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Spot metering


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#1 Gonalo Cardeira

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Posted 12 January 2013 - 12:49 PM

Hey,

I've been with a huge doubt on my head.

In digital cinema cameras the viewfinder tells us how the exposure will be right? if darker or brighter.
In film cameras that's not possible right? We see what the camera will record but not if it's darker or brighter.

So we need to use a spot meter, for example, in a scene where we film a face, we read the light but if we use that reading that won't be a good exposure because it's not a grey object right? How do I know how many stops i have to over or sub expose to get a good exposing?
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#2 Paul Bartok

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Posted 12 January 2013 - 01:30 PM

Hi Gonalo, It's abit hard to make out what your saying. If I read it correctly you want to know about spot metering.

A spot meter reads the light reading of a point you point it to, it is the actual reading from that point only and is not a calculated from the overall scene as usually your Video camera will. This is why you want a spot meter for the control, for example if I want to meter for the highlights I can point it to the brightest part and get my reading, the meter will have these options:

T priority and F priority T is Shutter speed and F is F-stop

So lets say its a low light scene but you want to expose for the face you punch in say T:1/60 or if you have a cine one 24fps
ISO: 500 we point it at the the face, and get a reading such as F:5.6. and you can go from there, see if you need to lift your ISO or F stop etc.It's kinda like a automatic calculator, so If your 2 stops under the correct exposure you can either put up your ISO two stops ie.2000 in this case or get a faster lens
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#3 Guy Holt

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Posted 12 January 2013 - 03:34 PM


 if we use that reading ( from a spot meter) that won't be a good exposure because it's not a grey object right? How do I know how many stops i have to over or sub expose to get a good exposing?



This is a complicated subject and every DP has their own approach. Basically there are two types of meters: incident and spot. Incident meters read the light falling on your subject. Spot meters read the light reflecting back from your subject. An incident reading gives you an exposure that would expose an 18% gray card as 18% gray if you held it in front of the camera in the same light. Incident meters enable you to peg the key tone (18% gray) in this fashion.

A spot meter is then typically used to take reflective readings to see how other objects will expose relative to the key tone that was pegged with the incident meter. The thing to remember about spot meters is that they want to expose everything as mid gray. For instance, if you expose a black piece of paper with the reading of a spot meter it will appear as mid gray after processing – likewise for a white piece of paper. But, if you place an incident meter down on the black piece of paper and expose for the incident reading the black paper will be black, and the white paper will be white, after processing because you pegged the key tone and exposed for mid gray by using the incident reading.

You use the reading from the spot meter to be sure that the object you are metering will be within the exposure range (characteristic curve) of the film stock you are using. If the stock has a nine stop range (four stops over before detail burns out, andr three stops under before detail blocks up), and your reading of a dark object is four stops under your key tone, you have two choices. You can open up and expose for the shadows (blowing out your highlights more) or you can throw some light into the shadows to bring the reflective value of the dark object within the exposure range of the film (onto the straight portion of its’ characteristic curve) without changing the key tone value or blowing out the highlights. In this fashion you fit the contrast range of your scene into the exposure range of the film emulsion you are using. Of course this is only the starting point. From this “correct” exposure a DP will further manipulate the relationship of the contrast range of a scene to the exposure range of the film stock to create a desired effect (see Bob Richardson’s work in Django.)

If you don’t have an incident meter, but want to peg the key tone under your subject's key light, an old trick is to take a spot meter reading of the palm of your hand under the key light and open up one full stop. This will give you a close approximation because the average Caucasian flesh tone is one stop more reflective than 18% gray. I hope this helps.

Guy Holt, Gaffer, ScreenLight & Grip, Lighting & Grip Sales and Rentals in Boston
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#4 Guy Holt

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Posted 13 January 2013 - 11:34 AM


 From this “correct” exposure a DP will further manipulate the relationship of the contrast range of a scene to the exposure range of the film stock to create a desired effect (see Bob Richardson’s work in Django.)
 



I have posted on my server at http://www.screenlig...ing_Example.jpg an example from “Miller’s Crossing” that might help make sense of my post above.

It is a common fallacy that dark scene’s like this are “underexposed.” This scene is not underexposed, but rather the reflective values of the objects in the scene are carefully balanced (placed on the film’s characteristic curve) relative to the key tone by lighting so that most of the scene remains dark but serves up the full contrast range the film emulsion is capable of. In other words, even though the scene is correctly exposed, nothing in the scene is “correctly” exposed. The flesh tones are underexposed and the lampshade is over exposed in order to create the mood of the scene.

In an instance like this, the DP would not use a meter (incident or spot) to find the exposure of the key tone; rather, he would choose the exposure of the key tone from the outset - say T5.6 for deep focus. And, having balanced the elements of the scene to that exposure using either his incident or spot meter, he will “lock it in” for lab timers or transfer colorists, by giving them the key tone (by properly exposing a chip chart with an 18% gray patch) as a reference at the head of the scene. Without providing the key tone, a timer or colorist will not know how dark the shadows should be or how bright the highlights should be because there is no other reference value at full exposure by which to calibrate the brightness of the scene.

Here are a few techniques, all of which are evident in this shot, that one could use to balance the lighting to create a dark scene without under exposing it.

