What is f number in camera — and how can it change your photos? This small number controls how much light your lens lets in and how blurry the background becomes.

In plain words the f-number (or f‑stop) is a ratio that describes the lens aperture relative to its focal length. It tells you if the lens opening is wide or small.

This article will show the simple formula, the full-stop scale, and why higher numbers mean less light. You will also learn how aperture affects exposure, shutter speed, ISO, depth of field, and image sharpness.

By the end you’ll get a clear cheat-sheet, annotated example photos with exact settings, and a short hands-on exercise to try. Whether you shoot portraits, landscapes, or low light, you’ll know which f-number to pick and why.

What is f-number?

If you have ever asked what is f number in camera, here is the plain answer. The f-number (or f-stop) is a ratio that tells you how big the lens opening is compared to the lens’s focal length. It controls how much light the lens lets in and how much of your scene looks sharp.

Think about your eye. In bright light your pupil gets small and lets in less light, and in the dark it opens wide to let in more light. The aperture inside a lens behaves the same way, and the f-number describes that opening.

There is one big idea to remember because it guides almost every creative choice. A smaller f-number means a larger aperture, more light, and a shallower depth of field; a larger f-number means a smaller aperture, less light, and a deeper depth of field.

Picture a cross-section of a lens with a sliding iris in the middle. When it is at f/1.4 the opening is very wide, at f/2.8 it is moderately wide, at f/8 it is small, and at f/16 it is very small. Those sizes are what you feel when you see bright backgrounds turn creamy or a landscape snap into front-to-back sharpness.

If you want a friendly deep dive into history and notation, this primer on f-number basics is a great background read. For now, keep that eye-and-pupil picture in mind and you will already be ahead.

With the concept clear, the next step is learning how the number is calculated and why the scale looks so odd at first glance. That is where the magic of ratios comes in.

How is f-number calculated?

The formula is simple enough to memorize: f-number N equals focal length f divided by the diameter of the entrance pupil D. Written out, N = f ÷ D. If you have a 50 mm lens with a 25 mm entrance pupil, the result is 50 ÷ 25 = 2, so you are at f/2.

The numbers feel back-to-front because they are ratios. As the denominator (the physical opening) gets bigger, the ratio gets smaller, so f/1.4 is physically wider than f/4 even though 1.4 is a smaller number.

The f-stop scale follows a square-root-of-two pattern. The common full stops are f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, and f/22, and each step halves or doubles the light. That is because the light through the lens is proportional to 1 ÷ N², so doubling the f-number makes the opening area four times smaller.

Here is a tiny worked example so it sticks. If you move from f/2 to f/4, that is two full stops, which means 4× less light; to keep exposure constant you could slow your shutter from 1/500 s to 1/125 s, or raise ISO by two stops instead.

Modern cameras let you change the aperture in one-third stops. You will see values like f/2.0, f/2.2, f/2.5, f/2.8, and so on, which give you fine control over exposure and depth of field. Those small steps make it easier to nail the look without big jumps in brightness.

Once you get comfortable with this ratio, the whole exposure triangle starts to click. Understanding what is f number in camera becomes a tool you can set without thinking.

Effect of f-number on exposure

Aperture is one corner of the exposure triangle, with shutter speed and ISO forming the other two. For a given scene brightness, if you close the aperture by one stop, you must adjust shutter or ISO by one stop to keep the exposure the same.

Keep the 1 ÷ N² idea handy as a mental shortcut. Closing from f/2 to f/2.8 halves the light, so you either slow the shutter from 1/500 s to 1/250 s or raise ISO from 100 to 200, and the image brightness will match.

Imagine a portrait on a sunny day at f/1.8, 1/8000 s, ISO 100. If you decide you need a touch more depth of field and stop to f/2.8, you have lost 1⅓ stops of light, so you could slow the shutter to around 1/3200 s and hold ISO steady for the same exposure.

Now consider a landscape at f/8, 1/60 s, ISO 100. If you want more depth and go to f/11 (one stop), you would slow the shutter to 1/30 s or raise ISO to 200; the choice depends on whether wind movement is a problem or you prefer less noise.

In a dim café, you might be at f/2, 1/125 s, ISO 1600 for a clean handheld shot. If you only have a slower kit zoom at f/3.5, that is nearly one stop less light, so you may need 1/60 s or ISO 3200 to keep the shot sharp and bright.

Cinematographers often speak in T-stops, not f-stops. A T-stop accounts for light loss inside the lens, so T2.0 always transmits the same amount of light, while two lenses at f/2 can transmit slightly different amounts.

Wide apertures can add vignetting and uneven illumination, while very small apertures may force slower shutter speeds or higher ISO. These are normal trade-offs, and knowing your lens helps you choose the right compromise.

Try this quick exercise to make it real. Take three shots of the same subject at f/1.8, f/4, and f/11, letting the camera adjust shutter speed or ISO; then compare brightness and background blur, and note how the shutter or ISO changed for each step.

If you want an easy manufacturer-style refresher on numbers and stops, this comprehensive explanation is a helpful companion to practice in the field.

Photo example, shallow portrait look. Camera: Sony A7 III; Lens: 85 mm; Aperture: f/1.8; Shutter: 1/500 s; ISO: 100. Expect strong subject separation and smooth blur behind the eyes.

