Author: Harry Guinness / Source: howtogeek.com
If you start to move from photography into videography, you’ll quickly start hearing about something called a t-stop, which are a combination of a lens’ f-stop and light transmittance value. Let’s take a deeper look at what that means.
What is An F-Stop?
In photography, an aperture is the hole in a lens that lets light into your camera. How much light your camera captures is measured by a combination of how long the shutter allows light through that aperture, and how big the aperture is. The aperture is measured in f-stops, and the number of each f-stop corresponds to the focal length of the lens divided by the diameter of the aperture. So, for example, a 50mm lens at f/2.0 has an aperture diameter of 25mm; a 100mm lens at f/2.0 has an aperture diameter of 50mm.
Whichever lens you use, f/2.0 will produce roughly the same exposure with the same shutter speed and ISO regardless of focal length, due to the inverse square law and the reduced field of view of the lens at longer focal lengths. A longer lens gathers more of the light from a smaller area while a shorter lens gather less of the light from a greater area. The result is that both gather the same amount of light.
You’ll note, however, I’ve been using the terms “roughly” and “about”. That’s because, while the physics remain the same, how each lens is constructed is different. And this is important for videography.
What is Light Transmission in a Lens?
Lenses—as we’ve covered before—aren’t perfect transmitters of light.
The different lens elements affect the light as it passes through, and one of their effects is to reduce the light. The elements in most lenses absorb (or deflect or otherwise waste) 10-40% of the light that’s passing through. This means that they only transmit 60-90% of the light that hits their front element.
The thing is, different lenses transmit different amount of light through the lens. A 50mm f/2.0 lens might have a lens transmittance of 70% while the 100mm f/2.0 lens might have a lens transmittance of 80%. This means that more light is going to hit the sensor if you use the 100mm…
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