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Exposing Video w/ spotmeter


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#1 Jack Brandhorst

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Posted 25 October 2007 - 12:41 PM

Ive been using my spotmeter for B&W still photography but now would like to use it to help expose my DVCAM (ikegami HLDV7w) on an upcoming shoot. How can I do this? What should I set the asa on the spot meter to and what do I do about my shutter speeds. I normally expose my video using zebras and my eye and cant afford a waveform. Any advice would be greatly appreciated. (im using a canon 9:19 zoom)

Thank you;
jack
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#2 Saul Rodgar

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Posted 25 October 2007 - 01:21 PM

"What should I set the asa on the spot meter to and what do I do about my shutter speeds. "

320 ASA at 60 shutter speed is what most people use at 30i, 48 shutter speed for 24p as well. Try experimenting with different shutter speeds for look, as one would with film, over and under exposing. You can rate your video a little lower if you prefer the underexposed look. 320 ASA for video seems to be the general preference, though. You can try reading a gray card with your camera built in light meter, and then comparing/ matching it to your hand held spot meter under the same light at 60 shutter speed for 30i. That would give you the correct ASA for your camera.
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#3 Jonathan Bowerbank

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Posted 25 October 2007 - 03:10 PM

Just going by the specs listed on the website, it says f11 at 2000 lux. Does anyone know under what settings that's probably taken from?

With 2000 lux reading f11, that gives the camera an effective ASA of 800...quite fast. Although I worked with a Panasonic HDX a few weeks ago, and it had an ASA of 640 at -3db, so it's feasible. You'd have to shoot your own tests under the settings you'll be using to make sure.

Set your zebras to the % you want, then shoot a white card and close down until the zebra lines go away, then read it with the meter and adjust the ASA until it matches the camera's f-stop. Then another way of doing it is shooting a grey card, auto exposing for the grey card and adjusting the ASA on your meter until it matches the f-stop.
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#4 Chris Keth

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Posted 25 October 2007 - 03:19 PM

Those lux specs tend to be taken with the camera set at it's absolute fastest so that the specs look good. I wouldn't trust them a bit. They're probably taken at maximum gain, lowest shutter, etc.
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#5 Saul Rodgar

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Posted 25 October 2007 - 04:01 PM

"You can try reading a gray card with your camera built in light meter ON AUTO IRIS, and then comparing/ matching it to your hand held spot meter under the same light at 60 shutter speed for 30i. That would give you the correct ASA for your camera.
[/quote]

Sorry I forgot the Auto Iris part, as Jonathan pointed out
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#6 Hal Smith

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Posted 25 October 2007 - 04:30 PM

Mike Butkus has a Pentax Digital Spotmeter manual on his website (GREAT SITE FOR MANUALS!).

There's a long section in it on using a spotmeter in a video environment, I recommend reading it.

http://www.butkus.or...l_spotmeter.htm
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#7 Jonathan Bowerbank

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Posted 25 October 2007 - 04:31 PM

Those lux specs tend to be taken with the camera set at it's absolute fastest so that the specs look good. I wouldn't trust them a bit. They're probably taken at maximum gain, lowest shutter, etc.


It's maximum gain is +48 I think (now I can't find that spec sheet from earlier), and it said it's able to read .12 lux at that setting, so it's not set at maximum gain, I don't think :/
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#8 Michael Nash

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Posted 26 October 2007 - 02:19 AM

Don't trust the auto iris for proper exposure of a gray card. Until you know what IRE the camera wants to put gray at (i.e., with a waveform monitor), you're doing yourself more harm than good. Many times the auto-exposure value is set way too high, and will vary with the gamma setting anyway.

Reason #101 not to use a lightmeter to determine exposure with a video camera...
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#9 David Auner aac

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Posted 26 October 2007 - 02:35 AM

Hi Michael,

but what about setting your zebra to 70 (or even 65 if you can) and then exposing a gray card so that it just hits zebras. Then get the f-stop and adjust the ISO on your meter until it matches the f-stop on the lens. Then you should have a pretty good approximation of the speed, right?

Cheers, Dave
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#10 Jonathan Bowerbank

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Posted 26 October 2007 - 10:46 PM

Don't trust the auto iris for proper exposure of a gray card.


That's why I added it as a sort of secondary method. It'd worked for me now and again.
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#11 Michael Nash

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Posted 27 October 2007 - 03:27 AM

Hi Michael,

but what about setting your zebra to 70 (or even 65 if you can) and then exposing a gray card so that it just hits zebras. Then get the f-stop and adjust the ISO on your meter until it matches the f-stop on the lens. Then you should have a pretty good approximation of the speed, right?

Cheers, Dave


If you change your reference luminance from 50% to 70%, you'd be "biasing" your ASA approximately one stop faster. Spot meters are calibrated to show 50% gray, which translates to about 50-55 IRE. But zebra stripes set to a known value are the closest thing you have to a waveform monitor in the eyepiece, so using that as a reference will at least be a constant.

However, 70% is actually very useful as a reference for video because it's a good balance between protecting the highlights from clipping and also getting normal-looking midtones. Film uses 50% midtone as a reference for exposure since it has a decent "straight line portion" of the gamma curve. Video typically has a "normal" curve below 50% but a dramatically steeper curve above 50, so "proper" exposure on video is sometimes biased toward the highlights (hence 70%, which also corresponds well with Caucasian skintones).

The problem with using a spot meter with a video camera is that the response curve of the camera is not linear. It's somewhat film-like in the mid-to-shadow region but very steep and contrasty in the highlights, so the under- and over-exposure ranges won't be even. Add to that they fact that the "knee function" compresses the highlight, and you've got another curve value to deal with. Add to that the fact that the knee is "dynamic" ( the "D" in "DCC" (Dynamic Compression Circuit)), means that the extreme highlight response will change with the scenic content. And then add to THAT the fact that the sensitivity of the chips(s) changes with the light level, kind of like a variable ASA.

Like I said, reason #101 not to use a light meter to determine exposure with a video camera.
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#12 Jack Brandhorst

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Posted 27 October 2007 - 05:58 PM

If you change your reference luminance from 50% to 70%, you'd be "biasing" your ASA approximately one stop faster.

Spot meters are calibrated to show 50% gray

, which translates to about 50-55 IRE. But zebra

do meters give 50% grey or 18%?

Edited by Jack Brandhorst, 27 October 2007 - 06:02 PM.

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#13 Michael Nash

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Posted 27 October 2007 - 06:22 PM

A gray card is 18% reflectance, meaning it only reflects back 18% of the light hitting it. It's not 18% "luminance." Don't let that confuse you. It's designed to be representative of "average midtone," or half-way (50%) between the darkest black and the brightest white that the film can capture.

This is a photo of a (more or less) properly exposed 18% reflectance gray card. The border around the picture is 50% luminance.
screenshot1.jpeg
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#14 Jack Brandhorst

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Posted 28 October 2007 - 08:59 PM

A gray card is 18% reflectance, meaning it only reflects back 18% of the light hitting it. It's not 18% "luminance." Don't let that confuse you. It's designed to be representative of "average midtone," or half-way (50%) between the darkest black and the brightest white that the film can capture.

This is a photo of a (more or less) properly exposed 18% reflectance gray card. The border around the picture is 50% luminance.
screenshot1.jpeg

Thanks Michael
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