using light meter to set the f-stop and so. I must say I am quite ignorant how to set the exposure on a video camera correctly or orthodoxly?

I wonder if you guys use light meter to determine the exposure when using HVX200 or any video camera? Then what is the ASA? How can I find out the ASA of each camera?

How about a wave form monitor, and do you use it to set the f stop? By the way, I wonder how a grey card is represented on a wave form monitor. In which level or reading?

A camera man told me that he just uses zebra to set the exposure. He just keeps everything not having zebra. It doesn't sound right, or at least scientific, to me. I think I should understand what Zebra mean more. What do you say?

Actually, what do they mean by 70% or 100% zebra? What are the differences? Someone told me that for a video camera, anything above 70% (or 80%, I don't remember) zebra has no details, so keep everything below this level. Like a color chart, we will set 7.5% and 11% as black (not sure the number either).

Please advise.

Thanks in advance.
King
 
ASA is the same thing as ISO which you should be familiar with if you have worked with film. Lower ISO is higher quality but less sensitive to light, higher ISO is vice versa. For example, for a recent shoot we always set the camera for 100 ISO if we were outside, 320 ISO if we were doing interior or night shots, and 500 ISO if all else failed. At 500 though, grain in the black areas was obvious.

An 18% gray card on a luminance waveform monitor will be right smack in the middle. A line at the top is pure white, bottom is pure black.

Zebras mark over exposed areas. Over exposure is dangerous in video because it contains no data. For example, when you bring footage into post production to do color correction you can correct for improper exposure as long as you can see color. If it is over exposed, and white, that area contains no color data, so adjusting levels during color correction will do nothing.

Zebras are an aide, and you shouldn't follow it completely. Naturally white objects on camera, for example, will have zebras on them because the camera sees the white as over exposure.

Hope this helps.
 
I've never used a light meter to determine exposure on a video camera, I usually just eyeball it. There's a meter on the camera that shows in increments your exposure level and what it "ideally" should be at, but as Luxalor said you shouldn't always trust the metering (his example was with the zebras) so there are likely some times when the camera's light meter can be misleading since it is often the sum and difference of a collection of light and shadowed areas in the frame - for instance, if you're shooting inside a car and you've framed the man driving the car in the drivers seat and exposed him properly, the car window will be blown out and that will probably tell the meter you're over exposed, when in reality, you aren't since you're not focusing on the window.

Just trust your eyes. Look at the meter for a guide, then decide for yourself how bright or dark you want it to be. As for zebras, the only time I ever use them is for weddings and that's because often times its too bright outside for me to see my monitor clearly, and so I trust the zebras to tell me what is exposed properly and what is overexposed. Generally, I find 85 to be a good setting for outdoors/sunny day shooting but I have never used them inside so I can't comment on a level for that.
 
I use a light meter when on set.

I'm not good enough to just go on it alone.

Using a monitor can help deturmine if your not totally f$cking up.

A waveform monitor is a great tool to have for digital, and thank god Panasonic is putting them into their new models such as the HPX-170 and etc.

So to keep this short, Waveform gives the Digital cinematographer overall exposure levels, and I'd recommend a light meter to more easily tell what is what as far as specific readings, because a busy scene on waveform can sometimes be hard to 'read'.

The 'marker' on the HVX is like a reflectance spot meter, and the zebras are nice too. Those will work in a pinch.
 
c3kings said:
Actually, what do they mean by 70% or 100% zebra? What are the differences? Someone told me that for a video camera, anything above 70% (or 80%, I don't remember) zebra has no details, so keep everything below this level. Like a color chart, we will set 7.5% and 11% as black (not sure the number either).

Please advise.

Thanks in advance.
King
Click to expand...

100% zebras + means it is 'blown out' and there is no data in that area of the picture, meaning if you try to bring it down in post, it will just be gray, no pixels will appear.

70% and 80% are not bad at all! They mainly will be the brighter things in the picture. No worries.
 
Im all histogram and light meter when working with the red but for HVX I use light meter for contrast ratios not exposer.
 
lairhair said:
Also just re read your question. If for whatever reason you want to
read exposer for the hvx I believe the asa is 320
Click to expand...

yeah, but that is default setting 1, I believe.


You scene file settings and values will certainly change the ISO/ASA. Best to find the look you want before production during tests, then, keeping those settings, do another test to find the 'new' and accurate camera rating. Crushes
 
I also was taught on film, and video is a lot harder to expose properly.

You can teach anyone how to expose for film in five minutes, and even if they're off by an entire stop (or two!) it'll still look okay by the time it gets back from the lab. You screw up with video and there's little you can do to save it.

I don't use a lightmeter now though, just zebras and occasionally a waveform monitor.
 
