metering for contrast ratio

[Elliot Rudmann]

I was recently hired as a gaffer for a low budget 16mm project at my school, and a few days ago we did some lighting tests with an SRII to see how the V2 200T stock would react to different ratios (key+fill: fill). However, the DP and I had some slight disagreements as to how to measure for those ratios.

 

We were lighting a subject with two lights - a key light and a gelled kicker somewhat behind him. The DP took meter readings by pointing the lumisphere toward the camera to measure the key, and then turned off the key to take a reading of the kicker, again pointing the dome toward the camera and then compared the ratio of them. I told him that it would be more accurrate to measure the key with the flat disc (pointing towards the key light) and to do the same with the kicker.

 

I told him that pointing the meter toward the camera in such noir-type lighting conditions would result in inaccurate exposure (at least a half stop to a stop of overexposure because of how the dome averages). Not that overexposure is bad when working with 16mm, but that you should at least know an accurate reading of the key before decide to overexpose. Especially since the DP wanted to expose for the key light.

 

What would be the best way to meter this, or in general, the best way to meter for any type of desired contrast ratio?

Sorry if this sounds confusing. Any suggestions are appreciated - thanks!

 

Elliot

[David Mullen ASC]

Well, you could use a dome and measure the key by pointing it towards the light and shading off the kicker with your hand -- you don't have to use a flat disc.

 

As for metering kickers -- I always set the brightness of backlights and kickers by eye anyway. Since it is a kick-light glancing off of a cheek, an incident meter reading won't be entirely informative anyway since the glossiness of the skin matters.

[janusz sikora]

... just point Disk to your Key and read footcandles. Point your Disk toward Fill (Kicker) read footcandles. Apply your formula.

And don't call it a Contrast Ratio as this is different story altogether.

[Jaan Shenberger]

i feel that using ratios to gauge a kicker is not a good idea, since a kicker is going to react very differently to different subjects/wardrobe/surfaces.

 

i'd advise spot metering the kicker's cast on the subject from the camera's direction, and then just adjusting it in relation to the key/fill exposure.

[David Sweetman]

stupid quick newbie question - what would be the difference between a kicker and a backlight?

[Stuart Brereton]

A backlight is positioned directly behind the subject, hence the name. A kicker is generally positioned behind and off to one side. It's also sometimes called a 3/4 backlight.

[Laurent Andrieux]

... just point Disk to your Key and read footcandles. Point your Disk toward Fill (Kicker) read footcandles. Apply your formula.

And don't call it a Contrast Ratio as this is different story altogether.

 

Why do you consider it would not give you the contrast ratio ?

[David Mullen ASC]

Point your Disk toward Fill (Kicker) read footcandles. Apply your formula.

And don't call it a Contrast Ratio as this is different story altogether.

 

A Kicker is not Fill.

 

I guess the contrast ratio is Key + Fill : Fill only, since the fill adds to the key level to some minor degree (at least, considering the low amounts of fill I tend to use.) I've never had a use for figuring out the ratio myself -- what's the point of having the mentally transcribe a 4:1 ratio, let's say, into relative f-stops? I just decide how many stops under key I want to use for the fill level.

[Laurent Andrieux]

From what I understand, a kicker is an effect. I would not consider it as the fill in either, but I think it's usefull to know the ratio and the effect value as well, just to maintin continuity along the scene, as well as to evenly shoot something that should match another day... But I think that deciding how many stops under key you want to use for the fill level (as well as for effects) is really the same thing. The contrast ratio has to be calculated from fc or lux, but having the values in stops is the same idea, to me. If you have a 1:2 ratio, it's 1 stop, etc.

[David Mullen ASC]

Yes, but let's say I set the fill level to be 2 1/2 stops below the key because that's what I like. What's the point then in calculating what the ratio would be? 5.4:1??? I don't see the practical value.

[Laurent Andrieux]

Oh yes, sure.

[Guest GARRETT HARTMAN]

Why do you consider it would not give you the contrast ratio ?

