http://www.filmtools.com/tiffen-round-81ef-neutral-density-filters.html3418Tiffen 4.5" Round 81EF Neutral Density Filters: http://cdn.filmtools.com/media/catalog/product/t/i/tiffen-packaging_2.jpg225.35USDInStock/Camera & Accessories/Filters & Accessories/Camera & Accessories/Filters & Accessories/Camera Filters/More/Shop by Trade/Cinematographers/Filters & Accessories<ul>
<li><strong>81EF Warming filter</strong> </li>
<li>One stop loss</li>
<li>Used to warm scenes</li>
<li>Partial color correction when using tungsten based film outdoors</li>
<li>Slight blue cast</li>
</ul> The Tiffen 81EF Warming filter has about a one stop loss and is used to warm scenes, or for partial color correction when using tungsten based film outdoors and the desired look is to still have a slight blue cast.