These slight differences aren’t often a big deal to many photographers. The differences may not be huge, but placed side by side, they’re often noticeable. Half a dozen different lenses at f/4, even from the same manufacturer, can often yield different exposures at the same settings. Today, though, there are so many different lens coatings and strange designs that light gets lost all over the place. Pick up a few different lenses of the same aperture, and they’d probably be virtually indistinguishable. When glass had little-to-no coatings, and lens design was very simple, there probably wasn’t much light lost.
In theory.Īt one time, this was probably an extremely reliable and consistent measurement. This is why a 50mm f/1.4 has a much smaller aperture than a 105mm f/1.4 but still allows (roughly) the same amount of light to hit the sensor. The focal length and aperture relationship allows a certain overall amount of light through the lens. Twice the diameter means four times as much area, so, two stops more light gets in. That same 100mm lens at f/2 has an aperture opening of 50mm. So, a 100mm lens at f/4 has an aperture opening of 25mm. A T-Stop is the actual measurement of light transmitted through the lens. It’s based on the focal length of the lens relative to size of the opening through which light is allowed. Essentially, an F-Stop is a mathematical equation.
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