02《把照片拍好！》——对焦
<P>《把照片拍好！》</P>
<P>----谈数码品质控制之摄影</P>
<P>艾 威 </P>
<P>上篇讲了【曝光】很多朋友来咨询。</P>
<P>由于篇幅关系,就曝光的几个特别情况尚未展开仔细探讨。<br>比如：特殊点穿，色过量未过等等...<br>有兴趣的朋友可以在网上我们来讨论。</P>
<P>本篇我要讲的是【对焦】</P>
<P>拍摄一张清晰的照片,<br>是作为摄影师最起码达到的要求.<br>特别是商业人像，一般顾客是很难接受模糊的影象，<br>尽管你怎么解释这个模糊是你的创意或称之为艺术。</P>
<P>但是到底怎么样算是清晰呢？<br>这就无法避免的要讲到一个问题：景深</P>
<P>大家都知道:</P>
<P>景深＝(2x弥散圆直径x镜头焦距的平方x光圈值x调焦距离的平方)/(镜头焦距的4次方-弥散圆直径的平方x光圈值的平方x调焦距离的平方) <br> <br> 合焦后，理论上，被摄物体上的点通过镜头会在焦平面上汇聚成为一个完美的点。而被摄物体前后的点在焦平面上尚未汇聚成点，依然是会聚锥体截面上的一个圆；或者会在焦平面之前就会聚成点，而在焦平面上重新发散成圆了。如果这两个圆小到放大到照片上也看不出来，我们就认为它们还是清晰的点，这个最大的能够视为点的尺寸就叫做弥散圆直径。</P>
<P><img src="attachments/dvbbs/2007-1/20071265122018776.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2007-1/20071265122018776.jpg\');}" onmousewheel="return imgzoom(this);" alt="" /><br> <br> 对这个尺寸的一般要求是在放大到正常大小的照片，在明视距离看不出来。卡尔.蔡司的规定好像是对焦线长度的1/1750(一说是1/1730)，也就是说，只要照片对角线分辨率达到1750点(大约相当于7英寸的显示器能够达到1500X1000分辨率)就可以。而光圈对于景深的影响与CCD、镜头焦距等等无关，因为它本身只是镜头焦距和光瞳直径的比值，所以只影响光线汇聚锥体与光轴的夹角，从而影响弥散圆出现的位置。</P>
<P><img src="attachments/dvbbs/2007-1/20071265122091570.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2007-1/20071265122091570.jpg\');}" onmousewheel="return imgzoom(this);" alt="" /></P>
<P>[本图来源：色影无忌论坛]</P>
<P>上述理论非本文讨论重点，只希望通过这个计算让大家知道：</P>
<P><br>★★★光圈越大，景深越小；光圈越小，景深越大；<br>★★★镜头焦距越长，景深越小；焦距越短，景深越大；<br>★★★距离越远，景深越大；距离越近，景深越小。</P>
<P>我们很多摄影师的拍摄习惯还停留在胶片时代，<br>对数码拍摄的操作手法未做改变。</P>
<P>数码相机与传统相机拍摄比较，最大的变化之一就是自动对焦系统的引入。<br>所以想获得清晰锐利的照片必须熟悉了解自动对焦系统的工作原理和局限性。</P>
<P>大多数数码相机在半按快门时，都能自动对焦，不同的相机其自动对焦系统在对焦能力，<br>对焦速度和对焦精度上有很大的差别。影楼用的相机基本比较专业，所以大多数的相机<br>自动对焦系统的对焦速度和精度都强于手动对焦。因为拥有自动对焦系统的数码相机不<br>象纯手工对焦的传统相机那样提供专用的微棱镜或裂像棱镜的对焦屏了，所以想靠肉眼<br>在取景器里判断对焦是否清晰不是那么容易了。</P>
<P><br>★★★下面我以S2为例讲一下使用自动对焦系统的一些注意事项</P>
<P>1/根据自己的视力情况，适当调整取景器的屈光度。<br><img src="attachments/dvbbs/2007-1/20071265122071746.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2007-1/20071265122071746.jpg\');}" onmousewheel="return imgzoom(this);" alt="" /></P>
<P>取走橡胶框，</P>
<P><img src="attachments/dvbbs/2007-1/20071265122028532.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2007-1/20071265122028532.jpg\');}" onmousewheel="return imgzoom(this);" alt="" /></P>
<P>通过取景器观察时，调节取景器调节键位置，<br>直到对焦框或图象在取景器中显示出高锐度。然后装上橡胶框。</P>
<P>2/注意镜头前有否安装特殊镜片，以免影响自动对焦系统。<br><img src="attachments/dvbbs/2007-1/20071265122067910.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2007-1/20071265122067910.jpg\');}" onmousewheel="return imgzoom(this);" alt="" /></P>
<P>比如线性偏振镜等，会迷惑自动对焦系统。</P>
<P>3/设置合理的对焦模式</P>
<P><img src="attachments/dvbbs/2007-1/20071265122063149.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2007-1/20071265122063149.jpg\');}" onmousewheel="return imgzoom(this);" alt="" /></P>
<P>一般数码相机都会看到三种基本的对焦模式：<br>S：单一AF伺服 对焦时只需要半按快门，等相机完全合焦或发出合焦指示即可拍摄。<br> S模式下相机处于[对焦优先]状态，只有相机确认对焦成功快门才能按下。<br>C：连续AF伺服 在这个模式下，半按快门时相机可以连续调整对焦点，直到快门完全按下。<br> 与S模式不同的是C模式允许快门随时触发。适合追踪拍摄动态，不易掌握。<br>M：手动对焦<br>所以人像拍摄适合选择S模式</P>
<P><img src="attachments/dvbbs/2007-1/20071265122087182.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2007-1/20071265122087182.jpg\');}" onmousewheel="return imgzoom(this);" alt="" /><br><img src="attachments/dvbbs/2007-1/20071265122015083.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2007-1/20071265122015083.jpg\');}" onmousewheel="return imgzoom(this);" alt="" /><br><img src="attachments/dvbbs/2007-1/20071265122041631.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2007-1/20071265122041631.jpg\');}" onmousewheel="return imgzoom(this);" alt="" /></P>
