If you have read our beginner's guide to aperture in photography, you already know that the f-number describes how wide the lens opens, and that it shapes both exposure and depth of field. Pick up a cine lens, though, and the aperture ring reads T1.4 or T1.8 instead of f/1.4. Both numbers describe lens aperture, but they measure different things. Here is what separates an f-stop from a T-stop, and when the difference matters for your photos and videos.
What Is an f-stop?
An f-stop is a calculated value: the lens's focal length divided by the diameter of its entrance pupil (the opening as seen from the front of the lens). It describes the geometry of the aperture, not how much light actually reaches the sensor.
This is what the f-number on a lens means. A 50mm lens with a 25mm entrance pupil is f/2; close the aperture until the pupil is about 3mm wide and you are near f/16. Because the number is a ratio, f/2 lets in roughly the same amount of light on a 24mm lens as on an 85mm lens, which is why photographers can move between lenses and keep the same settings.
The f-stop scale (f/1.4, f/2, f/2.8, f/4, f/5.6 and so on) is built so each full stop halves or doubles the light. With aperture explained this way, the number is a clean, theoretical reference point.
The video below walks through the practical differences between f-stops and T-stops, along with the math behind each.
What Is a T-stop?
A T-stop ("transmission stop") is a measured value. It tells you how much light a lens actually transmits to the sensor at a given aperture, after losses from glass absorption and reflections at each lens surface are taken into account.
No lens transmits 100% of the light that enters it. Every element absorbs a little, and every air-to-glass surface reflects a little, even with modern coatings. A lens with many elements loses more than a simple design. The T-stop is the f-number adjusted for that loss: T = f-number ÷ √(transmittance). A lens that is f/1.4 on paper and transmits around 80% of incoming light works out to roughly T1.6.
That is why a T-stop is always equal to or slightly higher than the f-stop of the same lens. Cine lens manufacturers test each lens and mark the aperture ring in T-stops so the number reflects real-world exposure.
f-stop vs T-stop: What's the Actual Difference?
The f-stop describes the size of the opening; the T-stop describes how much light comes through it. One is geometric, the other is photometric. The table below sums up the key points.
| f-stop | T-stop | |
|---|---|---|
| What it is | Focal length ÷ entrance pupil diameter | f-number corrected for measured light loss |
| How it is determined | Calculated from the lens design | Measured on the lens |
| Describes | Physical aperture size | Actual light reaching the sensor |
| Controls depth of field? | Yes | No, it describes exposure only |
| Typical lenses | Photo and hybrid lenses | Cine lenses |
| Exposure match between lenses | Close, may vary slightly | Consistent across lenses at the same T-stop |
Does a T-stop Affect Depth of Field?
No. Depth of field depends on the physical size of the aperture, which is what the f-stop describes, along with focal length, subject distance and sensor size. The T-stop only tells you about brightness.
In practice this means a lens rated T1.4 is physically a little wider than f/1.4, since its f-number sits slightly below its T-stop. If you are planning a shot with very shallow depth of field, calculators and depth-of-field charts should use the f-number. For exposure, the T-stop is the more reliable number. For most shooting the gap is a fraction of a stop, so a wide aperture lens will look similar either way; it only becomes worth tracking when you need precise control.
Why Do Cine Lenses Use T-stops Instead of f-stops?
Film and video crews often switch lenses several times within one scene. If each lens at "f/2" delivered a slightly different brightness, every cut would show an exposure jump, and the colorist would have to correct it. T-stops remove that problem: set any lens in a matched set to T2 and the image brightness stays the same.
This is one of the main reasons a cine lens is built differently from a stills lens. Other differences usually include a geared focus and aperture ring for follow-focus systems, a long focus throw, and a declicked aperture ring. A declicked aperture for video lets you adjust exposure smoothly during a take, for example when the camera moves from a shaded interior to a bright window, without a visible step in brightness. You can read more about these features in what defines a cine lens.
Photographers rarely need T-stops. The camera's metering reads the light that actually reaches the sensor, so small transmission differences are compensated automatically in most stills work.
How Are SIRUI Lenses Rated: f-stop or T-stop?
SIRUI rates each lens according to how it is designed to be used.
f-stop ratings (photo and hybrid autofocus lenses):
- Aurora 35mm and 85mm F1.4: full-frame autofocus primes for both photo and video. They are sharp wide open, and a clicked/declicked aperture switch lets the same lens work with click stops for stills or smooth aperture changes for video.
- Sniper F1.2: APS-C autofocus primes in 16, 23, 33, 56 and 75mm, suited to portraits, night scenes and vlogging.
T-stop ratings (cine lenses):
- Vision Prime: full frame cine lenses at T1.4 (24/35/50/75mm), T1.6 (15mm) and T4 (150mm macro), with consistent color across the set for easy lens changes.
- Astra T1.8: the world's first full-frame autofocus anamorphic lenses, in 50, 75 and 100mm. Although they focus automatically, they are rated in T-stops because they are built for cinema workflows.
Final Thoughts
The f-stop and the T-stop answer two different questions. The f-stop tells you how wide the aperture is, which governs depth of field. The T-stop tells you how much light gets through, which governs exposure. For photography, the f-stop is all you need, since your camera's meter handles the rest. For video, especially when you cut between several lenses in one scene, T-stops keep exposure consistent from shot to shot. Knowing which number you are reading makes it easier to choose the right lens and set it correctly.
FAQ
Is T1.4 the same as f/1.4?
Not exactly. A T1.4 lens transmits as much light as a theoretically perfect f/1.4 lens. Its actual f-number is slightly lower, so its physical aperture is a little wider than f/1.4.
Why is the T-stop number always higher than the f-stop?
Because every lens loses some light to absorption and reflection inside the glass. The T-stop accounts for that loss, so it can only equal or exceed the f-number.
Can you convert an f-stop to a T-stop?
Only if you know the lens's transmittance. The formula is T = f-number ÷ √(transmittance). Since transmittance has to be measured, manufacturers test cine lenses individually rather than converting from a spec sheet.
Do photographers need to care about T-stops?
Rarely. Camera metering reads the light that actually reaches the sensor, so small transmission differences are corrected automatically. T-stops matter most in video work with multiple lenses or external light meters.
