【翻译】2009年7月22日日全蚀--3.3 日食摄影
本帖最后由 ma007 于 2009-5-30 20:48 编辑这是“日食资料翻译组织人手”里我申请的3.3 日食摄影这一段，我花了几天时间全部翻译好，许多地方需用到意译，文章中所提到的表格图例也全部汉化好，但毕竟可能有不准确的地方，望大家积极指出，红色文字实在不明白意思，查找许多资料也无果，如有朋友能翻译将非常感谢（感谢gohomeman1在回帖中解释 ）
原帖http://www.astronomy.com.cn/bbs/thread-111450-1-1.html
原文http://eclipse.gsfc.nasa.gov/SEpubs/20090722/TP214169a.pdf
3.3 Eclipse Photography
The eclipse may be safely photographed provided that the above precautions are followed. Almost any kind of camera can be used to capture this rare event, but Single Lens Reflex (SLR) cameras offer interchangable lenses and zooms. A lens with a fairly long focal length is recommended in order to produce as large an image of the Sun as possible. A standard 50 mm lens on a 35 mm film camera yields a minuscule 0.5 mm solar image, while a 200 mm telephoto or zoom lens produces a 1.9 mm image (Figure 24). A better choice would be one of the small, compact, catadioptic or mirror lenses that have become widely available in the past 20 years. The focal length of 500 mm is most common among such mirror lenses and yields a solar image of 4.6 mm.
With one solar radius of corona on either side, an eclipse view during totality will cover 9.2 mm. Adding a 2x teleconverter will produce a 1000 mm focal length, which doubles the Sun’s diameter to 9.2 mm. Focal lengths in excess of 1000 mm usually fall within the realm of amateur telescopes.
Consumer digital cameras have become affordable in recent years and many of these may be used to photograph the eclipse. Most recommendations for 35 mm SLR cameras apply to digital SLR (DSLR) cameras as well. The primary difference is that the imaging chip in most DSLR cameras is only about 2/3 the area of a 35 mm film frame (check the camera’s technical specifications). This means that the Sun’s relative size will be 1.5 times larger in a DSLR camera so a shorter focal length lens can be used to achieve the same angular coverage compared to a 35 mm SLR camera. For example, a 500 mm lens on a digital camera produces the same relative image size as a 750 mm lens on a 35 mm camera (Figure 24). Another issue to consider is the lag time between digital frames required to write images to the DSLR’s memory card. Better DSLRs have a buffer to temorarily store a burst of images before they are written to the card. It is also advisable to turn off the autofoFcus
because it is not reliable under these conditions; focus the camera manually instead. Preparations must also be made for adequate battery power and space on the memory card.
If full disk photography of partial phases of the eclipse is planned, the focal length of the optics must not exceed 2500 mm on 35 mm format (1700 mm on digital). Longer focal lengths permit photography of only a magnified portion of the Sun’s disk. In order to photograph the Sun’s corona during totality, the focal length should be no longer than about 1500 mm (1000 mm on digital); however, a shorter focal length of 1000 mm (700 mm digital) requires less critical framing and can capture some of the longer coronal streamers. Figure 24 shows the apparent size of the Sun (or Moon) and the outer corona in both film and digital formats for a range of lens focal lengths. For any particular focal length, the diameter of the Sun’s image (on 35 mm film) is approximately equal to the focal length divided by 109 (Table 18).
