今天接到老朋友的信，互相拜年一番。想想上次联系的时间是去年春节还是前年春节都不记得了，时间过的真实太快。叹息之余心里也有安慰，朋友永远都不会陌生，无论何时何地。正所谓但愿人长久，千里共婵娟。

回头在计算机中找到当年彼此的通信，曾经因为朋友简单的打听，就竭力卖弄当时对照相机的了解。现在在翻开来看，竟然不算太肤浅。放在这里，也算个纪念。毕竟是十几年前的话题了，可能对有些人来说，连怀旧的感觉也不能体会了。

下面是其中的三封信，其中第三封还比较可读，所以把顺序倒过来。

给老同学的第二封信: 有关照相机的术语解释

陆兄:

你好!

来信收到,看信中的内容显然相机还没有买,这种速度很成问题,利率下调听说了吧,下一次经济腾飞或者叫INFLATION马上就要来了.赶快花钱吧.信中说让我科普一下,并不敢当,但见博士的问题实在太薄(对不起),忍不住要卖弄一番.

从信中所提的问题看,仍是情钟傻瓜相机,真傻瓜也.

照相机的发明起源于人们对针孔成像技术的发现和利用,早在宋代,我国的大聪明人沈括就在其所著的《梦溪笔谈》中介绍了这一现象,所以说发明照相机的光荣应属于中国人,后来我们的祖先忙于打仗抢粮食编故事写历史书,反被吃了生牛肉没喝中国茶叶不消化而坐立不安蠢蠢欲动的西洋人首先将这一技术应用于绘画,制成了临模箱,再用沥青最终是卤化银制成了底片,终于按照中国人的想象发明了,其实是仿制了实用的照相机.

可以想象针孔成像中的针孔愈细则成像愈清晰但也愈暗,因为相对于暗箱来说,针孔是成像的通道更是唯一的光源.后来随着技术的进步,人们又采用了凸透镜代替不实用的针孔,其最大的优点是可将照射在透镜表面上的所有光线,校正"缩小"至近于针孔般的一点,就是说照射在透镜边缘的光线与照射在中心的光线通过透镜后都仿佛来自同一点,从而既保证了成像的亮度又达到了一定的清晰度,但透镜并不能完全等价于针孔,其成像的清晰度有一定的范围和限制.从理论上说,透镜的有效通光孔径越小,其性能越接近针孔,但此时的成像也就越暗;透镜的有效通光孔径越大,其成像越亮,但清晰度的范围就越窄.这就是现代相机中光圈的作用.光圈的准确定义是焦距与有效通光孔径的比值,这二者与光强均为平方关系,所以光圈值倒数的平方与胶片所受光照强度成正比,再乘以一常数及曝光时间,即为曝光量,显然所得曝光量为单位面积的曝光量,这正好用来比较各种不同尺寸间胶卷的灵敏度.

由于实际使用的透镜并非如理想透镜那样厚度为零,所以在实际使用中就会不可避免地产生各种像差和畸变,所以为提高成像质量特别是镜头价格,人们饮鸠止渴地设计了许多种包含多个镜片的复合镜头,在复合镜头中,镜片们乱其八糟地摆放,每两个相邻空隙之间的镜片无论几个(如超过一个,一定是粘在一起的),都称为一组,所以镜头有9片7组,10片8组等等.镜头中镜片的多少并不能说明成像质量的好坏,如宣传广告中出现类似词句,除眩人耳目外,并无其他含义.

以前所有的透镜,其表面必定是某一半径球面的一部分(由于研磨所至),所以又称球面透镜.近年来有些聪明人想到采用非球面透镜来进一步提高成像质量并获得了成功,所以采用非球面镜片的镜头很出了一些风头,这与其说是因为其成像素质高,倒不如说是因为玻璃镜片的加工研磨费用昂贵所至.时至今日,多数相机配备了自动聚焦功能,用可怜的电池去推动沉重的玻璃镜片显然已不合时宜,且高分子材料日新月异,成像质量也是马马虎虎,所以很多镜片均是用塑料通过模具浇铸流水线生产而成,其质量也因其所用材料及加工设备的不同而相去甚远.此时的非球面镜片的加工费用已与普通镜片相差无几.制造商们所吹嘘的轻便相机镜头中的非球面镜片,其制造成本固然平平,而其对成像质量的贡献恐怕也是很有限的.

