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#1 nahid ahmed

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Posted 16 September 2005 - 02:35 AM

what happens if i only use the flat disc and not the white dome to measure lighting ratio? is it neccessary to use the dome to take final reading?
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#2 Dimitrios Koukas

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Posted 16 September 2005 - 07:09 AM

what happens if i only use the flat disc and not the white dome to measure lighting ratio? is it neccessary to use the dome to take final reading?

<{POST_SNAPBACK}>


Hello Ahmed
There is no problem using only the flat photodisk to do measure the light, it's just that u have to make more calculations.
Flat photodisk is been used rarely by me, but sometimes when I need to match scenes or sequences that I want the same light ratio then I use it.It's been used for helping the cinematographer to be sure about his lighting ratios of a scene, by getting separate readings from the subject.
U can do the same though in a dome photodisk, by covering with your palm the light that u want to exclude from the reading. (all lights are visible on the photo dome.)
With the photodisk:Measure your key light and then your fill light, right down or memorise the results,
Usually in a daylight scene I ll go with the fill light reading, and for night shots I will go with the key light.
But that's a general rule, and depends of what u re shooting.
Other wise u can just use an f or T stop, that comes excactly in between.
Let's says f/5.6 when your key is 8.0 and your fill is 4.0.
But this is also a general rule, cause u see i have no idea what is the subject u are filming.Also what are the lighting ratios that u want to use.
Regards
Dimitrios Koukas
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#3 Laurent Andrieux

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Posted 16 September 2005 - 01:22 PM

The flat disk is designed to meter an illumination, in either lux or foot candle. As it is a vectorial parameter, the flat disk allows you to meter an illumination in one direction.

Then you can can read the illumination provided by each sources in turn, pointing the disk in its direction.

When it comes to metering an exposure, meaning you need a F stop, you need to integer the different sources that light the point you are metering from, and you are then supposed to direct the light meter to the camera's direction.

That's the purpose of the dome, also called integerating dome.

The relationship beetween illumination and F-sop is given by the common rule :

N²/t = E.S/250

Where N is the T stop, t the exposure time (1/48, 1/50, 1/60 s...) ; E the illumination in lux, considering 1 fc is about 10 lux ; S the Iso sensitivity you work at.

In television, DoPs often work with illumination, then use the flat disk. In cinema, most DoPs look for a f Stop metering and then use the half-spherical dome.

As Dimitrios explained, you can also use the light meter as to always meter F-stop, and though you are using the dome, point at a precise source, not the camera, aloowing you to quantify contrast. It's then up to you to figure out what f-stop you set on the camera, though pointing the meter to the camera is supposed to give you the "good, average" metering.

Afterwards, it is a question of interpretation. As Dimitrios explained, according to the effect you're looking for, you will interprete the results as you wish...
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#4 dbledwn11

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Posted 17 September 2005 - 02:00 PM

i'd got the impression that one could use the dome for an 'intergrated reading' (as laurent might put it?) if the key light was less than 90degrees from the camera axis in which case light from key and fill are both falling on the same area (i believe this is total key or something?)

then getting a separate fill reading (blocking off key light) and from that determining your key/fill ratio.

now as an extension to the original question I was just wondering if professional DoPs ever truelly worry themselves with a key/fill ratio i.e would you approach a scene saying to yourself "i want a 4:1 ratio?"

I would assume that if it did happen to be a 4:1 ratio (2 stops) and the film stock u are using has a good latitude then you know the contrast won't be so big.

sorry to ramble but I am just starting out and keen to know if I am even on the right lines. thanks for your time
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#5 Michael Nash

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Posted 17 September 2005 - 04:11 PM

I was just wondering if professional DoPs ever truelly worry themselves with a key/fill ratio i.e would you approach a scene saying to yourself "i want a 4:1 ratio?"

I would assume that if it did happen to be a 4:1 ratio (2 stops) and the film stock u are using has a good latitude then you know the contrast won't be so big.

<{POST_SNAPBACK}>


I may be an anomaly but in 15+ years of shooting I have never, ever, ever, used "contrast ratio" when lighting. ;) I DO however think about how dark I want the fill side to go, as expressed in f-stops on the neg or luminance on the screen. So in essence, that's the same thing just expressed differently. It's actually more like the zone system, where I'll say "I want the fill side of the face 'half way' down," or about 2 stops under or zone 3.