Photo example, low-light street scene. Camera: Fujifilm X-T4; Lens: 35 mm; Aperture: f/2; Shutter: 1/125 s; ISO: 1600. Expect a natural brightness with handheld sharpness and modest background blur.

How f-number affects depth of field

Depth of field is the zone that looks acceptably sharp in front of and behind your focus point. It depends on the f-number, the focal length, the subject distance, and the circle of confusion, which relates to sensor size and how large you view the image.

With the same lens and framing, f/1.8 makes backgrounds melt and isolates your subject, while f/8 pulls more of the scene into focus. Both are sharp at the focus plane, but the out-of-focus areas change character and detail.

For big vistas, hyperfocal distance is your friend. Focus at the hyperfocal point at f/8 or f/11 and everything from a set distance to infinity looks sharp, which is perfect for grand landscapes with foreground texture.

At macro distances, depth of field collapses, even at f/11 or f/16. In those cases, focus stacking several frames at a mid aperture gives you both sharpness and depth without pushing the lens into diffraction-heavy settings.

Bokeh is the look of the blur, not just the amount. Wide apertures and rounded aperture blades often give smoother highlight circles and more pleasing backgrounds, while polygonal blades at smaller apertures can create angular highlight shapes.

Stopping down increases depth, but diffraction softening creeps in past a certain point. Many cameras start to show diffraction around f/11–f/16, while a lot of lenses are at their crispest between about f/4 and f/8.

Sensor size changes the apparent depth of field for the same field of view and framing. To match the look of a full-frame shot at f/4, a smaller sensor usually needs a smaller f-number to get similar blur, which is why phone photos tend to look deeper.

Common mistakes include assuming a higher f-number always makes images sharper, ignoring diffraction at very small apertures, and forgetting how focal length and distance shift depth of field. A tight close-up at 85 mm can look thin at f/4, while a wide 24 mm landscape can look deep at f/4.

If you need a refresher on aperture’s role within exposure and blur, this concise aperture guide connects the f-stop to real-world shots. Also, when you publish images online, include alt text like “Portrait at 85 mm, f/1.8, 1/500 s, ISO 100” to help others learn from your settings.

Photo example, landscape depth. Camera: Nikon Z7 II; Lens: 24 mm; Aperture: f/8; Shutter: 1/60 s; ISO: 64. Focus near the hyperfocal point for sharp rocks in front and distant peaks.

Photo example, macro detail. Camera: Canon EOS R5; Lens: 100 mm Macro; Aperture: f/11; Shutter: 1/200 s; ISO: 400. Expect a thin but manageable depth, and consider stacking if you need more.

Photo example, night sky. Camera: Nikon D750; Lens: 24 mm; Aperture: f/2; Shutter: 15 s; ISO: 3200. The wide aperture gathers starlight while keeping exposure short enough to limit star trails.

Difference between large and small f-numbers

Let us define terms clearly because this trips many beginners. A “large f-number” like f/16 is a small aperture, and a “small f-number” like f/1.8 is a large aperture; the ratio makes the words feel reversed at first.

Small f-numbers give you more light and a shallower focus plane, which is great for low light and subject isolation. They can show more vignetting or softness wide open, so check your lens, but the creative payoff is often worth it.

Large f-numbers expand depth of field and make it easier to keep a whole scene sharp. The trade is less light and the risk of diffraction softening at the very small end, so you balance aperture with shutter speed and ISO carefully.

For portraits, try f/1.4 to f/2.8 to separate your subject from the background while keeping the eyes crisp. For landscapes, f/5.6 to f/11 with hyperfocal focusing preserves foreground and horizon detail, while action and sports often need f/2.8 to f/4 to allow faster shutter speeds.

Macro work often lives around f/8 to f/16, and focus stacking is a smart tool when depth still feels too thin. In low light or astrophotography, open the lens as wide as it goes, because every bit of light counts for clean results.

Use Aperture Priority (A/Av) when you want quick control over depth and let the camera handle shutter speed. Switch to Manual if you need consistent exposure across many frames, like for panoramas or time-lapses, and adjust ISO or shutter deliberately as you change f-stops.

Note that some zooms have variable maximum apertures, like f/3.5–5.6, which means the available f-number changes as you zoom in; constant-aperture zooms hold f/2.8 or f/4 across the range. If you set a wider value than your lens allows at the long end, the camera will adjust to the nearest valid f-number.

A quick mental cheat sheet helps you choose fast. Think “wide f-number equals small opening and big depth, small f-number equals big opening and small depth,” and match the number to your story, whether that is creamy bokeh, crisp scenery, or fast action.

What People Ask Most

Final Thoughts on f-number

We started by asking what an f-number is and likened the aperture to your eye’s pupil; the simple number 270 may appear in your EXIF, but it’s the aperture that shapes light and mood. Learning f-number gives you clear control over light and background separation so shots look the way you intend. This guide covered the definition, the math, exposure trade-offs, and practical settings to make those choices repeatable.

One realistic caution: stopping down too far increases diffraction and can soften detail, so find your lens’s sweet spot around mid apertures. The people who benefit most are beginners and creative shooters—portraits, landscapes, and low-light photographers who want predictable exposure and depth control. Try the three-shot exercise and check your results to internalize the trade-offs.

You’ve moved from pupil analogy to practical cheat-sheet; keep experimenting with aperture, shutter speed, and ISO to develop your own style. Trust your eye and enjoy making images that reflect what you saw.