How to put together a Lighting Kit for Video (Part One): Quantity of Light
By Sareesh Sudhakaran
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This article is written for the absolute novice who wants to put together a basic lighting kit for video work.
A lighting kit (unlike a computer or a camera) is an evolving thing. Accept this, and you’ll be all right.
What you buy today might lay unused tomorrow. You can never have enough lights, and every variable (every video production) alters your perception of what a lighting kit should contain. And of course, the more you shoot, the more you learn – this changes your aesthetic sense of what work for you.
No matter what the lighting scenario, there are some fundamental principles that limit your choices and ‘box’ you into specific options. You cannot circumvent these factors – they are issues imposed on you due to science or technology. You will face these problems even when you have ‘trucks-full’ of the best lights in the world.
Let’s look at how the quantity of light required over a given area determines our choices.
Quantity of light
The quantity of light is governed by three factors:
Light intensity (measured in foot candles or lux)
Area (measured in square feet or square meters)
Distance between Area and Light Source
For an overview of these terms, read Units of Photometry Parts One and Two, and Camera Exposure.
For any given scenario, no matter what your artistic intention is, you have to base the quantity of light required (in lumens) over the area you want to cover, and according to the intensity of the light source. Look at this simple example:
The sun throws light in all directions (3600), and the orange patch is our ‘party strip’. The area of the orange patch decides how much light it gets, since we can’t control the intensity of the sun (actually we can, but that’s for another day).
Let’s keep things as simple as can be. The sun delivers a total of 100,000 lumens per square meter (lux). If our party patch had an area of 30 x 30 feet (900 sq.ft or about 82 sq.m), then the lumens falling on it is about 8 million.
Why we’ll always need ‘big’ lights
Let’s say we tried to light the same patch with a 100 Watt tungsten bulb with an LPW rating of 10 (if you don’t understand these terms, please read the links I’ve given above – it’ll make your life a lot easier). This gives us 1000 lumens. For simplicity’s sake we’ll assume all of these lumens are available for our patch, and there’s no light loss at all. The lumens per sq.m (or lux) is 1000/82 = 12.2 lux.
So, how many 100W bulbs do we need to replace the sun? About 8,000. That’s 820 KW of light.
How many 18K HMIs (at about 85 LPW) does it take to do the same thing? About five at full spot. Now you know why we need large HMI lights for exterior daylight sets. Here’s the bummer: No matter how good a camera ISO becomes, and no matter how many ND filters you use, if you want to match the sun, you need to match the sun in lumens. Slice it any way you like, with any technology practically available today for filmmaking – big expensive lights are here to stay.
An 18K HMI kit runs about $25,000, excluding grip and transportation. Five of them will cost $125,000. 8,000 $2 100 Watt tungsten bulbs will cost $16,000, plus the cost of rigging them. Don’t forget that 5 HMIs are about 90 KW, while 8,000 incandescent bulbs are about 800 KW. Before you jump up and down thinking 8,000 bulbs are way cheaper, please call a few rental houses and find out how much it costs to rent generators (and trucks, with drivers and grips) to cover 800 kW. After you’ve recovered, continue reading.
Light and Exposure
From everything we’ve covered, here is a simple inaccurate (but good enough for our purposes) table that plots exposure alongside f-number, area and light output (Click to enlarge):
A lot of this has been covered already in the above links, but to summarize, note the following:
As explained in the Chapter on lenses from the Comprehensive Guide, you might want to plan on using an f-stop of at least f/2.8 for low-light scenarios.
In What Lens to Get?, I’ve explained how the typical shooting area falls in either a 1,000 sq.ft area (Hall, studio, large office, etc.) or 100 sq.ft area (room or office). I’ve given the Lumens and Wattage measurements based on these loose numbers.
One can say 1,000 lux is typical of studio setups. It allows you to shoot at up to f/8. If you’re looking for shallow DOF at f/2.8, you can get by with about 150 lux. I haven’t given the lumens vs ISO reading because that depends on f-stop as well as the shutter speed.
If you look at the low-light row, a small office will need about 632 Watts of a tungsten source. Is it a wonder then, that both Arri and Mole Richardson have a 650 Watt (sometimes called a Tweenie) Fresnel in their arsenal, which also happens to be their most popular light? Don’t forget that this approximate value covers the entire 100 sq.ft. area, so it’s a very rough guideline. Also don’t forget that you can always choose to have an area light ‘brighter’ or ‘darker’ than a typical scenario, but you don’t learn this until you’ve done it many times.
Typically, the most ‘necessary’ lights are 1,000 and 2,000 watts, which is why a 1K or 2K Tungsten Fresnel is extremely popular.
When working outdoors, tungstens are hugely inefficient when compared to HMI lights, and also need to be daylight balanced to match sunlight. For this reason, when the sun is providing light, either directly or indirectly (shade or scrimmed), an HMI is always the right choice.
If you discount ‘distant night’, which is almost impossible to shoot, the typical exposure latitude (dynamic range) of a ‘well-lit’ production is 10 stops. Any camera that can cover more than 10 stops is good enough as long as the lighting is controlled, and is in the hands of an experienced DP.
How to determine the quantity of light
If you step back and assess your requirements, you will notice that most lighting scenarios can be bracketed into spaces similar to what’s given in the above table. This is why lighting manufacturers stick to traditional wattages. It’s not something they conjured up over the weekend. It’s taken them a hundred years of experience to arrive at these figures.
As a general guideline, you could find the total quantity of light in the following way:
Find what scenarios you’ll be shooting in. From the above table, you can determine the approximate lux ratings you’ll encounter.
You can get more accurate lux readings with a good light meter like the Sekonic series. However, for the beginner, it is pointless to be this accurate.
Once you’ve found the typical lux ratings, you can determine the dynamic range of each scene. Isolate the highest dynamic range. Don’t forget to take into account specular highlights and sky.
If your camera can handle this dynamic range, you know you can play around with the lighting. If your camera falls short, you know you’ll need to find a way to light the scene to keep everything within the acceptable dynamic range. This is the first set of limitations you’ll encounter.
Determine the f/stop. This number is typically set for aesthetic reasons, but you could have your own reasons. From the above table, you can determine the typical EV ratings for each lux rating. Using the shutter speed (determined by the frames per second), EV and ISO range of your camera, you will be able to determine the practical f-stop range for your scene.
Once all these factors are accounted for, and you’re sure everything is under control, you can start to quantify light by making a table of the typical areas (in square meters) you’ll encounter. It is important to note that even though I’m using area as a ‘floor’ measure, actual photography involves shooting ‘planes of focus’. You’re shooting vertical planes, not the floor. Still, we are only looking for general guidelines. It’s not critically important to be accurate.
After you have a full list of areas (room sizes or spaces), you can multiply this by the lux rating to get the typical lumens necessary. This will tell you what the ambient lighting will be like, and what you’ll need to augment or match it.
Always err on the side of caution. You can always find ways to cut down light, but you can’t do anything much if you fall short of the required intensity.
Slowly but surely, you’ll arrive at a set of lighting ‘classes’ for your scenes. Try it, it’s not that hard. Then, it’s only a matter of finding out if you have the budget to match your requirements.
But hold on, we’re not done yet.
The next step is determining the kinds of light you’ll be using. That’s what we’ll cover in Part Two.
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April 1, 2013