But regardless, you're right about the film stock having an impact on the contrast ratio. In time you learn how to light by eye more and use the meter less, and then make adjustments for the contrast of the film stock/processing/printing you're using.

When you're using a fairly "normal" film stock and techniques, you might meter more using the dome just to confirm a middle exposure and let the rest go (trusting your eye for contrast ratios). But if you're doing something a little more visually abnormal that might monkey with the contrast of the print (like doing a bleach bypass on the print, for example), then you'll probably use the disk or isolated measurements to make sure the fill level is in the correct range for your print.
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#6 Dimitrios Koukas

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Posted 18 September 2005 - 07:32 AM

i'd got the impression that one could use the dome for an 'intergrated reading' (as laurent might put it?) if the key light was less than 90degrees from the camera axis in which case light from key and fill are both falling on the same area (i believe this is total key or something?)

then getting a separate fill reading (blocking off key light) and from that determining your key/fill ratio.

now as an extension to the original question I was just wondering if professional DoPs ever truelly worry themselves with a key/fill ratio i.e would you approach a scene saying to yourself "i want a 4:1 ratio?"

I would assume that if it did happen to be a 4:1 ratio (2 stops) and the film stock u are using has a good latitude then you know the contrast won't be so big.

sorry to ramble but I am just starting out and keen to know if I am even on the right lines. thanks for your time

<{POST_SNAPBACK}>


Friend,
If you point to the camera and get a reading, you just get the average reading of the lights, based on a surface that reflects 18% of light.(skin) It is the total light we may say that falls on your subject.This will get you out of your shoot safely.

If you want to know the key/ fill ratio then u turn off the fill or block it from the photo dome, you read the f/stop , u turn the key off and u turn the fill on and u get your reading.This will give you the ratio.
A reading pointing the camera with all the lights on, will give you an average light reading of all the lights.Usually I block the backlight from the dome
, and also the skylight, overcast sky.

It's good to know the lighting ratio of a scene, especially if you are in a weird schedule, that u have to match a scene of same sequence after three months.
e.g, the actor (in real life)is on coke and he has to go in a hospital to get rid of his habits and then come back to continue...
Hehe, just jokin here, but you never now.

As all people do and suggest, I will suggest to you too, to do your tests, and don't rash in a production before knowing your film, your lenses, and the laboratory results.
Make some film latitude tests, grey scale shootings, and see some of this projected.
Like Mr. Nash said, eventually u will know it ''by eye''.
Other things u see in the scene, others thru the eypiece and others the emulsion sees.

For a normal observer, some things that you are shooting look much like the set is lit for a comedy, but the result is the drama u wanted in the first place.
All has to do with what point of the latitude u put your exposure.

I don't know what excactly do you mean by saying ''the contrast won't be so big''.
Do you mean that it will be soft? normal? or hard?
You can change the films contrast by only overexposing it (make it harder). With the same lighting ratio, but different f/stop.How much? It depends from the film.
I hope I have been helpfull.
Dimitrios
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#7 dbledwn11

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Posted 18 September 2005 - 02:08 PM

first of all thank you very much for the advice.

however, i'm just a little confused since my original post had been based on something i'd read in a lighting guide, which went something like this:

if the key light is less than 90degrees from the camera axis then you read the combined intensities of key and fill - this was where i came up with "total key"

then blocking the key light and reading fill would give the key/fill ratio.

it then goes on to say that if the key light is more than 90degrees from the camera axis the key and fill will no longer strike the same surfaces and will therefore not combine for greater brightness

in this case reading the key and fill separately, as you describe in your reply dimitrios, is the appropriate method.

also when i said ''the contrast won't be so big'' i mean like a ratio of 2:1 would be less of a contrast than 6:1. in that case i suppose you would call 6:1 "hard."

Edited by dbledwn11, 18 September 2005 - 02:09 PM.

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#8 Laurent Andrieux

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Posted 18 September 2005 - 03:07 PM

We've been considering pointing the meter towards the camera, until nnow, but one could just as well point it toward the source one wants to meter, instead of blocking the other lights only...

