Jump to content


Photo

Lightmeter equivelant of IRE setting?


  • Please log in to reply
4 replies to this topic

#1 grant mcphee

grant mcphee
  • Basic Members
  • PipPip
  • 93 posts
  • 2nd Assistant Camera
  • UK, Scotland

Posted 24 August 2007 - 08:53 AM

Say you were shooting a CU of a face.

If you had your zebras on a video camera set to 80IRE so the highlights were just showing, and your stop was showing 5.6.

Would your light meter be set at 100IRE? i.e. if I got a reading at one of the points where the zebras were just showing on the video camera would the image exposed on 16mm be under exposed campared to the video image?

thanks
  • 0

#2 Tim O'Connor

Tim O'Connor
  • Sustaining Members
  • 860 posts
  • Other
  • Boston, Massachusetts

Posted 24 August 2007 - 09:58 AM

Say you were shooting a CU of a face.

If you had your zebras on a video camera set to 80IRE so the highlights were just showing, and your stop was showing 5.6.

Would your light meter be set at 100IRE? i.e. if I got a reading at one of the points where the zebras were just showing on the video camera would the image exposed on 16mm be under exposed campared to the video image?

thanks


This is a good question. If I use the zebras for skintone to set my "correct" exposure, then
what would the correct exposure be in shooting film?

I think that somebody else may be able to give you a more technical reply but in general I think
that the correct exposure is that which exposes for what is most important and if you have
say windows blowing out you either address that or leave it alone as the case may be.

I wouldn't think of it as having the light meter reading as equaling 100 IREs but there's some sense
in that analogy. If 5.6 is your correct exposure, then I think that in a way, despite the number of
of IREs, you're thinking of that as 100 percent right on. If that holds true, then using
your light meter to expose for the skintone, say with a spotmeter, should lead you to a 5.6 as well
(for an equivalent ASA.)

Incidentally, I think that 80 IREs would be considered somewhat hot for this example. I know that
some Mini-DV cameras have zebras that can't be set below 80 IREs (why?) but a starting point for
me with a "typical" caucasion face is around 65-70 IREs just showing on the forehead or tip of the
nose. Even then I just worked with two actors and one is so white that he's practically a walking
reflector. He would be way too hot in the same light as the other caucasian actor next to him whose
exposure had been set first, so I would have to adjust the lighting on one or the other to make them
both like okay. If I used the same light to key them both, I would use at least a single net on the
lighter guy.

It might be fun to experiment if you have some friends who could help you, especially a mix of
skin colors, not all caucasians.
  • 0

#3 Charles MacDonald

Charles MacDonald
  • Sustaining Members
  • 1157 posts
  • Other
  • Stittsville Ontario Canada

Posted 24 August 2007 - 08:25 PM

Let me see, an IRE unit is 1/100 pf a volt. Video uses 1.4 volts , .4 for the sync, and 1 for the Picture.

The only time I sayw a pro set exposure on Video, he put the hilights at 100, and the shadows in the noise. on that unit (a Marconi Orthicon Camera- it was a long time a go) he could set the lens opening, and also the gain of the tube. He just looked at the vidoe dispalyed on teh scope and fdged things until he had signal all over the range and he had not too much compressed hilights, and the shadow detail he wanted was out of the "grass" as he called the Noise. and it did look like Grass on a P1 Scope tube)

Now in filming we take meter readings from teh midtones, as both the high and low light levels will be compressed cleanly. the deep shadows will be masked by Grain rather than snow, and the highlights will not tend to a uniform white if you don't overexpose "too Much"

If you are looking to expose for the White highlights, and put them at the top of the useable range, I would imagine that is what you mean by shooting at 100 Insitute of Radio Engineers units? the IRE was renamed after the 1.4 Volt video standard was commonplace.)

