And an 85mm lens with a lens opening diameter of 50mm would have an F-Stop of F1.7 (85/50 = 1.7). This means that a 200mm lens with a lens opening of 50mm will have an F-Stop value of F4.0 (200/50 = 4). The equation for this relationship can be thought of as: F-Stop = Focal Length / Diameter. Put differently, an aperture value is a result of the size (or diameter) of the lens opening when compared to the focal length of the lens. Where things get a bit more mathematical however, is when you start to look at how an F-Stop is calculated as there is a scientific relationship between aperture and focal length. A wide open, fast lens might open up to F1.4 and close down to F22, meaning of course that the iris is able to open op to a very wide setting, and close down significantly. We all know that F-Stop represents the aperture setting on the lens, and ultimately controls how much light is hitting the sensor. F-Stopīefore I explain the simple (but important) difference between the two, let’s briefly discuss the science behind what an F-Stop really is. I realized after this exchange that this is a very common question amongst filmmakers (especially those that came from a DSLR background), and felt that it warranted a blog post to give some clarity to anyone else who might not understand the differences. I quickly explained to him that he was not making an accurate comparison as the cinema lens is measured differently than the stills lens, and pointed out the T-Stop markings on the Super Speed. More specifically, he wanted to know why his 35mm Zeiss Superspeed lens only opened up to 1.5, while a Zeiss stills lens at the same focal length can hit 1.4. I had a recent conversation with an up and coming DP who was shooting on cinema prime lenses for the first time, and he asked me why some cinema lenses appear to be slower than their still lens counterparts. Read on to hear why this is the case, and to understand the big differences that it can make while shooting. In film however, aperture is measured in T-Stops which is why authentic cinema lenses will always have their markings based around a T-Stop, not an F-Stop. DSLR lenses are of course intended for stills photography first and foremost, and the standard measurement of aperture in photography has always been an F-Stop. So many younger cinematographers today have come up during the DSLR revolution, and as a result have never used lenses that feature T-Stop markings.
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