Works great for metering the fill. As fora source tha would be further away than 90°.

The direction you point the meter at is a question that you should think of in every cas. The metering instructions you've read are especially accurate for still photo, as light meters are basically designed for it.

Imagine a shot where the camera is tracking and travelling around a subject, though you want an iris set the same all the way thru... This is when this 90° problem shows its limits, pointing "towards the camera".

Also, when you cut and match shots, the keylight that was less than 90° away from the camera might be much more further in another shot.

This problem is even obvious when you have the KL that happens to be backlight for one or several shots in the sequence...

The only way to meter correctly the light that comes to a point in such situations, is to point the meter to the source's direction better that towards the camera (where the hell is it bt the way ?), even though you eventually block some other sources actually...

It also is a matter of experience...
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#9 Paul Bruening

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Posted 18 September 2005 - 03:17 PM

Hey db,

Contrast ratios are a matter of asthetic taste. However, you do find common practice out there. A 2:1 (key plus fill at 2 and fill only at 1) is a commonly used average. This allows the shadows to sculpt facial and bodily features thus improving the illusion of depth. 3:1 is considered a high ration that allows the dark side to get at least a little exposure and not burn the high side. 4:1 and over means losing the dark side almost completly to darkness.

Even with 2:1 being a rough "middle ground" it still throws powerful shadows. 1.5:1 can be a little more gentle when you want slighter shadows. Glamour lighting in color can be 1:1. Adding a fill light under the lens is a common way to hide facial lines in older female subjects.

Hope that is useful.
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#10 Glenn Hanns

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Posted 18 September 2005 - 07:28 PM

what happens if i only use the flat disc and not the white dome to measure lighting ratio? is it neccessary to use the dome to take final reading?

<{POST_SNAPBACK}>


The dome was intended to represent the shape of a face, So yes, it is used mainly to give your final exposure reading which takes into account all your light sources. If you use the dome to read individual lights you might get a lower reading than using the disc. This is because of a phenomena called the "limb" effect. Using the disc to identify individual light readings is a far more accurate way.
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#11 Dimitrios Koukas

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Posted 21 September 2005 - 03:35 AM

We've been considering pointing the meter towards the camera, until nnow, but one could just as well point it toward the source one wants to meter, instead of blocking the other lights only...

Works great for metering the fill. As fora source tha would be further away than 90°.

The direction you point the meter at is a question that you should think of in every cas. The metering instructions you've read are especially accurate for still photo, as light meters are basically designed for it.

Imagine a shot where the camera is tracking and travelling around a subject, though you want an iris set the same all the way thru... This is when this 90° problem shows its limits, pointing "towards the camera".

Also, when you cut and match shots, the keylight that was less than 90° away from the camera might be much more further in another shot.

This problem is even obvious when you have the KL that happens to be backlight for one or several shots in the sequence...

The only way to meter correctly the light that comes to a point in such situations, is to point the meter to the source's direction better that towards the camera (where the hell is it bt the way ?), even though you eventually block some other sources actually...

It also is a matter of experience...

<{POST_SNAPBACK}>


Laurel,
Very well puted here, that u can turn the meter away of the camera to do any measurement, I am using this teqhnique a lot, it's just it's (as u say it) a matter of experience.
U see what lits the dome so u can tell what u read...I do the same with backlights too, But if you want to be more accurate I am suggesting to use a flat disk, instead of the dome, for separate readings.
As for the lights that overlap when less than 90 degrees,it's something that happens but, if u have a face looking towards your lens, it's nose will block the fill off for example, or if the fill is placed at 90 degrees then they mostly don't overlap.
You see we are talking about general principles here, so all the comments we make have a subjective nature.
Everything is about what you are shooting, and where, how many lights an actor crosses, and how many ''key'' lights or fills will cross in a scene.
I can guess u walk the scene with your photometer and u get a tracking readiing like most of us do.
Then u correct any hot spots if u don't want them, or u add some more.
Dimitrios
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#12 Laurent Andrieux

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Posted 21 September 2005 - 06:06 AM

Who's Laurel :huh: :blink: ??

:)
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