事实上,塑料镜片的大量泛滥,也是摄影迅速普及的要求,目前的自动聚焦已然如此的完善且便宜,使得轻便相机无不趋之若鹜,而塑料镜头的最大的好处就在于可使小巧的轻便相机配备大范围的变焦镜头并利用有限的电池驱动.目前轻便相机的自动聚焦均是通过发射红外线,然后检测反射回来的红外线的角度相位等等判断物体的远近.由于装置的简陋,所处理的红外线实际上是较粗且发散的光柱(这正好适用随便的傻瓜相机),所以自动聚焦有一粗略的范围.最唬人的是所谓的智能自动聚焦,脸皮薄的厂商称为多点自动聚焦(MULTI AUTO FOCUS),即向取景画面中的三个区域(即照片中最容易有人的地方)同时发射红外线并测定距离然后取其中距离最小者为依据驱动镜头,缺德的是所有的厂商均不公开其"智能聚焦"的范围,使得使用轻便相机的聪明用者在拍摄立式照片时心中惴惴不安,即使横握相机拍摄如门内人物之类的画面时也不得要领.与之相比,轻便相机推广初期所设置的单点自动聚焦功能(SPOT AUTO FOCUS)反倒稳妥得多,这种相机要求在对焦时先将对焦框瞄准希望的物体或人,半按快门锁定焦点,保持半按快门状态并重担新构图,最后释放快门.显然这种方法不易出错,这可能也许是不少厂商在其重要品牌相机上虽然都采用了多点对焦功能,但同时也设置可随时转换的单点聚焦功能的原因.

不论多点或是单点自动聚焦,半按快门时焦点(物距)即被锁定,等到快门最后释放时则以锁定的焦点调整镜头,所以需要一定的快门延迟时间,而在测距到全按快门的这段时间内主体的位置发生变化,则倒霉活该.厂商大概觉得这样太不成话,所以又设计了所谓的连续自动聚焦功能(CONTINUE AUTO FOCUS),在此方式下,半按快门后,相机即以每秒若干次的速度不停地测量主体距离,同时镜头也随之调节,其最大的好处是快门延迟时间短,适用于高速动体摄影.

现代相机特别是轻便相机的另一个特点是其自动曝光功能,其实是自动测光功能.一般是将取景画面分成三五七份,分别测量其亮度,然后根据所谓的专家经验强加一权重值,最后得出曝光值.这中间有很多诸如模糊逻辑之类的唬头,但这必然有一个问题,就是假如一个智商平均的傻瓜拍照,显然没有问题.如果是一个特别傻的傻瓜或是不太傻的傻瓜的话,结果就很难说了.另外需要揭发的是所谓的自动逆光检测功能,实际上就是比较画面中心与边缘部分的亮度差,如果中心部分暗于边缘部分且超过一自作聪明的限定值时,相机即作出主体处于逆光的判断并自动加以补光,举例来说,江泽民和李鹏与站在中间的曼德拉用轻便相机合影时,半按快门,内藏闪光灯就会自动充电,并在快门释放时对主体加以补光.

总而言之,现代相机,包括其他产品在内,其更新换代的过程及方向,集中体现了其设计者对人类平均能力的越来越清醒的认识,或者说是越来越傲慢的偏见.一个有力的佐证就是现代相机中所谓的DX码自动识别系统.所谓的DX码,是柯达公司最先炮制的,具体就是暗盒输片口旁的两列黑白相间的方块,白色方块为裸露的金属部分,黑色则为涂漆绝缘部分,方块的不同排列方式分别表示胶卷的感光度及张数等诸如此类的信息,在相机的暗盒仓内与每个方块对应的位置上都设有一对金属触点.通过这种方法自动设定胶卷的主要是感光度等信息.其实当前我等所用均为ASA100胶片,最多少数高级人物偶尔眩耀一下ASA400胶卷,这只需普及幼儿园教育即可解决或替代这一问题,实在不必如此脱了裤子放屁.况且轻便相机中具备此功能后已不能手动设定,也没有任何显示,不单目前国产黑白胶卷都不能使用.假如你领着女朋友出去玩同时又眩耀新买的照相机,偏偏胶卷暗盒上的信息区或被粘污或被划伤,则相机必然以错误的曝光组合工作,整卷胶卷不是曝光过度就是不足,最不幸的是沾沾自喜的你毫无知觉,对即将产生的严重局面及其衍生出来的更严重的后果一无防范.

