How to Find out Your Lens ‘T-Stop’ Transmission

If you didn’t know, the marked f-stop on your lens is probably a lie. The actual light transmission of lenses is almost always less than the marked f-stop. I’m not talking about the lens edges and corners, where transmission is even worse. Real lens light transmission (in the center, at least) is given by its T-stop, where T stands for transmission.

Often, cinema lenses are marketed by their T-stop, because you need to be able to swap lenses and be able to get the same light level at a given f-stop setting. Movie makers and editors are much more demanding than still photographers, it appears.

You can still use the marked lens f-stop for determining the depth of focus; it just won't reliably indicate how much light your camera sensor is seeing.

To find out your lens T-stop, you need to either look up the T-stop information on the web, or else you need a ‘reference lens’ with a known transmission for a given f-stop. A really handy website that gives T-stops for many lenses is DxO Mark . This excellent website has much more information than just T-stops, of course.

As an example, I have a ‘reference lens’, which is my Nikkor 85mm f/1.4G AF-S lens. This isn’t actually an f/1.4 lens, but an f/1.6 transmission lens or T 1.6.

DxO Mark website Nikkor 85mm f/1.4G AF-S lens data

As shown above, my 85mm f/1.4 is really an 85mm f/1.6 lens.

If you can’t find a website that has already analyzed your lens for transmission, then you need to figure out the T-stop yourself, starting with a reference lens that you can access.

If you can locate a reference lens with a known T-stop, then you can take a photo of a smooth evenly-lit surface, such as a wall, using your reference lens to establish the exposure. Make sure your camera is in manual mode at a fixed ISO value, where ideally you set the same marked aperture you’ll use on your ‘unknown’ lens under test. Note the shutter speed required for correct exposure.

By the way, you’ll need to switch to ‘spot’ metering, and make sure you are looking at the center of the field of view when metering. Otherwise, the camera meter will be influenced by the (dimmer) edges and corners of nearly every lens. Most camera meters only give you a resolution of 1/3 stop, so don’t expect a more accurate T-stop estimate than that.

For these procedures to work, your camera needs to be able to measure the exposure with the lens aperture actually stopped down to the requested f/stop. My Nikon mirrorless cameras automatically focus and measure the exposure at the requested aperture, and not wide-open (through f/5.6).

Make sure that the tested lens has its focus set to infinity. Lenses get dimmer as you focus closer, and lens T-stops should always be referenced to infinity focus.

If your lens under test doesn’t have the same (marked) widest aperture, then you’ll have to assume that your reference lens has consistent transmission loss at each aperture. Set the reference lens to the same marked aperture as your unknown lens under test, and then note the shutter speed for correct exposure.

Now, switch to your unknown lens at the same marked aperture as your reference lens, and adjust the shutter speed to get the correct exposure. Compare this shutter speed from your shot with the reference lens at the same aperture. This change in shutter speed between lenses can be converted into an Exposure Value, or EV, to see how much different your tested lens is, compared to your reference lens. 

I use a handy website called ScanTips.com. This website has an exposure calculator to convert f-stops into the equivalent EV value. There are many apps available that can do this same thing. To do the math on exposure changes, you need to convert from f-stops into EV values. An EV change of 1 is the same as the change of 1 f-stop. In the calculator ‘A’ side, set your reference lens marked f-stop, shutter speed, and ISO. Also enter your unknown lens marked f-stop, shutter speed, and ISO in the calculator ‘B’ side. This will give you the EV change between lenses. Add this EV change with your reference lens T-stop, and you now have your unknown lens T-stop.

To calculate the Exposure Value (EV) difference between my 85mm marked f/1.4 aperture and its actual f/1.6 T-stop, the calculator was set up like this:

EV change from f/1.4 to f/1.6 is 0.33EV

For example, my 85mm lens at the marked f/1.4 aperture had an exposure of 1/1000 second at ISO 400.  My TTArtisan 50mm f/0.95 lens, when set to its marked f/1.4 aperture and ISO 400 also needed a shutter speed of 1/1000 second for the correct exposure.

This means that the TTArtisan f/0.95 lens at f/1.4 would also be T-1.6, having the same transmission as the Nikkor 85mm f/1.4 lens at the same marked f/1.4 aperture.

I set the TTArtisan 50mm lens to a marked f/1.0 (one stop wider than f/1.4), and the camera meter indicated it needed a shutter of 1/1600s.

TTArtisan 50mm f/0.95 lens at f/1.0

The results above indicate that a change from f/1.4 and 1/1000s to f/1.0 and 1/1600s means that the TTArtisan lost another 0.33 stops of light transmission. I was expecting the correct exposure to be 1/2000s at f/1.0.  When I then changed to the marked f/0.95 aperture, the meter indicated correct exposure needed a shutter speed of 1/2000s.

Since this TTArtisan wide-open at f/0.95 transmits twice the light of my 85mm Nikkor wide-open, that means that its T-stop would then be T 1.1, which is 1 stop faster than the Nikkor 85mm’s T 1.6.

I guess this lens loses some of its bragging rights, going from “f/0.95” to the actual transmission of T 1.1.