A solar filter must be used on the lens throughout the partial phases for both photography and safe viewing. Such filters are most easily obtained through manufacturers and dealers listed in Sky & Telescope and Astronomy magazines (seeSect.3.2, “Sources for Solar Filters”). These filters typically attenuate the Sun’s visible and infrared energy by a factor of 100,000. The actual filter factor and choice of International Organization for Standardization (ISO) speed, however, will play critical roles indetermining the correct photographic exposure. Almost any ISO can be used because the Sun gives off abundant light. The easiest method for determining the correct exposure is accomplished by running a calibration test on the uneclipsed Sun. Shoot a roll of film of the mid-day Sun at a fixed aperture (f/8 to f/16) using every shutter speed from 1/1000 s to 1/4 s. After the film is developed, note the best exposures and use them to photograph all the partial phases. With a digital camera, the process is even easier: shoot a range of different exposures and use the camera’s histogram display to evaluate the best exposure. The Sun’s surface brightness remains constant throughout the eclipse, so no exposure compensation is needed except for the narrow crescent phases, which require two more stops due to solar limb darkening. Bracketing by several stops is also necessary if haze or clouds interfere on eclipse day.
Certainly the most spectacular and awe-inspiring phase of the eclipse is totality. For a few brief minutes or seconds, the Sun’s pearly white corona, red prominences, and chromosphere are visible. The great challenge is to obtain a set of photographs that captures these fleeting phenomena. The most important point to remember is that during the total phase, all solar filters must be removed. The corona has a surface brightness a million times fainter than the photosphere, so photographs of the corona must be made without a filter. Furthermore, it is completely safe to view the totally eclipsed Sun directly with the naked eye. No filters are needed, and in fact, they would only hinder the view. The average brightness of the corona varies inversely with the distance from the Sun’s limb. The inner corona is far brighter than the outer corona so no single exposure can capture its full dynamic range. The best strategy is to choose one aperture or f/number and bracket the exposures over a range of shutter speeds (e.g., 1/1000 s to 1 s). Rehearsing this sequence is highly recommended because great excitement accompanies totality and there is little time to think.
Exposure times for various combinations of ISO speeds, apertures (f/number) and solar features (chromosphere, prominences, inner, middle, and outer corona) are summarized in Table 19. The table was developed from eclipse photographs made by F. Espenak, as well as from photographs published in Sky and Telescope. To use the table, first select the ISO speed in the upper left column. Next, move to the right to the desired aperture or f/number for the chosen ISO speed. The shutter speeds in that column may be used as starting points for photographing various features and phenomena tabulated in the ‘Subject’ column at the far left. For example, to photograph prominences using ISO 400 at f/16, the table recommends an exposure of 1/1000. Alternatively, the recommended shutter speed can be calculated using the ‘Q’ factors tabulated along with the exposure formula at the bottom of Table 19. Keep in mind that these exposures are based on a clear sky and a corona of average brightness. The exposures should be bracketed one or more stops to take into account the actual sky conditions and the variable nature of these phenomena.
Point-and-shoot cameras with wide angle lenses are excellent for capturing the quickly changing light in the seconds before and during totality. Use a tripod or brace with the camera on a wall or fence because slow shutter speeds will be needed. In addition, disable or turn off the camera’s electronic flash so that it does not interfere with anyone else’s view of the eclipse. If the flash cannot be turned off, cover it with black tape.
Another eclipse effect that is easily captured with pointand- shoot cameras should not be overlooked. Use a straw hat or a kitchen sieve and allow its shadow to fall on a piece of white cardboard placed several feet away. The small holes act like pinhole cameras and each one projects its own image of the eclipsed Sun. The effect can also be duplicated by forming a small aperture with the fingers of one’s hands and watching the ground below. The pinhole camera effect becomes more prominent with increasing eclipse magnitude. Virtually any camera can be used to photograph the phenomenon, but automatic cameras must have their flashes turned off because this would otherwise obliterate the pinhole images.
For more information on eclipse photography, observations,and eye safety, see the “Further Reading” sections inthe Bibliography.
