If you’re even somewhat serious about your photography you are most like very familiar with the term “F-Stop.” We are all used to seeing the F-number on our lenses which tells us the maximum F-stop that the lens is capable of. We know that we can manipulate our depth of field by adjusting the F-stop. However, this is only part of the story. There is another, much less understood term: T-stop.
If you have a cinema background you should be familiar with this. However, for many photographers this is a principle they may not have come across. The F-stop is the measure of the diameter of the opening of the aperture describing a ratio. The T-stop is a measure of the amount of light reaching the imaging plane. So why is this important?
If you have shot with a lot of different glass over the years you may have noticed a F2.8 lens does not always seem to be the same brightness. Particularly if you’re using lenses from different manufacturers. Lets start with what does F2.8 actually mean, beside being fast glass. As mentioned above the the F-stop is a ratio based on the equation F-stop=Focal Length/Aperture Diameter. For a 50mm F2.8 the you have F2.8=50mm/17.8mm. But just because the ratio of light going through the aperture is the same doesn’t mean the amount of light reaching the imaging plane is equal as well.
Whenever light passes through a glass element a tiny fraction is lost, no matter how good the glass you are shooting with. Some light is absorbed and lost as it passes through. Additionally tiny portions are reflected back from the glass surface. This was a problem in early film making so they developed the T-stop to have a constant standard between manufacturers. Cinema cameras tend to have a standard lens mount, the ARRI PL mount regardless of who made the camera unlike photography where every manufacturer tends to have there own proprietary mount. Thus in order to use different lenses from different manufacturers during the same lighting set up the T-stop was created.
T-stop is expressed as T-Stop=F-Stop/Lens Transmission Percentage. By measuring the amount of light reaching the images plane you can be sure what exactly you are getting. Every lens has both an F-Stop and a T-Stop. Why then is it important to understand what is being measured?
In the images above I was using a flash to help light the front to of the image because it was heavily backlit. I used an ambient light meter in the process. The Canon 100mm Macro IS F2.8 was used but the lens has a T-Stop of 3.3, giving the lens a half stop difference between the two. So when I am setting up the meter and other factors I have to account for the .5 stop loss of light.
The two main macro lenses I use are the Canon 100mm F2.8 IS and the Zeiss 50mm F.2.0 Makro. The Zeiss has an F-Stop of 2.0 and a T-Stop of 2.2. At the same F-Stop on both lenses I have the Zeiss coming out 1/3 of a stop brighter than the Canon. Now if I set up the shoot and switch lenses from Canon to Zeiss the images with the Zeiss will be a 1/3 over exposed. Now I did not change lenses in the image above but if I had I would have to adjust part of the exposure by a 1/3 to create the same image.
Knowing the T-Stop value of your lens is important. Head over to dxomark.com to look it up. They do some incredible lens testing which provides a wealth of information. If you are doing a lot of shoots with lighting and different lenses knowing the T-Stops of your lenses becomes invaluable in order to create the same exposure every time. Even when you are using one lens and ambient light know what is getting to the sensor will tell you what your exposure truly is.