[求助]为什么斑点图会这样变化

<P>设计了一个扩束镜,焦距为1E08,当像面与最后一面的距离为零时,斑点如下图所示,</P>
<P><img src="data/attachment/forum/dvbbs/2004-5/2004527151437794.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(\'data/attachment/forum/dvbbs/2004-5/2004527151437794.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />
但斑点图的直径随着像面与最后一面的距离加大而加大,下图是距离为5000时的斑点图,</P>
<P><img src="data/attachment/forum/dvbbs/2004-5/200452715162831.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(\'data/attachment/forum/dvbbs/2004-5/200452715162831.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />
从图中看出,光斑直径变大,而且出现了光环,为什么会出现这种情况呢?难道是衍射造成的吗?</P>
<P>可是光斑很大呀,直径有12mm.最后说一句,没有任何像差.
</P>

<DIV class=quote><B>以下是引用<I>liufukui</I>在2004-5-27 15:18:12的发言：</B>
<P>设计了一个扩束镜,焦距为1E08,<FONT color=#000000>当像面与最后一面的距离为零时</FONT>,斑点如下图所示,</P></DIV>
<P>像面和最后一个面的距离为零的话，这个就相当于最后一个面和光线的相交点了，弥散这么大有什么好奇怪的。</P>
<P>最后说一句：软件是不能设计东西的，它只会按你的设计去做！</P>