 

Well, tell me if I'm wrong, jansik, but contrast ratio is usually applied to film. Contrast is the difference b/t the darkest and the lightest part of a picture. So it's usually used to describe the product of all the lighting. Now, lighting ratio, that's what I figure all this discussion is all about...

 

GH

[Laurent Andrieux]

I know of object contrast ratio(what is being discussed here, that you also call lighting ratio) and of image contrast ratio, my self.

[Guest GARRETT HARTMAN]

I know of object contrast ratio(what is being discussed here, that you also call lighting ratio) and of image contrast ratio, my self.

 

Well, when you start talking about an object's contrast ratio, it may just as well be moot, considering that when you measure a lighting setup, you're probably using a grey card, or a incident light meter (white dome). So the lighting ratio (of the lights themselves) and the object contrast ratio (of the grey card) will be the same -- or else why use a grey card as a measuring stick? But when you get out of the very controlled environment of grey cards, and start having to deal with different objects' _reflected_ light measurments, then the distinction between the lighting ratio and the various objects' is all sorts of screwed up. That's when the DP -- correct me if I'm wrong -- has to make the decision of what is important in the scene, and what style she's going for.

 

GH

 

 

some references:

http://www.filmmakers.com/features/cinematography/cine7.htm

http://www.acecam.com/magazine/gray-cards-3.html

http://www.sekonic.com/IncidentVsReflect.html

[Laurent Andrieux]

Yep sorry, object contrast ratio takes reflectance in account, (I'm a bit puzzled these days, for diffrent reasons, I should not post before I recover... :( )

[David Mullen ASC]

Most DP's, when lighting a scene, will try and keep a consistent key-to-fill ratio, which sort of suggests incident meter readings -- i.e. you're not adjusting the fill levels for each object's reflectivity in each set-up, but providing a consistent fill level for all the coverage of the scene regardless of the object's individual reflectivity.

[Mario C. Jackson]

Sometimes I take my readings by pointing the sphere towards the camera. I dont see anything wrong with that.

Mario Concepcion Jackson

[David Mullen ASC]

Sometimes I take my readings by pointing the sphere towards the camera. I dont see anything wrong with that.

Mario Concepcion Jackson

 

If your key is coming from the side and you're pointing your dome towards the camera, you are averaging a reading between the key and shadow more than reading the key alone, which is fine if you want the key to be slightly overexposed. Whatever gets you the results you want.

[Jonathan Bowerbank]

You should definitely meter that kicker with a spot/reflective meter. Using your incidental reading and pointing the globe directly towards the light will give you an idea, but to be more accurate the spot meter is the way to go.

 

And be sure to meter that kicker from the camera's vantage point.

 

good luck!

[Stephen Williams]

Sometimes I take my readings by pointing the sphere towards the camera. I dont see anything wrong with that.

Mario Concepcion Jackson

 

Hi,

 

It's how you apply the reading you get thats important!

 

Stephen

[Paul Bruening]

Hey guys,

 

Can I add my own brand of confusion to this thread? I think David's presentation is the more common approach. You use the globe and block the lights you don't want in the formulation. Ratios are typically presented in textbooks as key + fill vs fill. The reading for the impact of kicker will usually be outside of that formulation. As another poster mentioned, backlights and kickers are usually an effect. Backlights help to separate subjects from backgrounds by rimming the subjects' outline with light. This was a bigger concern in the B&W days when subjects could fuse with backgrounds more often. Kickers can do the same job as backlights but are more often used in night exteriors to give an outline on the non-key (often) side of a subject but seem more naturally motivated than a backlight. That kicker is often run at a stop brighter than the key. Yet, since it is a "rimming" light, it usually is not added in the normal ratio formula. If the kicker comes so forward of the subject that it impacts the key then it's not a kicker anymore. It becomes a multiple key or a fill that is brighter than the key and therefore, then, becomes the key.

 

Does that make sense or am I totally full of it?

[Laurent Andrieux]

Sounds good to me. I think its just what Jansik ment. I see the point by now. Thanks.