Thank you all of your wonderful, insightful and helpful replies, and Jimmy's thread even shows me how to run a ISO test.
It seems to me if we are in a situation required very accurate exposure, a waveform monitor maybe the best tool rather than a light meter in terms of a video shoot.
But I am still not sure how 70% and 80% zebra corresponding to a waveform monitor.
Also, I heard about that for video signal if you over or under certain IRE, it just means no details, and it is what zebra about.
Lastly, just to confirm, IRE50 is 18% grey card. Am I correct?
Thanks a lot in advance.
 
pmark23 said:
I also was taught on film, and video is a lot harder to expose properly.

You can teach anyone how to expose for film in five minutes, and even if they're off by an entire stop (or two!) it'll still look okay by the time it gets back from the lab. You screw up with video and there's little you can do to save it.

I don't use a lightmeter now though, just zebras and occasionally a waveform monitor.
Click to expand...

I think it is appropriate for digital video, to think like one is shooting positive film, that is the highlights blow out, so with positive film, one shoots for the highlights, and either 'lets the shadows fall where they may", or use something to soften the shadows.

It is also unfortunate that most video camera manufacturers attempt 'better the competitor specs', by obfuscating exactly what their camera is capable of. Heck, most of the 'lens' specs are made 'relative to 35mm' so that people don't have to calculate out what the approximate field of view, or the like. It would be nice if they could do the same for light sensitivity... still digital cameras all are rated with some form of ISO equvalent, or varable equivalents...

It would also be nice if they had a table for the different gain settings as well...

One of my projects to do, is to calibrate my meter to the digital video cameras I have, perhaps just for the exercise... but definitely to become more accurate in exposure... as well as estimating needed light for a desired effect!!!
 
I am still puzzling that why camera has a pre-set 80% for marker or zebra. Maybe, it sounds strange, but my question is why "80%?" What 80% signify? Skin-tone?
Thanks.
 
There are a lot of good books on this subject and also I wish I could find another good link.

Basically, if you know the zone system from film whearas zones 1-10 equals 1 as pure black and 10 as pure white, same goes IRE (zebra) in the video world. Except in the video world you don't have as much dynamic range as with a film emulstion. So as with film each zone equals 1 stop, in video 20% IRE equals one stop (aproximatly, depending on gamma ect).

With HD video you have approximately 5-6 1/2 stop of dynamic range, so you fit that in the IRE range.

Zone 5 or 18% grey is at 50 IRE or 50% zebra. Zone 6 or white skin tone is between 70 and 90 IRE (70 to 90 zebra). Dark skin tone is Zone 4 or 30-50 IRE. ect. You have about 2 stops above and below 50% IRE or about 2 zones each way.

Practically speaking you want to expose white skin tone right at 70% watching for overexposure in other parts of the frame. 80% is kinda right in the middle of the white skin tone "zone." I find it hard to expose for 80% because usually somthing else in the frame will blow out at that point.

I still use a light meter for finding ratios and location scouting, if you do the ISO test (for each gama setting or scene file you will use by the way) its pretty acurate.

http://en.wikipedia.org/wiki/Zone_system
 
Jimmy Moss said:
Basically, if you know the zone system from film whearas zones 1-10 equals 1 as pure black and 10 as pure white, same goes IRE (zebra) in the video world. Except in the video world you don't have as much dynamic range as with a film emulstion. So as with film each zone equals 1 stop, in video 20% IRE equals one stop (aproximatly, depending on gamma ect).
Click to expand...

Most photo papers since the 40-50's, had only about 8/7 'zones'. One would calibrate film and paper, by exposing the film in steps, then printing those steps. until one observed a 'difference' between the black of just 'filmbase+fog', and the step. That would set the minimum exposure for that film+paper combo. One would use that ASA/ISO value, and then make another series of exposers doubling the esposure, and then print again, and note at what 'stop' the printed frame and the 'white' of the paper were indistingishable. That set the film+paper range.

For most papers one would see only about 8 stops between 'black' and 'white'... If one did densitometer readings on the negative, one could get more... but the output device, the paper, was more limited.

In the case of the video camera, and IRE, if 20% corresponds to 1 stop, then one only has 5 stops...

Further, some films could be manipulated to give better 'high value response', because they had the ability to record wider dynamic range. Who ever shot Tri-X at 400 (or 360???) for most people I know... the effective ASA/ISO was 200 (or 160)...

The way I've thought about digital imaging (video or stills), is that it is much more like slide film than negative film. In slide film one would shoot for the highlight, that is set the exposure for what would yield detail in the highvalues, and either kick light into shadows...or live with the result, since slightly detailed areas of 'shadow/black' 'look' better than large areas of clear slide...
 
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