目前在市场上占据统治地位的高档轻便相机多具有内置2-3范围的变焦镜头,这不仅带来了方便更增添了乐趣.变焦范围的选择除了有关价格的考虑外,更应从中国的实际出发,选择有中国特色的照相机.在中国,不像外国那样,稀稀拉拉的没几个人,有人也不往一起凑,中国讲求的是团结,要的是凝聚力,所以公园景点人山人海,如想单独留影一张,非把相机镜头杵到被摄者的鼻子上不可.此时方显短焦距镜头之妙用,故此我一口咬定变焦范围一定要从28毫米开始.盖镜头焦距f,像距v和物距u有如下牵扯:1/f=1/v+1/u,物距太大,所以像距与焦距近似相等,设d为胶片画面上对角线长度,则以d的两个端点及镜头的中心点可作一三角形,其高度为v,而其顶角即为成像最大涵盖角.底边d长度恒定,则高度v越小,成像角度越大也.

轻便相机的最大目标是追求完全的自动化,恨不能把照相机设计成机器人.遗憾的是这一目标还很遥远,目前的相机在卷片,对焦和曝光等功能都基本上自动了,最碍眼就数变焦了,设计大师们穷思竭虑,终于有所交待,即所谓的人像功能(PORTRAIT MODE).人像功能的具体实施方法是设定此功能后,相机测定主体距离,根据此距离调节镜头焦距,调节的依据是使不同距离的人在底片上的成像大小趋于一致,如证明像,半身像像之类的统一格式,这是一很令人费解的功能,如采用此功能拍满一卷胶卷的话,出现在底片上的岂不是一排一模一样的呆瓜?

轻便相机得以迅速普及的重要武器之一即是其内藏的闪光灯,使得房间内及夜间摄影变得轻而易举,虽然照片糟糕无比.事实上就连高级单反相机的设计者们也发现了这一愚蠢并可供利用的想法,纷纷在许多极具端庄及严肃的单反相机内装上了内置闪光灯以迎合广大的傻瓜,也不介意相对少数的不太傻的傻瓜望而却步.不过设计者的本能仍使其在高档轻便相机中普遍设计了闪光灯取消功能.无论如何,闪光灯已成为轻便相机的招牌之一(目前市场上的轻便相机中唯一不设置闪光灯的是柯尼卡出的高档机,英文名记不清了大概HEXER类似,中文名叫"巧思"的,万把块钱).其功能也令人头晕眼花,唯一较为实用但又不为人称道的是闪光灯自动变焦功能(AUTO FLASH ZOOM),在拍摄中闪光灯的照明角度无疑应稍大于镜头的涵盖角,但在高档轻便相机中由于变焦镜头的关系,镜头的涵盖角度有一个非常大的变化范围,如将闪光照明角度固定在最大值上无疑会造成很大的浪费,故此特将闪光灯灯室的结构设计成可变的,使射出光线的角度可以在某种程度上调节,而这一调节是与镜头变焦同步进行的,其优点是既节约了电能又增加了闪光灯的使用范围.

聪明人都知道黑暗中直射的单灯闪光照明必然会产生呆板单调的影像外加讨厌的黑影,对此某些设计者采用了掩耳盗铃的方法,设计出了所谓的"柔和闪光(SOFT FLASH)"功能,在此功能下工作的相机采用漫射屏或散射屏附加在原来的透射屏上,使光源由原来的闪光管扩大至整个闪光屏,用心虽然良苦,效果却十分有限.以前摄影书中介绍过的用手帕蒙在闪光灯上以减小反差的方法便是此项设计的指导思想.

