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Using Sekonik 308s Light meter for DV


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#1 MattGrover

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Posted 04 December 2005 - 11:20 AM

Hi all,

I'm working on a short, to be shot on DV (probably Z1) and want to know how best to set up and use my Sekonik 308s, which I've recently purchased.

I was just reading the manual and trying to work out the settings. It seems that (on the 308) i can't set the shutter speed to 50, which the camera will be at (shooting PAL, 25fps, UK) to match the power/lighting.

Would I be correct in thinking then if I set the 308 to 25fps, and match the ISO to the one set on the camera, the meter will then give me the correct stop for exposure?

Many thanks :)

Matt
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#2 Matt Irwin

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Posted 04 December 2005 - 02:48 PM

I know this sort of thing has been addressed here before, but using a meter with video is not really necessary.

A calibrated reference monitor (and/or a waveform monitor, budget and camera permitting) would be better tools for judging exposure on video. I usually light to a reference monitor and then use a meter to keep the footcandles on the key as well as the key-to-fill ratio consistant during coverage, but not to determine exposure.

That's both the blessing and the curse of video: You can see exactly how your image will be exposed before you roll.

Edited by Matt Irwin, 04 December 2005 - 02:50 PM.

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#3 Laurent Andrieux

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Posted 04 December 2005 - 07:39 PM

Yes, sure, but if one wants to prep the set and doesn't have the camera nor talents, it's alos usefull to work with the meter, as a first step. Also, in a prep situation, it's not often as easy to frame, go to the monitor all the time, as you are on the set, talking with your gaffers etc.

Anyway, if you want to set your meter accordingly to your camera specifications, set it at 25 fps, don't change the "shutter" setting (if ever there is one on this meter, I don't know) and set the ISO as specified on your camera's book, if there is such a sensitivity specification.

Usually it's not the case, you'd find a required stop for a given illumination, for instance : 200 fc for F : 11and rely on the old "1-2-4" rule, that says that 100 ISO, 200 fc, N : 4 or 100 fc, 200 ISO, N: 4. That would give you an equivalent 800 ISO for 200 fc at f : 11.

But mind that it's not always easy to find the "typical" sensitivity in these books, because manufacturers like much better giving a "minimum" sensitivity (like 1 fc, f : 4 or whatever of the kind) for commercial reasons, including the maximum of electronic gain and anything that would rise up the sensitivity (and lower the image quality), which doesn't interest you...
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#4 MattGrover

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Posted 09 December 2005 - 05:20 PM

Hi guys,

Thanks for the replies.

laurent.a - I haven't come across the 1-2-4 rule before (which seems odd to me(that I haven't))

100 ISO - 200 fc - N: 4 , what does the N indicate, is that the stop?? If so how come it's N and not F?

Cheers :)
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#5 Laurent Andrieux

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Posted 10 December 2005 - 07:26 AM

Yes, it's the Stop. The T-stop actually. In france we use N, as well, sorry I just had put this too fast...
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#6 Chris Keth

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Posted 10 December 2005 - 08:27 AM

Yes, it's the Stop. The T-stop actually. In france we use N, as well, sorry I just had put this too fast...



You can quite easily figure out the sensitivity of your camera. Just set the zebra stripes to 70IRE and set the camerta up looking at a grey card. Adjust the stop so that the greycard is all in zebras. Meter the card (preferably a spotmeter), and figure out what ISO at 25fps (or whatever framerate you're shooting at) will give you the stop your camera told you was correct.

Edited by Christopher D. Keth, 10 December 2005 - 08:29 AM.

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#7 MattGrover

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Posted 10 December 2005 - 01:29 PM

Yes, it's the Stop. The T-stop actually. In france we use N, as well, sorry I just had put this too fast...


No worries, thanks :D


You can quite easily figure out the sensitivity of your camera. Just set the zebra stripes to 70IRE and set the camerta up looking at a grey card. Adjust the stop so that the greycard is all in zebras. Meter the card (preferably a spotmeter), and figure out what ISO at 25fps (or whatever framerate you're shooting at) will give you the stop your camera told you was correct.


Sounds good, I'll do that! Thankyou!!
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#8 Laurent Andrieux

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Posted 10 December 2005 - 04:28 PM

Wait a minute...

The 18 % grey card is not supposed to be zebra highlighted if the exposure is good...
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#9 Matt Grover

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Posted 11 December 2005 - 07:38 AM

Wait a minute...

The 18 % grey card is not supposed to be zebra highlighted if the exposure is good...


I thought u use the zebras to mark the correcly exposed area?? eg. Zebras set @ 70, stripes on the brightest third of the of the face, you've got a well exposed picture.
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#10 Laurent Andrieux

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Posted 11 December 2005 - 04:50 PM

Zebras set @ 70, stripes on the brightest third of the of the face, you've got a well exposed picture.


Exactly, so why would you get your 18 % grey card striped with zebras if well exposed ?
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#11 MattGrover

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Posted 12 December 2005 - 04:02 AM

Exactly, so why would you get your 18 % grey card striped with zebras if well exposed ?