More typical is to take an average tone, and set that at 18% grey. someone who has done video coult proably tell you wher ethat fits , but I giuess it would be probaly between 18 and 50 IRE units. In Film you expose for the most important part of your scene, and let the very light and very dark fall away. they don't end up as Snow or Gloss ghost white like in Video.
perhaps someone else can relate to your question.
  • 0

#4 Michael Nash

Michael Nash
  • Sustaining Members
  • 3330 posts
  • Cinematographer
  • Pasadena, CA

Posted 25 August 2007 - 12:42 AM

If you had your zebras on a video camera set to 80IRE so the highlights were just showing, and your stop was showing 5.6. ... Would your light meter be set at 100IRE?


I'm not even sure what "light meter set at 100 IRE" means! And re-reading this I'm not even sure what you're asking. So let's back up...

IRE values translate into "screen brightness." Think of 80% IRE as meaning "zone 8" in terms of print brightness. 80 IRE = zone 8 = slightly better than half-way between middle gray and pure white (100 IRE). (This is not strictly true; video display gamma is different than a film print. But let's overlook that for the moment). With a light meter you're usually trying to find the midtone reference (50% brightness = 50 IRE = zone 5), not the pure white reference. And since the highlight response of video is dramatically different that film, you can't use the same standard of reference for exposure. Exposing whites the same for both media would result in different midtone brightnesses, and vice versa. Maybe that's what you were asking.

If your video exposure looks good with the brightest part of skin tones touching 80% and your camera is at 5.6, you'd set your incident light meter to whatever shutterspeed you like as a reference (1/48 or 1/60) and adjust your ASA until your meter shows 5.6. You can then use your light meter to approximate exposure on your video camera, or find the effective ASA of your video camera. If you're using a spot meter you could spot-read the highlights and expect them to be show about an 8/11 split.

And 80% does sound a little hot for Caucasian skin highlights. For broadcast video I put Caucasian skin just barely touching 70%, if showing any 70% zebras at all. I'll push those highlights higher in high contrast lighting (like outdoor sunlight) when I'm trying to balance exposure between highlight and shade.
  • 0

#5 Tim O'Connor

Tim O'Connor
  • Sustaining Members
  • 860 posts
  • Other
  • Boston, Massachusetts

Posted 25 August 2007 - 12:33 PM

I would add to these two posts that I use my light meter often when shooting video but I never calibrate
it to the camera's apparent "ASA" although it is possible as Michael described.

I like to use the meter to measure light levels when I'm comparing different parts of a room, or existing
light levels when I'm scouting and for both of those uses I measure in footcandles. When shooting film
I simply make the appropriate settings although you could figure out your stop if you know that ASA's
response for a certain number of footcandles at a given stop.


For example, most people rate the HVX-200 at about 320 ASA but when you are shooting in lower light the
sensitivity is closer to 125 to 200 and there are even other factors that can affect your "ASA". The ASA
of a film stock is based on the chemistry of the film and is consistent.

Unlike film, the ASA rating you give to a video camera is not going to be linear and therefore I never
even think of metering that way. For exposing video I use zebras and a waveform monitor.
  • 0


Media Blackout - Custom Cables and AKS

Willys Widgets

Metropolis Post

Visual Products

Wooden Camera

CineTape

Broadcast Solutions Inc

Tai Audio

Rig Wheels Passport

Ritter Battery

CineLab

Glidecam

Gamma Ray Digital Inc

The Slider

Abel Cine

rebotnix Technologies

Aerial Filmworks

Technodolly

FJS International, LLC

Paralinx LLC

Opal

Metropolis Post

The Slider

Gamma Ray Digital Inc

Paralinx LLC

Tai Audio

Ritter Battery

FJS International, LLC

Media Blackout - Custom Cables and AKS

CineTape

Rig Wheels Passport

Aerial Filmworks

rebotnix Technologies

Technodolly

Glidecam

Visual Products

Wooden Camera

Willys Widgets

Broadcast Solutions Inc

Opal

Abel Cine

CineLab