HD Camera Measurements and getting the most out of your camera


Introduction
Image quality all starts with the correct alignment of the camera which is used to capture the scene either on location or in the studio. The conversion of light into electrical signals involves many processes which can affect the quality of the image. Understanding these various adjustments that are necessary, using a suitable camera chart and waveform monitor are critical in ensuring a high quality image and
preventing mistakes from occurring – mistakes that are costly to fix in the post production process.
When looking into camera setup procedures, a number of different measurements are performed to ensue that the camera can perform to its full specification regarding dynamic range for colour and luma or linearity of electric signals produced. In a reasonable amount of cases, directors and cameramen would want to exploit certain effects to create various different looks to reinforce a feeling or mood of the video produced. In those cases, camera technicians and operators must assist to ensure that the output signals maintain the desired look and feel while still meeting specifications.
In the course of this series of small introductions to camera measurements, it is the objective to provide guidance to the novice user and provide a useful refresher for the seasoned expert in this field. In particular, we will cover white shading fundamentals, white shading measurements, setting up correct colourimetry and exposure, greyscale adjustments using a waveform monitor, colour adjustment using a vectorscope and camera matching. These five topics will be covered in a series of five short and easy to read contributions.
White shading
In setting up a camera, white shading is an important function that is often overlooked or even worse, performed by an inexperienced person using inadequate equipment. The basic tools required are a high quality High Definition (HD)
waveform monitor such as the WFM7120/7020 or the portable WFM5000 and a very even light source such as the diffuser panel of the DSC Ambi Illuminator. For optimum signal accuracy it is preferred to use an HD-SDI output from the camera, but the process can be similarly performed in standard definition (SD).
Every camera operator should be familiar with white shading techniques. Overall brightness must be even across the whole image. In the case of studio cameras with fixed lenses and shooting in the same controlled environment, white shading
will invariably need to be checked less frequently than a digital filmmaker on location who is changing lenses frequently. These measurements should be done in a controlled environment where a qualified operator can ensure each channel is
properly calibrated. The resulting adjustments can then be saved as scene files so that they can be quickly recalled on location when changing lenses or filters.
White shading is the process of electronically compensating for errors in brightness or colour, primarily generated by the optical system of the camera. A frequent problem that is encountered is the centre of the image appears brighter than the edges. This type of aberration is caused by differences within the camera’s optical system components such as the prism or optical block and is most commonly found within older lenses. Differences may also occur when changing from one filter to another. Note that there is a difference between shifts in colour and neutral shifts in evenness of the image.
For instance, adding a Wratten neutral density filter will invariably lower the brightness and give the image a warmer look. Changing lenses can also change colouration, but this effect is typically minimal. Larger differences in edge to edge
brightness are often more common, with a hot spot in the centre of the image.
To make white shading adjustments of the camera, a very even light source (such as the Ambi Illuminator from DSC Labs) will provide a precise illuminated field with evenness that can be controlled to close tolerances. Although a number of light sources could be used on the Ambi Illuminator, it is purposely illuminated by the industry standard 3200K tungsten source. Tungsten illumination has a spectral
curvature that is flatter than an HMI (Hydrargyrum Medium-arc Iodide) or fluorescent. Gas discharge light sources may not evenly light the Ambi Diffuser and are not recommended for white shading. In the next issue, the measurements performed for white shading are discussed and described.


Tags: hd camera | test and measurement | iss026 | camera measurements | alignment | engineer | technician | white shading | cameraman | wratten neutral density filter | tektronix | N/A
Contributing Author N/A

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