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One aspect of the Twilight camera that we do need to give a little bit of an explanation for is its use of this Focal Length value. Now, in photographic terms, the focal length of a lens is defined as the distance in millimeters from the optical center of the lens to the focal point, located on the sensor or film, if the subject at infinity is in focus. The values associated with focal length are based on the 35 millimeter film format. Now this means that choosing the appropriate focal length can get a little tricky in the digital arena, as many of today's cameras, even many SLR models, do not use what are called full-frame sensors-- that is sensors that are equivalent to 35 mm film.
In such cameras the cropped, or reduced, size of the sensor means that the given size of a lens--for example, a 60 millimeter lens--does not produce the same result as a 60 millimeter lens when mounted on either 35 millimeter film camera or a camera that has a full-frame sensor. In the words, composition, framing, even depth of field will not look the same on two cameras using different sensor sizes, even though we may use the same lens on each of them. Now, of course you're probably wondering why we're throwing all of this information your way.
Well, simply put, Twilight is essentially working as a cropped-sensor render engine. You may already have noticed that the reported focal length of your SketchUp viewport does not match the reported focal length inside the Twilight Render dialog. If we just make certain that we have our Zoom tool selected and then come down to the bottom-right of our SketchUp interface, you can see we have our reported focal length, which is 35 millimeters. This is the setting that was used to render this particular image, but you'll notice that Twilight is reporting a completely different focal length value.
Here, we're getting 24.306. This is happening because Twilight is basing its values on an expected film size of 25, not 35 millimeters. Now again, you may wonder why this is the case. Well, the Twilight engine is built on the Kerkythea Echo 2008 Render Engine. Kerkythea was built on the premise of a 25 millimeter film size, hence the reason that Twilight uses the same camera model. Now, the good news is that all of this doesn't really alter what we will get in our final render.
If we have used Twilight's Scene View tool to setup our SketchUp viewport, then our composition, our framing, will all be preserved in our final Twilight render. We do, however, need to keep in mind that when we are choosing focal lens for our shots, SketchUp will be using established photographic conventions, whereas Twilight will be reporting its focal lengths based on a 25 millimeter film format.
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