Every lens has an f-number printed on the barrel — f/1.4, f/2.8, f/4. Most photographers understand what those numbers mean in practice: smaller numbers let in more light and produce shallower depth of field. But where does that number actually come from, and what does it have to do with the physical lens you’re holding? And what’s a T-stop, and why does it exist if the f-stop already tells you the aperture?

This guide answers all of it, cleanly and without unnecessary complexity.

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What the f-ratio means in practice

The f-ratio — also called the f-number or f-stop — tells you two things about a lens: how much light it can let in at its maximum aperture, and how strong a background blur (bokeh) it can produce relative to other lenses.

A lens with a maximum aperture of f/2.8 lets in twice as much light as a lens with a maximum aperture of f/4. That’s one full stop of light — a meaningful difference in low-light shooting, flash power requirements, and minimum usable shutter speed.

The same f/2.8 lens will also produce stronger background blur than the f/4 lens when both are shooting at the same distance from the subject. The wider aperture creates a shallower depth of field, and the out-of-focus elements behind the subject dissolve more completely.

For a deeper dive on how aperture works across the full exposure triangle and its creative implications, see our complete aperture guide.

How the f-ratio is calculated

The f-ratio has a precise optical definition: it’s the focal length of the lens divided by the diameter of the entrance pupil — the effective diameter of the aperture opening as seen from the front of the lens.

The formula is simply: f-ratio = focal length ÷ entrance pupil diameter

Using the Canon EF 50mm f/1.8 as an example: 50mm divided by an entrance pupil diameter of approximately 27.8mm gives you f/1.8. A 50mm lens with a larger entrance pupil — say 35.7mm — would produce f/1.4. A smaller entrance pupil of 17.9mm would produce f/2.8.

This is why the f-number is based on a physical measurement. The relationship between focal length and aperture diameter is fixed by the geometry of the lens. This also explains why a 200mm f/2.8 lens has a physically much larger front element than a 50mm f/2.8 lens — to maintain f/2.8 at a longer focal length, the entrance pupil diameter must be proportionally larger. 200mm ÷ 2.8 requires an entrance pupil of approximately 71mm. 50mm ÷ 2.8 requires only about 18mm.

F-stops vs. T-stops: what’s the difference and why does it matter

The f-ratio is a geometrically calculated number based on the physical dimensions of the lens. It tells you how much light the aperture opening theoretically admits. What it doesn’t account for is what happens to that light as it travels through all the glass elements inside the lens.

Every glass element a light ray passes through absorbs and reflects a small fraction of the light. A complex lens with many elements — a 70-200mm zoom, for example — loses more light to absorption and internal reflection than a simple prime with fewer elements. The f-ratio doesn’t account for any of this.

The T-stop (Transmission Stop) is the measurement that does. It represents the actual amount of light that arrives at the sensor after passing through the entire optical system. If a lens is rated at f/1.4 but its glass absorbs enough light that only the equivalent of f/1.6 actually reaches the sensor, then the lens has a T-stop of T/1.6.

For still photographers, this distinction is almost never practically relevant. Your camera’s metering system measures light at the sensor, not at the aperture opening, so it accounts for any light loss automatically when determining exposure. You dial in f/1.4 and the camera’s meter compensates for whatever the actual transmission is.

For cinematographers, however, T-stops matter significantly. When shooting video across multiple camera setups or cutting between different lenses, exposure consistency between cuts is critical. Two lenses rated at f/2.8 might have different T-stops — T/2.9 and T/3.1 — which would produce a visible brightness difference in a cut between them. Cinema lenses are rated and calibrated in T-stops precisely to avoid this problem, ensuring that T/2.8 on one lens delivers exactly the same exposure as T/2.8 on another.

One important clarification: the T-stop has no effect on depth of field. Depth of field is determined by the physical f-ratio — the actual geometry of the aperture opening — not by the amount of light that makes it through the glass. A lens set to f/1.4 produces f/1.4 depth of field regardless of whether its T-stop is 1.5 or 1.8.

What this means for choosing lenses

For most still photographers, the f-ratio is the number that matters for lens selection, exposure decisions, and depth of field control. The T-stop is a behind-the-scenes consideration that your camera handles automatically.

Where T-stop awareness becomes useful for still photographers is in understanding why two lenses with the same maximum aperture might behave slightly differently in low light — particularly when using older or lower-quality optics with more significant light loss through their elements. Modern lenses with high-quality coatings minimize this gap considerably, keeping T-stops very close to the stated f-ratio.

For more on how aperture affects your images in practice — including its relationship with depth of field, bokeh, and the exposure triangle — see our aperture guide and the complete Learn Photography hub.

Frequently asked questions about f-ratio and t-stops

What does f/1.4 mean on a lens?

It means the lens has a maximum aperture with an f-ratio of 1.4 — the focal length of the lens divided by the entrance pupil diameter equals 1.4. In practical terms, it tells you the lens can admit a relatively large amount of light and produce significant background blur at its widest opening. A smaller f-number always means a wider aperture, more light, and shallower depth of field at any given focal length and shooting distance.

Does a higher f-ratio mean a better lens?

No. A lower f-ratio (wider maximum aperture, like f/1.4 or f/1.8) means a faster lens — one that lets in more light and can produce shallower depth of field. Whether that’s “better” depends entirely on what you shoot. A landscape photographer shooting on a tripod at f/11 has little use for f/1.4. A wedding photographer shooting in dark receptions needs every stop of light available. Lens quality is determined by sharpness, distortion control, bokeh character, build quality, and autofocus performance — not the f-ratio alone.

Should I care about T-stops when buying lenses for photography?

Generally no, for still photography. Your camera’s metering system compensates for actual light transmission automatically, so T-stops don’t affect your exposure decisions. Where T-stops become relevant is in video and cinema work, where consistent exposure across different lenses in a cut matters. If you’re building a dedicated cinema kit, T-stop ratings become an important consideration. For stills-focused shooters, the f-ratio, optical quality, and autofocus performance are more meaningful selection criteria.

Why are cinema lenses so much more expensive than photo lenses with similar specifications?

Cinema lenses are calibrated and measured in T-stops rather than f-stops, meaning their actual light transmission is precisely tested and labeled rather than just geometrically calculated. They’re also designed for manual focus with smooth, linear throw, minimal breathing (focal length shift during focus), and consistent mechanical behavior for follow focus systems. They’re built for frame-by-frame reliability across long shooting days and often in challenging environments. All of that precision manufacturing and testing adds significant cost over photo lenses that don’t require the same consistency standards.