3.3 日食摄影
如果（严格）遵守前面所说的防护措施，那么你就能非常安全地拍摄日食。几乎任何种类的相机都能用于捕捉这个稀有的事件，但是，单反相机提供了可换镜头以及变焦的功能，为了使底片上的太阳拍到的足够大，一个长焦距的镜头是必须要的，在35mm相机的底片上，一个50mm的标准镜头所拍摄到的太阳极其微小，直径才0.5mm左右，然而一个200mm的远摄镜头或变焦镜头提供的图像直径就有1.9mm大小了（图解24）。一个更好的选择就是使用短小精悍的折返镜头，它在过去20年间广泛的被用到，折返镜头中500mm的焦距是非常普遍的，它拍摄到底片上太阳的成像直径可达4.6mm。
由于包围在太阳周围的日冕有1个太阳半径宽，全食的阶段将会覆盖底片直径9.2mm。当使用一个2倍增倍镜来提供1000mm焦距后，可以使太阳在底片上的直径翻倍达到9.2mm。焦距超出1000mm的镜头就应列入业余望远镜的行列了。
消费类数码相机在近几年已可以承受得起，他们中的许多都可以用来拍摄日食。对35mm单反相机所提出的多数建议也一样适用于数码单反相机上。主要的不同点就是大多数数码单反内（他这里指aps-c画幅的数码单反）的成像芯片大小只有35mm胶片的2/3（请查阅相机的技术规范说明）。这意味着在数码单反上所拍摄到的太阳相对会大1.5倍，因此，与35mm单反相比之下一个焦距略短的镜头也可以得到相同的覆盖角度。例如，在数码单反上一个500mm焦距的镜头所拍摄的图像尺寸与一个750mm焦距的镜头在35mm单反上所拍摄的图像尺寸相对来说是一样的（图解24）。另外一个要考虑的问题是从按下快门的那刻到照片写入数码单反内的存储卡之后存在一个滞后时间。优秀的数码单反有一个缓存来临时存储那些在写入存储卡之前突然激增的图片。另外，关闭自动对焦是一个明智的选择，因为在这种情况下它不怎么可靠；可以用手动对焦来代替。充足的电量和足够容量的存储卡在拍摄之前必须准备好。
如果要想拍摄到日食阶段中完整大小的太阳，那么在35mm单反上镜头焦距的长度不能超过2500mm（数码单反上不能超过1700mm）。更长的焦距所拍摄到的只是整个太阳中放大的一个部分。如果为了拍摄全太阳的日冕，那镜头焦距的长度不能超过1500mm（数码单反上是1000mm）；然而，一个比1000mm焦距短的镜头（数码单反是700mm）只需较少的关键帧因此它能捕捉到更长的冕流。图解24 说明了各种长度的焦距在底片和数字成像芯片上所呈现的太阳（或月亮）以及外部日冕的外观尺寸。对于任何特定的焦距，太阳所成像的直径（在35mm底片上）近似等于焦距除以109（表格18）。
太阳滤镜必须在日食的吞食阶段使用来用于拍摄和安全的观察。通过联系制造商和《天空望远镜和天文学》上的销售列表这类滤镜是很容易得到的（请看3.2章节 “太阳滤镜的购买渠道”）。这类滤镜的特点是将太阳的可见光和红外线减弱到原先的1/100000。然而，滤波系数和iso感光度的选择对决定正确的曝光起到了关键的作用。几乎任何iso感光度都能使用因为太阳散发出了充足的光线。通过在还未发生日食的太阳上进行校准测试是最早能决定正确暴光的方法。将每个固定的光圈（f/8-f16）配合使用从1/1000秒到1/4秒的各种快门的这个方法来拍摄正午的太阳一卷胶卷，之后换上新胶卷，记住最佳曝光值然后用他们来拍摄日食各个阶段。数码相机的话整个过程就更简单了：拍摄多次不同的曝光并用相机内的柱状图显示来估算最佳暴光。在日食过程中太阳表面的亮度始终是一样的，所以，除了由于太阳边缘变暗而需要2次以上改变光圈的非常狭窄的月牙相位外，其他过程曝光补偿是不需要的。如果日食天受到了烟或云雾的妨碍，用几种光圈进行包围曝光也是很有必要的
