Getting the Exposure Right
The best looking video is video that is exposed properly. The key concept to understanding exposure involves the amount of light that reaches the camera’s sensor – too much light and the image will be blown-out, or overexposed; too little and the image will be dark, or underexposed. Bad exposure is simply unpleasant to watch and is, therefore, not acceptable to a professional broadcast.
Getting the right amount of light to enter the lens will produce excellent results. It’s not as hard as you think, not if you commit to memory the following checklist in this order:
- Neutral Density (ND) filter
- Shutter Speed
How you set each of these items in the checklist will help you control the amount of light that enters the lens.
First, make sure the camera is set to MANUAL operation mode. On the lower left side of the camera, slide the AUTO MANU button to MANU. Don't trap yourself into thinking that AUTO will make operating the camera easier. AUTO is useful only under certain circumstances when it becomes difficult to manually adjust exposure. But when you want the most control, always use MANUAL.
Neutral Density Filter (ND)
The ND Filter is the first thing you should check on your camera. Think of the ND filter as “sunglasses” for the camera - you put sunglasses on outside on a sunny day and take them off inside. The filter is used only in bright environments, typically outdoors on a sunny day. It’s not necessary to use the ND filter indoors, so make sure it’s set to OFF so you can get a fuller range of f-stops.
Strictly speaking, Gain electronically boosts the video signal under low light conditions to artificially make the image look brighter. The common misconception is that Gain allows more light to enter the lens - it does not! The Gain is applied only in low light conditions and only as a last resort to get more brightness out of the image.
Because Gain is signal strength, it’s measured in decibels (dB). You can see the Gain values displayed in the viewfinder. Zero dB means there’s no gain. Every 6dB of gain doubles the brightness of the picture. But the more gain gets added, the more noise appears in the image, which is another reason why Gain is used only as a last resort.
Gain value appears in the LCD Monitor. In this illustration, the Gain is set to 6dB.
It’s important to understand that Gain only amplifies the video signal, but it doesn’t add detail that is currently not visible. At high gain settings, the image will start to look grainy.
Before you can adjust the Iris and Shutter Speed for proper exposure, always start by turning the Gain off. Then open the Iris to its maximum setting and use a slower shutter speed to allow as much light to enter the lens as possible.
Remember, you’re not the only one who checks out these cameras; the student before you might’ve left on the Gain and that could ruin your shot unless you do something about it first.
Gain on the camera is adjusted between L, M and H settings. The L setting will always be 0dB, or no Gain. The M setting will likely be set to 6dB and the H will be 12dB. These values can be changed in the menu settings.
You can also use the SELF/PUSH SET thumbwheel control on the camera to cycle to the GAIN settings. With the Gain value highlighted in the viewfinder, move the thumbwheel up or down, which will apply gain (or decrease it) in 1dB increments for more precise control.
If you have to use Gain to make the picture look brighter, then in all likelihood, your image will be underexposed and look like rubbish. Seriously, getting the exposure right is crucial to a professional production. Add lighting whenever possible, which is a must especially when shooting interviews indoors.
Think of the shutter as a gate that opens and closes; when closed, light is prevented from reaching the sensor. Video shooters typically think of using the shutter only when they are recording scenes with fast action, such as sports events. Changing the shutter to a faster speed helps prevent motion blur. But faster shutter speeds means the gate isn’t open long enough to allow as much light to enter the lens. To compensate, video shooters add more light to get the same level of exposure (opening the iris further to allow more light to enter the lens). When the shutter speed is slower, the gate is open longer and more light can enter the lens, but at the slower speeds there’s the risk of seeing motion blur even when people are walking or waving.
In almost all normal circumstances, the minimum value to set Shutter Speed, and avoid motion blur, is 1/60. If you’re shooting under fluorescent lights, you definitely need to keep the shutter speed at 1/60. In North America, fluorescent lights always flicker at 60Hz frequencies (In Europe the frequency is more like 50Hz). Changing the shutter speed to anything other than 1/60 might cause noticeable orange bands or scrolling waves in your video.
Become aware of the Shutter Speed settings on your camera before you change the Iris so you can get the most light to enter the lens.
This is the last item on your checklist, which you can adjust only after you’ve ascertained the other items. The aperture is simply the hole in the lens where light can enter; the Iris is the mechanism that controls the size of the aperture and the amount of light that gets through. The size of the aperture is measured in f-stops, which describe how much light enters the lens.
F-stops are numbered in the following sequence: f/1.4, f/2, f/2.8, f/4, 4/5.6, f/8, f/11 (these values vary with the type of lens). The smaller f-stops correspond with a larger aperture, therefore, an f/2 admits more light than an f/8. With each setting, or stop, half as much light enters the lens. In other words, an f/2 admits half as much light as an f/1.4.