I thought, the zebras (on camera) show correctly exposed, not over exposed???
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#12 Laurent Andrieux

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Posted 12 December 2005 - 08:19 AM

Thezebras are ment to show other 70 % reflectance objects. So if one imagines a portait of a person holding a grey card in his hands, the zebra should only show in the highlighted parts of the skin, if ever, not on the grey card.

So determining the proper exposure, without a waveform monitor, cannot be done by setting zebras on a 18 % grey card.
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#13 MattGrover

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Posted 16 December 2005 - 09:00 AM

OK. forgive me for the basic question here, I'm unable to locate my notes from film school :huh:

What does the 18% Grey card represent?

I've been working on the basis that it (thinking hard now) represents a similar level of reflection to caucasian skin, I'm now starting to think that this is completely wrong and I really should find those notes!

cheers

Matt
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#14 Laurent Andrieux

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Posted 16 December 2005 - 03:28 PM

the 18 % grey reference reflectance is not the reflectance of the caucasian skin.

It is the "middle grey", that is perceived like being midway beetween black and white, by eye.

It is, also, the geometrical mean beetween common reflectance values for black and white.

Remember that a 1 stop step in reflectance, is equivalent to dividing/multipliying by 2 this reflectance.

For instance, there is one stop in density, beetween a 10 % and a 20 % reflectance.

The 6 grey card, that is going from black to white by one stop steps, is usually 2.5 % (black), 5 %, 10 %, 20 % (that is very close to 18 %), 40 % and the "white " is 80 %. On the english chart, you have a seventh value, that is about 30 % as a reference for caucasian skin.

If you have ever seen a 18 % grey, you will, without a doubt, see easily that a "white skin" is much lighter.

(about 2/3 of a stop).

These values (2.5 % for black, 80 % for white) are "old values" based on the principle that a photographic system can be unable to distinguish details if reflectance is under 2.5 % and that a refelcting object hardly reflects more than 80 %.

In fact, in exterior day, we see higher values, but that's how this chart and middle grey are calibrated.

Since the reflectance are in geometrical serial, not arithmetical (like decibels for instance), it's not an arithmetical mean (that would give you 50 %) that gives you the "middle grey", but the geometrical mean (I.e. root mean square of the multiplied values). Some people say 16 %, some say 18 %...

Anyway, as the reflective (spotmeter) light meters are calibrated on this value, they would give you the same value if metering on a 18 % grey as with an incident light meter at the same position.

One can easily check this up, as well as one can easily check that metering on a white skin won't give the same result.
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#15 Chris Keth

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Posted 16 December 2005 - 05:04 PM

I thought u use the zebras to mark the correcly exposed area?? eg. Zebras set @ 70, stripes on the brightest third of the of the face, you've got a well exposed picture.



OK, I apologize for that. I read the approach I detailed in The DV Book and assumed it was right.
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#16 Laurent Andrieux

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Posted 17 December 2005 - 08:33 AM

This is right, but the skin reflectance, that is about 30 %, but can go up to 60 to 70 % in the lightest parts, making the zebra to highlight these values, is not 18 %.

It's the proposition you made for determining the camera's sensitivity that I was more like "criticising", since you based your determination on zebras that should appear on a 18 % grey chart. If so, that means you would be about 2 stops (or at least 1 and 2/3) overexposed...
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#17 Hunter Sandison

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Posted 28 January 2006 - 08:11 PM

Laurent,
What if you set up a properly calibrated monitor then after making sure the 18% grey card is evenly lit adjust your iris until the grey on the monitor matches the your eyeball judgement of the actual card, then take an incident reading of the light falling on the grey card, and then simply change the ISO on your meter untill you get the stop that you set on the lens? Would this give you an accurate ISO of your video camera or am I missing something critical? If so could you or someone else please set me staight? Thank you and sorry for the run-on sentence.
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#18 Laurent Andrieux

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Posted 28 January 2006 - 09:36 PM

There is no need to apologize ! (even though I don't get the expression "run-on sentence"... :lol: )

Yes, your approach is good to me, and you actually don't need a grey card for this :

for anything you frame adjust your iris until the image on the monitor matches your eyeball judgement, then take an incident reading of the light falling and then simply change the ISO on your meter untill you get the stop that you set on the lens.

And if you are satisfied by your camera's image when taking an "auto" iris value, you can also use this one .

It will give you a good idea of your camera's sensitivity. But I woudn't say it's too accurate. You should, actually, doing this, find a "range" where your image is "good", I would say this range could even be a 1 stop range, since the iris rings on video cameras have some sort of "play" if you see what I mean. The "auto" setting should give a value in that range.

And if you have a chart in the frame, the zebras should show by the white chart as well as if you have someone in the frame, the zebras should show in the highlited areas of the skin.

Also, what you can do, if you have your camera's specifications, as I said in the first posts of this thread, is to set the specified illumination, let's say 200 fc, and since your meter can read fc, if I'm correct (but you are supposed to use the flat disc for metering fc, not the bulb), find the sensitivity that gives you the specified F-stop for this illumination.
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