The f-number runs backwards: smaller number, bigger hole, more light, less depth of field. On your kit lens the widest hole shrinks as you zoom — f/3.5 at 15 mm, f/5.6 at 45 mm — and nothing you press changes that.
f/3.5-5.6 min f/22 7 rounded blades sharpest ≈ f/8
Why the numbers look strange
The f-number is a ratio, not a size: focal length divided by the diameter of the hole. At 45 mm, f/5.6 is a hole about 8 mm across; at 15 mm, f/3.5 is about 4.3 mm across. The physically bigger hole is on the longer setting, yet both deliver the brightness their number promises — which is exactly why a ratio is the useful thing to print on a lens.
The sequence 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22 is each number multiplied by about 1.414. Area scales with the square of diameter, so multiplying by the square root of two halves the area, and halving the area halves the light. That is one stop.
Your lens’s real maximum aperture
| Focal length | 35 mm equivalent | Max aperture | Status |
|---|---|---|---|
| 15 mm | 23 mm | f/3.5 | published |
| approx. 18 mm | 27 mm | approx. f/4 | approximate |
| approx. 23 mm | 35 mm | approx. f/4.5 | approximate |
| approx. 30 mm | 45 mm | approx. f/5 | approximate |
| 45 mm | 69 mm | f/5.6 | published |
Where to set it, and why
| Goal | Aperture | Reason |
|---|---|---|
| Most background blur this kit allows | Widest available, at 45 mm, subject close | Shallowest depth of field the lens can produce |
| Most light indoors | Widest available | Buys shutter speed and keeps ISO down |
| Two or three people in a row | f/5.6 to f/8 | You need depth to cover them all |
| Landscape, sharp throughout | f/8 | Deep focus plus the lens’s peak sharpness |
| Near foreground and far building | f/11 | More depth, slight sharpness cost |
| Avoid | f/16, f/22 | Diffraction softens the entire frame |
Common mistakes
- Setting f/3.5 and then zooming in, expecting it to stay. Set the aperture after framing.
- Using f/22 for landscapes because more depth of field sounds better. Diffraction makes the whole frame mushy.
- Shooting everything wide open for blur. At f/5.6 the blur is modest and wide open is the lens’s softest setting. Get closer instead.
- Blaming the lens for a busy background. No f-number rescues a wall 30 cm behind your subject. Move the subject.
Assignment: the aperture ladder
Put a subject 1.5 m from you with a background at least 5 m behind. Zoom to 45 mm, mode A, Auto ISO. Shoot the same frame at f/5.6, f/8, f/11, f/16 and f/22 without moving. Then go to 15 mm, walk back until the subject is the same size, and shoot f/3.5 and f/8.
Two things to notice. The background firms up less dramatically between f/5.6 and f/11 than you expect — a useful reality check. And the 15 mm f/3.5 frame has a more detailed background than 45 mm f/5.6, despite the wider aperture. Focal length matters as much as f-number.