Vật sáng AB đặt trước thấu kính hội tụ, cho ảnh <span class="MathJax_Preview" style="color: inherit;"><span class="MJXp-math" id="MJXp-Span-1"><span class="MJXp-msup" id="MJXp-Span-2"><span class="MJXp-mi MJXp-italic" id="MJXp-Span-3" style="margin-right: 0.05em;">A</span><span class="MJXp-mo MJXp-script" id="MJXp-Span-4" style="vertical-align: 0.5em;">′</span></span><span class="MJXp-msup" id="MJXp-Span-5"><span class="MJXp-mi MJXp-italic" id="MJXp-Span-6" style="margin-right: 0.05em;">B</span><span class="MJXp-mo MJXp-script" id="MJXp-Span-7" style="vertical-align: 0.5em;">′</span></span><span class="MJXp-mo" id="MJXp-Span-8" style="margin-left: 0.333em; margin-right: 0.333em;">=</span><span class="MJXp-mstyle" id="MJXp-Span-9"><span class="MJXp-mfrac" id="MJXp-Span-10" style="vertical-align: 0.25em;"><span class="MJXp-box"><span class="MJXp-mi MJXp-italic" id="MJXp-Span-11">A</span><span class="MJXp-mi MJXp-italic" id="MJXp-Span-12">B</span></span><span class="MJXp-box" style="margin-top: -0.9em;"><span class="MJXp-denom"><span><span class="MJXp-rule" style="height: 1em; border-top: none; border-bottom: 1px solid; margin: 0.1em 0px;"></span></span><span><span class="MJXp-box"><span class="MJXp-mn" id="MJXp-Span-13">2</span></span></span></span></span></span></span></span></span><script type="math/tex" id="MathJax-Element-1"> A'B'=\dfrac{AB}{2}</script>.