1) Edge light objects in frame. Use reverse keys for talent and underexpose flesh tones by at least two stops or more. As long as you define the contours of your subject with subtle underexposed edges, don’t be afraid to let your talent fall off into black. There is a scene beautifully lit by James Merifield in the “Deep Blue Sea” of Rachel Wiesz and Harry Hadden-Paton standing in a dark alley way. They are back light by a practical at the end of the alley. Their contours are defined by the rims motivated by the practical, but otherwise their flesh tones fall off to complete shadows. James Merifield probably used a spot meter and negative fill to make sure that their flesh-tone would fall off the emulsion’s characteristic curve and reproduce as a pure silhouette. Sonnenfeld probably did the same in this scene to assure large parts of the frame had minimal detail.

2) I personally believe you should always have a hot spot in a frame – a practical in the scene or something in the deep background. You can shift your overall exposure in the camera or in post to create a dark scene, but without a hot spot reference in the frame it will lack contrast and look underexposed. A hot spot in the frame serves as a reference point and creates contrast. Practicals should be close to clipping and appear to be the source of light in a scene.

3) Don’t try to light your talent with only practical’s because they will blow out – the hot spot in your scene has to look natural. Not only is supplemental lighting required to light your talent, but you must also treat the practicals to make them look realistic. I find that practical lamps never look convincing unless one treats the lampshade as well as boost the bulb wattage. That is because if you stop down to keep the shade from burning out, the output of the practical, on the table it sits on or the wall its on, looks rather anemic. I find you get a more realistic look if you boost the wattage of the bulb and line the inside of the shade with ND gel. It is a delicate balance to obtain.

You can obtain this delicate balance without a monitor, by using the old school method with incident and spot meters and a selection of practical bulbs including PH 211, 212, and 213 bulbs. Years ago Walter Lassaley, BSC, instructed me to balance practical’s such that an incident reading of the direct output one foot away from the bulb is one stop over exposure. I have found that rule of thumb gives a realistic output to the practical - the light emitted downward onto the table top and upward onto the wall or ceiling is realistic. After establishing the practical’s output using an incident meter, you then use a spot meter to determine how dense an ND gel is needed to line the inside of the shade to place the brightness value of the shade on the characteristic curve of the emulsion so that it does not too hot and without detail.

4) Define the edges of your frame with a little detail. As long as you define the edges of your frame with a little detail, as Sonnenfeld does here, you can leave most of it black without it looking under exposed.

5) Soft sources like China Balls and Kinos are the wrong kind of fixtures for this kind of scene. You will need fixtures that you can easily control because you will need to cut them off large parts of your set. It will be hard to keep china balls and Kino Flos from spilling light all over the place and filling shadow areas that you want to keep dark. Fresnels with light diffusion inside the doors, cut with flags and nets, will give you the control you need. Spot meter readings of objects on the edge of the frame, like the upholstered chair on the left, will tell you if they are within the exposure range (characteristic curve) of the film. If they are not, use a little light to bring out detail that will define the edges of the frame as Sonnenfield has done here with the chair.

Guy Holt, Gaffer, ScreenLight & Grip, Lighting and Grip Equipment Sales and Rentals in Boston.
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#5 David Cunningham

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Posted 24 January 2013 - 10:06 AM

I just wanted to say that these explanations are fantastic and very helpful. Also, that frame grab is amazing! That is perfect mood lighting that I dare a digital DP to reproduce without major work in post.

Guy, I'm in Lowell, MA. I'll be sure to contact you if I need lighting in the Boston area!

Dave
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#6 Chris Mirden

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Posted 27 January 2013 - 12:31 AM

Another great explanation of what you are researching can be found in Ansel Adams' book #2 "The Negative". Adams basically invented the "Zone System" which is what spot metering is. He was dealing with B & W negative still film but it is the foundation of of spot metering. The book is still very much in print and very much worth the read.

One more thing to keep in mind is that exposing for negative film is almost the opposite as exposing for digital film when it comes to if you should overexpose or underexpose when in doubt. Film is much more forgiving at the high end (overexposing) and digital is not. With negative film you almost want to over expose 1/2 a stop and will typically have no problem darkening the image. But with digital, when you overexpose or "clip" the film, that's it. The information is gone - you can't bring it down. And conversely if you underexpose film by mistake, it's card to bring it up - because with it you will also bring up the grain (which looks like digital noise).

I made the HUGE mistake of shooting an entire 16mm film about 1/2 stop underexposed because I was concerned about overexposure. You don't want to make the same mistake I did. (If you are shooting positive film - like a lot of what 8mm is) Then go for the best exposure you can, it's not the same as negative film - and your approach might be a bit more like shooting digital ...... sort of
:)

Edited by Chris Mirden, 27 January 2013 - 12:34 AM.

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#7 Guy Holt

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Posted 27 January 2013 - 08:30 AM

If you are shooting positive film - like a lot of what 8mm is... then go for the best exposure you can, it's not the same as negative film - and your approach might be a bit more like shooting digital ...... sort of


Reversal film stocks (both 8mm & 16mm) are more like digital in the sense that they can dig deeper into shadow areas. If a negative stock has a nine stop range (five stops over before detail burns out, and four stops under before detail blocks up) a color reversal stock, like Kodachrome, will have a seven stop range (three stops over before detail burns out, andr four stops under before detail blocks up.)

Guy Holt, Gaffer, ScreenLight & Grip, Lighting & Grip sales and rental in Boston

because they have a greater exposure range
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