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Chương 2: Ánh sáng.
Cũng như Chương 1 là Âm thanh, Chương 2 của Media College nói về phần kỹ thuật ánh sáng. Nhưng vì là Media nên bài viết thiên về ánh sáng kỹ thuật của thu hình (video), không phải Stage lighting. Tuy vậy, về phần cơ bản, 2 loại hình cũng giống nhau, đáng để tham khảo. (tuyenphuc).
Phần 1: Thuật ngữ ánh sáng.
Common Lighting Terminology
The light already present in a scene, before any additional lighting is added.
Ambient light means the light that is already present in a scene, before any additional lighting is added. It usually refers to natural light, either outdoors or coming through windows etc. It can also mean artificial lights such as normal room lights.
Ambient light can be the photographer's friend and/or enemy. Clearly ambient light is important in photography and video work, as most shots rely largely or wholly on ambient lighting.
Unfortunately ambient light can be a real nuisance if it conflicts with what the photographer wants to achieve. For example, ambient light may be the wrong color temperature, intensity or direction for the desired effect. In this case the photographer may choose to block out the ambient light completely and replace it with artificial light. Of course this isn't always practical and sometime compromises must be made.
On the other hand, many of history's greatest photographs and film shots have relied on interesting ambient light. Unusual lighting can turn an otherwise ordinary shot into something very powerful.
Light seen directly from a light source (lamp, sun, etc).
Light seen after having bounced off a surface.
A standard of measuring the characteristics of light, measured in kelvins.
Colour Temperature Chart
Colour temperature is a standard method of describing colours for use in a range of situations and with different equipment. Colour temperatures are normally expressed in units called kelvins (K). Note that the term degrees kelvin is often used but is not technically correct (see below).
Colour temperature means the temperature of an ideal black body radiator at which the colour of the light source and the black body are identical. (A black body is a theoretical radiator and absorber of energy at all electromagnetic wavelengths.)
Colour Temperature in Video
For video operations the relevant temperatures range from around 2,000K to 8,000K — these are common lighting conditions. In practical terms this usually means selecting lights, gels and filters which are most appropriate to the prevailing light or to create a particular colour effect. For example, a camera operator will select a "5600K filter" to use outside in the middle of a sunny day.
- When referring to the unit kelvin, it is not capitalised unless it is the first word of a sentence. The plural is kelvins (e.g. "The light source is approximately 3200 kelvins").
- The symbol is a capital K (e.g. "The light source is approximately 3200K").
- When referring to the Kelvin scale, it is capitalised (e.g. "The Kelvin scale is named after William Thomson (1824 – 1907), also known as Lord Kelvin".
According to the The International System of Units (SI) , colour temperatures are stated in kelvins, not in degrees Kelvin. The "degrees" part of the name was made obsolete in 1967. However, the "degrees" reference has remained in common use in media industries.
The difference in brightness between the brightest white and the darkest black within an image.
Contrast Ratio is a measurement of the difference in brightness between the whitest white and the darkest black within an image. A ratio of 300:1 means the brightest point in the image is 300 times as bright as the darkest point. A higher contrast ratio therefore means a larger difference in brightness.
Contrast ratio is of interest in two situations:
- Cameras: When recording an image (video, film, photography)
In video and film work it is important to understand what sort of contrast ratio your camera is able to reproduce. A high contrast ratio means that brighter and darker areas of the image will be recorded with more accuracy and apparent detail.
Most people don't need to know the actual specifications but video makers need to be aware that video has a relatively low contrast ratio. If an image includes extreme light and dark, the camera will struggle to reproduce both. Bright areas will appear over-exposed and dark areas will appear "crushed" (all black, lacking detail).
The example on the right illustrates a common problem for sports coverage in stadiums — the difference between the sunlit areas and the shadows is significant.
The best way to minimize problems with contrast ratio is to avoid having very bright and very dark objects in frame at the same time. When dealing with human subjects it makes sense to avoid white and black clothing. Also, be wary of caps and sunglasses in strong light — they can create terrible over-contrast on the face.
If you can't alter the framing, either add lighting to the dark areas or filter the bright areas.
When all else fails, the standard approach is to expose for the subject. If some parts of the picture are too bright or too dark it's not the end of the world — people are used to seeing this.
Note: Film cameras generally perform better than video, but still do not reproduce the same range that the human eye would see.
- TVs, Monitors, etc. When choosing or setting up a playback device (TV, computer monitor, etc)
Contrast Ratio is a specification given with most good televisions and monitors. It refers to the device's ability to reproduce different levels of brightness.
A higher contrast ratio is more desirable — 500:1 is quite good.
Unfortunately the contrast ratio specification given by TV manufacturers has become somewhat abused. There is a limit beyond which this specification loses significance, as further improvements aren't noticeable in the real world. Do not be too concerned with figures higher than 2000:1, and figures over 10,000:1 don't have much relevance at all.
Note: Monitors also vary in their ability to display levels of gray, i.e. their grayscale performance. This can also effect the detail in darker areas of the image.
The main light on the subject, providing most of the illumination and contrast.
A light placed to the side of the subject to fill out shadows and balance the key light.
A light placed at the rear of a subject to light from behind.
Light directly from a source such as the sun, traveling undisturbed onto the subject being lit.
The Standard 3-Point Lighting Technique
The Three Point Lighting Technique is a standard method used in visual media such as video, film, still photography and computer-generated imagery. It is a simple but versatile system which forms the basis of most lighting. Once you understand three point lighting you are well on the way to understanding all lighting.
The technique uses three lights called the key light, fill light and back light. Naturally you will need three lights to utilise the technique fully, but the principles are still important even if you only use one or two lights. As a rule:
- If you only have one light, it becomes the key.
If you have 2 lights, one is the key and the other is either the fill or the backlight
This is the main light. It is usually the strongest and has the most influence on the look of the scene. It is placed to one side of the camera/subject so that this side is well lit and the other side has some shadow.
This is the secondary light and is placed on the opposite side of the key light. It is used to fill the shadows created by the key. The fill will usually be softer and less bright than the key. To acheive this, you could move the light further away or use some spun. You might also want to set the fill light to more of a flood than the key.
The back light is placed behind the subject and lights it from the rear. Rather than providing direct lighting (like the key and fill), its purpose is to provide definition and subtle highlights around the subject's outlines. This helps separate the subject from the background and provide a three-dimensional look.
If you have a fourth light, you could use it to light the background of the entire scene.