Why does a big number not mean a big aperture?

Because the **f-number** is a mathematical ratio, not an absolute diameter. The formula is: *f/N = focal length ÷ aperture diameter*. N is the denominator. So a small f-number (f/1.4) means a large aperture diameter, and a large f-number (f/16) means a small one. Same logic as a fraction: 1/2 of a pizza is bigger than 1/16. Once that reflex sets in, the f/1.4 → f/22 scale becomes intuitive: *the higher the number, the more you close the tap*.

How do I find a lens's sweet spot?

The **sweet spot** is the aperture where the lens delivers its best sharpness, contrast, and lowest aberrations. Rule of thumb: it sits **two to three stops down from wide open**. A 50 mm f/1.8 is at its best around f/4–f/5.6; an f/2.8 zoom rather around f/5.6–f/8. To confirm it on your copy, photograph a detailed scene (brick wall, newspaper) at every aperture, on a tripod, and compare at 100% in the center and the corners. Sites like DxOMark and Lenstip publish these curves for most optics.

Which aperture for Reels or vertical photos?

For vertical mobile formats (Reels, Stories, TikTok), reading happens at small size on screen: **f/2.8–f/4** is often the best compromise. Open enough to isolate the subject and get that *cinematic* look that works in scroll, but closed enough to keep the face fully sharp despite micro-movements. At f/1.4, depth of field is so thin that the slightest shift of camera or subject blows the focus — a risk you rarely take handheld in portrait mode. Save f/1.4 for staged, controlled shots.

EXIF Data

Aperture

Aperture is the lens's light tap — and the most cinematic lever on the body.

Photo : Bernard Hermant / Unsplash

Definition

What is aperture?

Aperture is the diameter of the diaphragm inside the lens — that ring of metal blades that opens or closes to meter the light reaching the sensor. It's expressed as an f-number, the ratio between focal length and the effective aperture diameter. The counter-intuitive consequence: the smaller the number, the larger the opening. f/1.4 lets in plenty of light, f/22 very little.

The standard scale follows a geometric progression: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22. Each step (or stop) doubles or halves the amount of light. Going from f/2.8 to f/4 cuts light in half; going back from f/8 to f/5.6 doubles it.

A lens's maximum aperture defines its speed: a Leica Summilux 50 mm f/1.4 or a Zeiss Otus f/1.4 are called fast lenses. At the other end, the optical sweet spot — where the lens delivers its best sharpness — usually sits two or three stops down from wide open, around f/5.6 to f/11 for most modern optics.

Example

In practice

Three concrete scenarios to lock in the reflex.

Portrait, 85 mm at f/1.8. Every portraitist's go-to lens. At this aperture, depth of field shrinks to a few centimeters: the eyes are sharp, the ears already drift into blur, and the background dissolves into creamy bokeh. The subject literally lifts off the backdrop. Watch out: at f/1.4 the margin gets so thin that a slight movement from the model is enough to lose focus on the eye.

Landscape, 24 mm at f/8–f/11. Here you want the opposite: everything sharp, foreground to horizon. f/8 is the sweet spot for most wide-angles; f/11 extends the zone of sharpness without giving up too much optical quality. To push further, calculate the hyperfocal distance rather than stopping down to f/22.

Low light, 35 mm at f/1.4. Concert, night street, indoors without flash. The wide aperture lets you keep ISO reasonable and shutter speed fast enough to freeze the gesture. It's the third corner of the exposure triangle: opening up means buying light you wouldn't otherwise have.

Common mistakes

What we see too often

f/1.4 by default — the Instagram reflex. Buying a 50 mm f/1.4 and never closing it again. The problem isn't diffraction (you're nowhere near it), it's a ridiculously thin depth of field: on a portrait, the eye is sharp and the tip of the nose is soft. On a restaurant plate, the fork is crisp and the dish behind has vanished. Stopping down to f/2.8 or f/4 gives volume back to the subject without killing the bokeh.

f/22 to "get everything sharp." Logically, the more you close down, the deeper the sharp zone. Except that from f/16–f/22 on, diffraction kicks in: light bends as it passes through too small an opening, and the entire image loses bite. On a full-frame sensor, the drop-off is visible from f/11 with some lenses. Prefer f/8–f/11 plus hyperfocal focusing.

Confusing aperture with lens speed. A zoom with a constant f/4 aperture (like a 24-70 mm f/4) and a 50 mm prime at f/4 share the same number, but not the same optics. More importantly, an f/3.5-5.6 zoom sees its maximum aperture close down as you zoom in — a classic trap in low light.

How Focalis evaluates

How Focalis evaluates aperture

Focalis-X reads the aperture value from your file's EXIF data, then cross-checks it against the detected scene type. A portrait at f/11 will trigger a note about a lack of subject isolation; a landscape at f/2 about insufficient depth of field; a night shot at f/16 about its inconsistency with high ISO. The analysis also combines aperture with focal length and focus distance to estimate the actual depth of field. Analyze a photo →

FAQ

Frequently asked questions

Why does a big number not mean a big aperture?
Because the f-number is a mathematical ratio, not an absolute diameter. The formula is: f/N = focal length ÷ aperture diameter. N is the denominator. So a small f-number (f/1.4) means a large aperture diameter, and a large f-number (f/16) means a small one. Same logic as a fraction: 1/2 of a pizza is bigger than 1/16. Once that reflex sets in, the f/1.4 → f/22 scale becomes intuitive: the higher the number, the more you close the tap.
How do I find a lens's sweet spot?
The sweet spot is the aperture where the lens delivers its best sharpness, contrast, and lowest aberrations. Rule of thumb: it sits two to three stops down from wide open. A 50 mm f/1.8 is at its best around f/4–f/5.6; an f/2.8 zoom rather around f/5.6–f/8. To confirm it on your copy, photograph a detailed scene (brick wall, newspaper) at every aperture, on a tripod, and compare at 100% in the center and the corners. Sites like DxOMark and Lenstip publish these curves for most optics.
Which aperture for Reels or vertical photos?
For vertical mobile formats (Reels, Stories, TikTok), reading happens at small size on screen: f/2.8–f/4 is often the best compromise. Open enough to isolate the subject and get that cinematic look that works in scroll, but closed enough to keep the face fully sharp despite micro-movements. At f/1.4, depth of field is so thin that the slightest shift of camera or subject blows the focus — a risk you rarely take handheld in portrait mode. Save f/1.4 for staged, controlled shots.