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We’ve heard in the past that Sigma is working on super telephoto lenses, namely a 500mm f/4 OS. Now a patent has appeared for a 400mm f/2.8 OS lens from Sigma. I think the biggest challenge for Sigma in this area is going to be autofocus. Super telephoto lenses are the least forgiving when it comes to focus accuracy in most situations.</p>
<p><strong>Patent Publication No. 2015-64436 (Google Translated)</strong></p>
<ul>
<li>Published 2015.4.9</li>
<li>Filing date 2013.9.4</li>
</ul>
<p>Example 1</p>
<ul>
<li>Focal length 390.00mm</li>
<li>Fno. 2.91</li>
<li>Total angle of view 2ω 6.29 °</li>
<li>Image height Y 21.63mm</li>
<li>Overall length of the lens 353.00mm</li>
<li>Positive, positive positive and negative negative 5-group configuration</li>
<li>Inner Focus (fourth group)</li>
<li>Anti-vibration (part of the fifth group)</li>
</ul>
<p>Source: [<a href="http://egami.blog.so-net.ne.jp/2015-04-13" target="_blank">EG</a>]</p>