In most cases, you won’t need to know specific f-stop values to get the right exposure. Just understand that f-stops describe how much light enters the lens. In a low-light environment you would need to use a lower f-stop value to get as much light to enter the lens as possible.
Press the IRIS button, located just behind the IRIS ring, which is at the base of the lens. Note the f-stop values in the LCD Monitor/Viewfinder. Turn the IRIS ring to change the values. When the values say OPEN, the aperture is as open as it will get under the present lighting conditions.
Automatic or Manual?
The camera is capable of judging how much light is necessary to expose the picture properly. Auto-exposure can free you up to concentrate on other things like composition and focus, but professionals rarely rely on this, and neither should you. It’s crucial that you understand how to manually control the exposure, which lets you decide what’s the most important element in the scene to expose for. Also, preventing the camera from changing the exposure whilst recording is another compelling reason to use manual exposure. However, you can certainly use Auto-Exposure as an assist to let the camera judge what it thinks of your exposure settings.
It’s easy to quickly pop into Auto-Exposure mode by pressing the IRIS button. In the LCD Monitor/Viewfinder, the letters STD will appear next to the f-stop value - this is for the Standard Auto-Iris control. Pressing it again will switch your camera back into manual mode, which will then lock the exposure to what the camera decides. But be careful with this – you might expose on an area in close-up, but when you widen out other areas of the shot will be over-exposed.
Don’t judge the exposure using strictly the LCD monitor. In fact, it’s a bad idea to trust what you see in the LCD monitor. The reason why concerns the backlight setting for the LCD, which changes the brightness of the image in the monitor. If your LCD monitor represents video as brighter than it is, you could be underexposing the video without realising it. If the LCD monitor is too dark, then you could end up overexposing the image simply to make it look brighter on the monitor.
So if you can’t rely on the LCD monitor, then what are your options? It’s important that you remove the subjective guesswork in what is a properly exposed image. Fortunately there are some tools to help you do just that.
Zebra Stripes – USER 6 button, also labelled ZEBRA on the camera
You can apply a monitoring tool called Zebras that show up in parts of the image that are brighter than a predetermined level. Zebra stripes help you know at a glance what areas are too bright, and possibly overexposed. You can then reduce the iris to eliminate Zebras, but it’s not crucial to eliminate them entirely; it might be unavoidable in some things like the sun or light bulbs, but it's best to avoid Zebras showing up in the sky and even on white buildings or cars. When shooting an interview, you don’t want to see Zebras on the face except maybe on little bits of the forehead or nose where they tend to be shiny.
Press the ZEBRA button located at the lower left side of the camera near the rear. Pressing the button will display zebra stripes over areas of the picture that are overexposed.
The stripes are drawn only on the viewfinder image and are not recorded into the picture.
Pressing the ZEBRA button again will turn them off. But pressing the button also allows you to cycle between two brightness values at ZEBRA 1 80% and ZEBRA 2 100%, which represent the minimum and maximum brightness values that will trigger the Zebras. 80% is used as a guide for exposing faces and 100% is used to help prevent what are called "blow outs", which happen when parts of the image are too bright for the sensor to resolve so the video signal overloads. For faces, highlights on the forehead and on the nose might be bright enough to trigger the Zebras, but shouldn't be overblown.
Waveform Monitor – USER 7 button, also labelled WFM
The DVX200 uses a built-in Waveform Monitor (WFM) to help you judge your video’s exposure levels. The WFM basically shows the relative brightness across the image in a 2-D scale. Learning to read the WFM at a glance will tell you whether the image is over-exposed, underexposed or clipped, and it’s a far more useful tool than Zebras.
The graph is essentially a mathematical representation of pixels according to the brightness (luminance) of the image; the higher up the pixels go on the scale, the brighter the image – the lower the pixels, the darker.
The scale uses measurements in IRE (Institute of Radio Engineers). The brightest portions of the image might reach up to 100 IRE (the top solid green line) and dark images might be almost down to 0 IRE (the bottom solid green line), or pitch black. Levels about 50 to 55 IRE represent a medium grey. Ideally, you want to see the brightness distributed across the full scale.
Over-exposed parts of the picture will show more pixels grouped at the top of the graph; underexposed parts will have more pixels grouped at the bottom. A properly exposed image should show the pixels more or less evenly distributed throughout the image.
To display the Waveform Monitor in the LCD Monitor/Viewfinder, press the WFM button, located above the ZEBRA button. Press again to restore the display to normal viewing.