<P>4/选择最佳对焦点<br><img src="attachments/dvbbs/2007-1/20071265122041034.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2007-1/20071265122041034.jpg\');}" onmousewheel="return imgzoom(this);" alt="" /></P>
<P>在AF单一区域有五点可供选择作为焦点，目前数码相机已经有9点11点13点供选择，<br><img src="attachments/dvbbs/2007-1/20071265122018923.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2007-1/20071265122018923.jpg\');}" onmousewheel="return imgzoom(this);" alt="" /></P>
<P>所以你完全可以通过键盘，选择最佳点对焦，而不是一味的选择中心点。</P>
<P>5/正确的掌机姿势<br>影楼摄影师和多不好的拍摄习惯，最多体现在拍摄姿势上了。<br>数码相机重量越来越轻，又有快门滞后。所以摄影师的动作对清晰度也会有影响。<br>很难想象：一边大声叫着：“美女看这里！”一只手还给方向，一只手掌机，<br>不顾站姿，呼吸，这样能拍摄出清晰的照片，真的难为摄影师了。呵呵！</P>
<P>★★★特别推荐对焦方法★★★</P>
<P>1/转盘到自定义</P>
<P><img src="attachments/dvbbs/2007-1/20071265122096472.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2007-1/20071265122096472.jpg\');}" onmousewheel="return imgzoom(this);" alt="" /></P>
<P>2/在CSM9 AE/AF锁定按钮，选择4：AF操作<br>3/设置拍摄模式为手动</P>
<P><img src="attachments/dvbbs/2007-1/20071265122053576.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2007-1/20071265122053576.jpg\');}" onmousewheel="return imgzoom(this);" alt="" /></P>
<P>4/调整测光模式为点测光。</P>
<P><img src="attachments/dvbbs/2007-1/20071265122058347.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2007-1/20071265122058347.jpg\');}" onmousewheel="return imgzoom(this);" alt="" /><br></P>
<P>5/现在你可以直接按下AE-L/AF-L键进行对焦。完成后按下快门拍摄。</P>
<P><img src="attachments/dvbbs/2007-1/20071265122080094.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2007-1/20071265122080094.jpg\');}" onmousewheel="return imgzoom(this);" alt="" /></P>
<P>这个好处在于当AE-L/AF-L键被按下时，曝光和焦距是被锁定的。<br>快门只有拍摄功能，无半按对焦功能，这样可以避免“走火”。<br>而选点后的焦点锁定更方便你二次构图，而不会失焦。</P>
<P>十大容易造成自动对焦距失败的现象：<br>1/低对比度的场景：被摄主体与背静色相同；<br>2/低光照：太暗的环境，辅助照明范围不够；<br>3/场景重叠：如笼中鸟，动物等；<br>4/几何图形：如格状物，头纱，建筑等；<br>5/强光比：如被摄物一半强光一半阴影；<br>6/反光表面：如汽车，玻璃制品等；<br>7/另类构图：如被摄主体在边缘或过小；<br>8/细节场面：全画面的沙滩，珠子，鲜花等；<br>9/前景：拍摄时前景过大或过近，位置又在居中；<br>10/操作：使用AF辅助灯时，灯前手指或相机带挡住。</P>
<P>【后记】</P>
<P>当然影响照片清晰度原因还有很多。<br>比如：拍摄格式和曝光，还有相机锐化设置等也会影响清晰度。<br>但是关键还是在于操作者。<br>我见过摄影师室内用F2.8 1/4s手持拍摄，我实在想不出这样拍摄的理由。<br>要知道数码相机拍摄的照片，用软件你可以作出任何光圈效果的景深。<br>拍摄清晰了，要模糊还不容易吗？<br>特别外景拍摄摄影师为了强调意境，采用大光圈拍摄，结果背景是虚化了<br>但是白纱过曝，人物不清晰等等得不偿失。<br>其实摄影师完全可以和后期沟通，数码后期可以轻而易举的完成。<br><br><img src="attachments/dvbbs/2007-1/20071265262947097.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2007-1/20071265262947097.jpg\');}" onmousewheel="return imgzoom(this);" alt="" /></P>
<P>拍摄模糊</P>
<P><img src="attachments/dvbbs/2007-1/20071265262939640.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2007-1/20071265262939640.jpg\');}" onmousewheel="return imgzoom(this);" alt="" /></P>
<P>后期锐化修整</P>
<P><img src="attachments/dvbbs/2007-1/20071265124293523.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2007-1/20071265124293523.jpg\');}" onmousewheel="return imgzoom(this);" alt="" /></P>
<P>拍摄清晰</P>
<P><img src="attachments/dvbbs/2007-1/20071265125332186.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2007-1/20071265125332186.jpg\');}" onmousewheel="return imgzoom(this);" alt="" /><br>后期模糊处理</P>
<P>就如从画画到摄影；<br>摄影从黑白到彩色，<br>数码的进入是摄影技术的进步！<br>固守传统概念不放，只能永远停留在胶片时代。</P>
<P><FONT style="BACKGROUND-COLOR: #000000" face=楷体_GB2312 color=#ffff00 size=5>本文已经刊登《人像摄影》2006第3期</FONT></P>
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