当然日食过程中最壮观最令人惊叹的就是太阳被全部遮盖的阶段。在短短几分钟甚至几秒内，太阳那珍珠白的日冕，红色的日珥以及色球都是可见的。最大的挑战就是拍到一套捕捉到这些飞逝现象的照片。最重要的一点要记得就是在这整个阶段中，任何太阳滤镜都不能使用。日冕的表面亮度比色球层要暗1百万倍，所以为了拍到日冕必须移除太阳滤镜。此外，直接用肉眼观看全食阶段的太阳是绝对安全的。日冕平均亮度的变化和它到太阳外圈的距离成反比。内部的日冕远远要比外部日冕亮得多所以单次曝光捕捉不到它完整的动态范围。最好的方法就是选择一个光圈然后用一系列的快门速度来进行包围曝光（比如从1/1000秒到1秒不等）。强烈要求事先排练一遍这个动作因为到了全食阶段巨大的兴奋会冲击着你到时会没有一点时间来思考。
表格19总结了使用不同iso感光度，光圈来组合曝光的时间以及太阳表面的特征（例如色球，日珥，内部、中间以及外部的日冕 ）。这个表格是由F. Espenak根据许多拍摄的日食照片同时也从发表在《天空和望远镜》的日食照片上所得出的结论。下面说一下怎样使用这个表格，首先从最左上一栏选择一个iso感光度，然后向右移动到光圈数值一栏选择一个想要的光圈数值。在下面快门速度这一栏中的快门就能作为一个起点来拍摄列在最左下角事物这一栏中不同的特征和现象。例如，拍摄日珥用iso400与f/16的光圈组合，表格中就建议快门速度为1/1000。另一个选择就是，配合列在表上的“Q”因子数与在表格19底部的曝光公式一起使用，也可以计算出推荐快门速度。记住这些曝光时间都是基于晴朗的天空以及平均亮度的日冕所采用的。考虑到天空的实际状况和这些变化的现象，需要用不同的光圈来进行1次或更多的包围曝光。
拥有广角镜头的傻瓜相机在捕捉全食阶段快速变化的光线方面是非常优秀的。使用一个三脚架或者将相机支在墙上或围栏上因为会用到慢速快门。另外，关闭相机的电子闪光使之不干扰其他观看日食的人。如果闪光灯不能关闭，那就用黑色的胶带遮住他。另外一个能被傻瓜相机轻松捕捉的日食效应也不应该被忽视。用一个草帽或一个厨房的漏勺放置在一块白色硬纸板几英尺高的地方并将他的投影落在纸板上。细小的圆孔就像针孔照相机一样并且每个圆孔都投射出它自己的日食成像。你用手指弯成一个圆孔对着地面看也可以做出同样的效果。针孔相机的效应随着食分的增长会越来越明显。实际上任何相机都能用来拍摄这个现象，但是自动相机必须关闭它的闪光灯要不然就会拍不出针孔图像。
如需更多日食摄影，观测数据以及眼睛保护方面的信息，请看在文献资料部分的“阅读更多”这一节
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连图和表格都做了处理，很敬业啊。看了再交流下。 从上下文看，我认为那句话的意思是
无论如何，1000mm的短焦距镜头（数码单反是700mm）仅需较少的关键帧（就能拍摄整个日蚀）并且能拍到更长的冕流。
长焦镜头看见的不是整个日面，自然需要更多的照片。短焦镜头当然就不需要这么多张照片了。 从上下文看，我认为那句话的意思是
无论如何，1000mm的短焦距镜头（数码单反是700mm）仅需较少的关键帧（就能拍摄整个日蚀）并且能拍到更长的冕流。
长焦镜头看见的不是整个日面，自然需要更多的照片。短焦镜头当然 ...
gohomeman1 发表于 2009-5-27 23:10 http://www.astronomy.com.cn/bbs/images/common/back.gif
哈，非常感谢，原来“关键帧”指这个意思，我一开始就想偏了
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