Do you know what is the difference between f/stops and t/stops is? And do you know what both of them mean for photographers and cinematographers? Tony Northrup, the co-host of Tony & Chelsea Northrup, discusses these answers to those questions and also discuss how you can apply f/stops and t/stops to different lenses.

F/stop is the ratio of the focal plane to the diameter of the opening and a lens. And the lower the number of an f/stop is the blurrier the background gets. Northrup says that this is important for photographers to know because they obviously would like to have the creative choice of blurring their picture’s background or not.

But what about exposures? A lot of us may think that f/stops determine how much light your camera is gathering, but Northrup says f/stops sort of do that, but only as a shortcut. “In practice, f/stops cannot necessarily have a direct relationship, have like a perfect relationship with the exposure. That measurement is actually t/stops.”

F/stops are measuring the light that hits the lens and t/stops are measuring the light that comes out the back of the lens.

The reason why filmmakers use the term t/stops and photographers use f/stops is because back when filmmakers just started to make film, they discovered that they could cut 2 f2.8 lenses together so they could physically stitch the film together. But they noticed that they would have different brightnesses, because of how the light was passing through the lense (some lenses allow light to pass in pretty smoothly, while others have light bounce around inside it or the light gets absorbed or it can be blocked.) Even though the f/stops were the same, they exposures looked different, so this is when t/stop began to be used. They began to measure the light that comes out the back of the lens.

To Northrup, the reason why photographers still use the term f/stop is because if a picture’s exposure looks a little different than the previous one, it isn’t a big deal. With film, it isn’t desirable to have light exposures change suddenly in a scene, so that’s why filmmakers refer to t/stops.

Lens manufacturers sometimes use the confusion between f/stops to t/stop to their advantage. Northrup remembers a time when he was super excited to try out the Nikon 50mm f/0.95, thinking that it would be great in low light. However, when he actually tried it, he found that it didn’t perform any better than the Canon 50mm f/1.4. He discovered that the Nikon lens that open at f/0.95 was actually T1.4, meaning that it was producing the same brightness as a f/1.4 lens even though their f/stops were different.

To summarize, the difference between f/stops and t/stops is like the difference between measuring the light hitting the lens and measure the light that exists the back of the lens. Northrup believes that every photographer should know both the f/stop and t/stop, primarily to avoid being mislead by manufacturers.

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Sean Berry
Sean Berry is a blogger and Videomaker Contributing Editor.