由于前些年轻便相机,特别是简易傻瓜机的迅速普及,作茧自缚的厂商们已开始尝到市场提前饱和所带来的苦果,现在只得拼命地花样翻新以吸引消费者,甚至不惜以摒弃辛辛苦苦攒起的家底为代价,推出所谓的APS(ADVANCE PHOTOGRAPHY SYSTEM),其实就是带磁带的胶卷,来吸引消费者.原来的体系也作垂死挣扎,推出了所谓的PANORAMA摄影方式,即在24X36毫米的底片上拍成13X36毫米的格式以求刺激,其厂商的发言人毫不脸红地承认,该方式就是要刺激消费者以新的格式拍摄以增加胶卷的销售.自然,新出的拍马屁冲印系统兼容了这一格式.

另外再有一事相烦,我因拍幻灯片上瘾,特别是翻拍图片的喜欢.欲买一摄影灯,在大连看有两种规格单灯头和双灯头的,我觉得价格有些宰人,故请你在逛专业店时看一看北京的价格,如相差不大的话我就买了.牌子是雅刚(ARKON?),名子是卤素灯或摄影灯,灯头的形状有些像首饰柜台内的照明灯,同时替我看一下放大机,洗片鼓的规格及价格,再就是幻灯片框的价格.希望你早去逛商店,我能早得回音.

别不多写

祝身体健康!思想活泼!

1996.7.12

老同学之前的回信摘要:

仍想买轻便照相机，希望详细介绍以下广告中常见的术语：

光圈与透光量、景深之间的关系，镜头的结构，镜头焦距与视角的关系，非球面镜头，AUTO FOCUS, CONTINUE AUTO FOCUS, MULTI AUTO FOCUS, SPOT AUTO FOCUS, 智能测光，自动补光，智能自动聚焦，DX码自动识别，PORTRAIT MODE，AUTO FLASH ZOOM，SOFT FLASH，APS，PANORAMA。

给老同学的第一封信: 照相机的选购

July 9, 1996

Dear Dr.

It is always a great pleasure to share a common topic with a friend. I will be very glad to give you all of my experiences and collections. But first of all, I must inform you that to be an amateur in photography will turn to be very expensive, time-consuming and, in general, fruitless. In fact, this hobby will take a considerable part of your time and salary, in return of which, gives you a lot of proud photos which may seem meaningless to the other persons.

To answer your question about the selection of camera, I suggest you to purchase a camera as good as you can afford now, I mean, with all possible money you can arrange. Remember what your opinion was about 1000 yuan two years ago? and four years ago? and imagine what it will to be after two years. How can you tolerate regretlessly with your Seagull DF-300? A normal camera which is usually designed to last 15 years can not meet the different situations that its owner may suffer during the same time.

For an amateur of photography, the SLR (Single Lens Reflex) camera is an inevitable equipment while the compact camera is in the second range. For you want to practice what you have learned, i.e., to try the combinations of shutter speed and aperture index, the change the depth-of-field, to set the exposure compensation, et al. You buy a camera because you want to TAKE pictures other than to GET pictures, so the more complex the camera is, the more pleasure you will get.

The SLR camera, compared with the compact camera, is more seriously and precisely. When you are plunged in this hobby, you may need to photograph instruments and samples (rocks), or to take picture from screen, magazine and drawing, in which the compact camera can not even to be imagined. The only default of the SLR camera is that sometime it will seem to be a little exaggerating to carry the awful camera and exchange lenses in the daily life, the only exception of which is to be companied by your girl(s).

Your choice of the SLR camera can begin at:

(1) MINOLTA X-700 (~2,500.00)

* Programmed AE (Auto Exposure).

* Manual exposure with lighting meter indicator.

* Programmed flash TTL (Through The Lens) controlled exposure.

* Xsync is 1/60 sec.

* +/- 2 EV exposure compensation.

It is the most classical and common manual focus SLR camera in China, the famous Seagull camera was its unsuccessful replica. It has the most large amount of exchange lenses commercially available than other trademark, for instance, the owner of Canon AE-1P camera will take great effort to find a usable lens (for this reason, I dislike MINOLTA personally).

(2) RICOH XR-8 S/D (~2,000.00)

* Metal body with professional taste.

* Multi exposure.

* Xsync is 1/125 sec.

