When Is An Aperture Of F1.2 Not Actually A F1.2? By the time the light gets to your sensor, that’s when.
One of the first things we learn as photographers are F stops and how we can use them to properly expose a photograph, but there is also such a thing as T stops and we don’t always give them the attention they deserve. Of course, a T-stop may not be essential knowledge on every photo you take, but understanding what a T stop is will give you a better understanding of light, which is never a bad thing for a photographer to have. (It’s also helpful information to have in your bag if you’re going to be lens shopping soon!). And Matt Granger does an amazing job of explaining the difference.
The F-Stop vs The T-Stop
F Stop – The F stop is the measurement of the opening of the lens.
T Stop – The T stop (T= Transmission) is the measurement of how much light has passed through the aperture and actually made it to the sensor.
As we all know, a lens is made up multiple glass optics, or individual lenses inside the lens body. As light travels through the lens, each of those glass optics absorb part of the light, effectivly reducing the amount of light that actually hits your sensor. This happens regardless of your f stop, so even though you may be shooting at an aperture of f 1.2, your sensor may only be seeing an amount of light equivalient to f 1.4. It may not seem like there’s a big difference between a 1. 2 and 1.4, but it’s actually half a stop!
Check out Matt Granger’s video tutorial (below) as he eloquently breaks down the T stop, then you can head over to DXO Mark and check out the t-stop values for all your lenses.
Video Breakdown
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