* Aperture priority AE.

RICOH is the most common trademark because of its price, but the quality and durability of which (both body and lens) can not be trusted. It perhaps a good idea to buy a RICOH body and a SIGMA lens with RICOH mount, so the quality of photo can be guaranteed but the cost is lower. Similarly, there are RICOH SOLAR/D, KR-5III/D, XR-7MII/D selectable, all of which have alike qualities and properties though with many different models.

(3) NIKON FM10 (~4,000.00 with 35-70 zoom lens)

* Metal body with professional taste.

* Manual exposure with lighting metering indicator.

* Xsync is 1/125 sec.

* Multi exposure.

NIKON is the best camera of Japan. NIKON FM10 is a simplification of the famous FM2. It is worth purchasing if the body and lens can be sold separately (the zoom lens involving in set is considered not favourable).

(4) NIKON F801s (~6,500.00)

* Auto focus.

* Programmed AE.

* Xsync is 1/250 sec.

* Multi exposure.

Auto Focus (AF) is the coming thing. It can be predicted that in the near future, most of the major camera manufacturing company will stop producing Manual Focus (MF) lenses, for example, Canon has already. So it will be reasonable to begin with an AF camera.

The major characteristics and specifications of NIKON F801s will be described in a separated paper.

All above is my opinion to choice a SLR camera. If I were you, I will select from NIKON F801s to RICOH XR-8 according to my savings, but a Seagull DF-300 which seems to be cheaper but actually not when considering the cost of the coming repairing, will be certainly out of the question.

For the compact camera, as shown in its popular name, which is designed for housewife or other person similar, can never meet the needs of a man with knowledge and brain. But the compact camera can be a perfect supplementary according to its great convenience, the shooting speed of which is also much superior to the SLR cameras.

The fixed focus compact camera is comprehensible durable and cheaper. But the fixed focus, generally 28 or 32 mm (this is the most important focal length for living photographing), will be very difficulty go give a clear portrait without distortion. So, mainly for the same reason, zoom compact camera in which the focus covers as long as 76 mm or more has been developed and produced.

Several years ago, the zoom ranges of the compact cameras in the market are all begin at 38 mm and end at 76 to 135 mm respectively, it is because of the difficulties from designing and especially producing. As mentioned above, in living photographing, a shorter focal length as 28 mm is necessary to cover a large area or more persons at close distant, for instance, at a crowed park. So many of the famous camera companies have advanced their new kinds of compact cameras recently, like Canon PRIME SUPER 28V, PENTAX ESPIO 928, MINOLTA RIVA ZM 28W et al., in which a shorter focal length of 28 mm are involved.

For some odd reasons, I do not like the SLR cameras with built-in flash, since I believe that direct flash can only produce dull pictures. In the compact camera, however, all of which are equipped with flash, then I despise the kind in which have the jump-out flash (unfortunately, your pre-camera happened to be of this kind, I am sorry), for I believe the jumping (or its cause of flash will certainly consume some energy of battery, and what is more, the jumping will possibly do harm to the camera. In some kinds of compact cameras, the flash zooms synchronously with the lens, so the camera with a jump-out flash must involving a complex and then something dangerous construction.

So much is my opinion to compact camera, the most important of which is the zoom range must cover a 28 mm focal length. So the choice is greatly simplified, for only few cameras can meet this claim, they are Canon PRIME SUPER 28V, PENTAX 280P, PENTAX ESPIO 928, PENTAX ESPIO 628, PENTAX ESPIO W, NIKON TW ZM85, Olympus AZ 220 SZ, RICOH RZ105 ZOOM, MINOLTA RIVA ZM28W et al. I have bought a PENTAX ESPIO 928 two years ago and was satisfied by its performance.

With this letter, I have enclosed a comparison table of some compact cameras and two articles about compact camera written in the last winter, which had been submitted and not published nor even accepted. So you may be the only serious reader of mine. Some contents of the articles may be a little outmoded and need to be replaced, for example, the Canon S 28V should be involved in the articles.

The definition your demanded are list after the articles.

I have never been discussing such and interesting topic with a friend like you, so I would greatly appreciate your continuation.

Your sincerely