A dark box, a hole, and an upside-down world. Light travels in straight lines. A small hole lets only a thin bundle of rays through from each point of a scene, so they paint a picture on the back wall.
A dark box, a hole, and an upside-down world. Take a real camera apart in 3D and learn what aperture, shutter speed and ISO really do.
CameraClearOpened 6 Jul 202615 min to playFree · no sign-up
In 60 seconds
Light travels in straight lines. A small hole lets only a thin bundle of rays through from each point of a scene, so they paint a picture on the back wall. It lands upside-down because rays from the top cross over to the bottom.
A bigger hole gives a brighter picture but a blurrier one. A lens bends every ray from one point back to one point, so you get a wide opening that is still sharp. Focusing means moving the lens until those rays meet exactly on the sensor.
Iris blades set the size of the opening. A wide aperture (f/2) lets in lots of light and blurs the background. A narrow one (f/16) lets in little light and keeps near and far sharp. That in-focus zone is the depth of field.
Two curtains slide across the sensor. A fast shutter (1/1000 s) freezes motion. A slow one (1/15 s) collects more light but smears anything that moves, including your own shaking hands.
The sensor counts photons in millions of tiny wells. ISO amplifies that signal after the fact. It brightens a dark photo, but it amplifies the random grain too.
Aperture, shutter speed and ISO all trade against each other. Close the aperture one stop and you need twice the shutter time, or twice the ISO, for the same brightness. Every photo is a choice about which side effect you'd rather have.
Laws at work here
The full explanation
Chapter 1
A dark box, a hole, and an upside-down world.
Every camera is a dark box. Light from each point of the flower travels in straight light rays. A tiny pinhole lets through only a thin bundle from each point, so they paint a picture on the back wall — the sensor.
Follow the coloured rays: light from the top of the flower goes down through the hole, and light from the bottom goes up. That's why the image is upside-down.
Chapter 2
Take one apart. Click every piece.
A modern camera is a light-tight body with a lens on the front and a sensor at the back. Everything in between exists to control how much light reaches the sensor, for how long, and how sharply.
Light's journey: lens elements bend it → the aperture limits how much gets through → the shutter opens for a moment → the sensor records it.
Chapter 3
Making the rays meet exactly on the sensor.
A lens catches a whole cone of rays from each point of your subject and bends them so they meet again at a single point behind it. Focusing means moving the lens until that meeting point lands exactly on the sensor.
If they meet too early or too late, the point is recorded as a disc — the blur circle. Big disc, blurry picture.
The lens equation 1/f = 1/u + 1/v tells you where they meet: closer objects (small u) focus further back (big v), so the lens has to move forward to focus close.
Chapter 4
The adjustable hole that controls light — and blur.
Inside every lens is an iris of overlapping blades. The hole they form is the aperture. It's measured as an f-number: the focal length divided by the hole's diameter.
So small f-number = big hole. f/1.4 is wide open; f/22 is a pinprick. Each step in the sequence f/1.4 → 2 → 2.8 → 4 → 5.6 → 8 → 11 → 16 → 22 halves the light — one stop.
With Av mode on, the camera adjusts the shutter speed to keep brightness the same, so you only see the blur change. Switch it off to see the brightness change too.
Chapter 5
How long the light gets in — and what moves meanwhile.
The shutter is a pair of curtains in front of the sensor. The first curtain drops to start the exposure; the second follows to end it. The time between them is the shutter speed.
At very fast speeds the second curtain chases the first so closely that only a slit sweeps across the sensor — every row still gets the same tiny exposure. This is a focal-plane shutter.
Auto ISO is on, so brightness stays constant while you explore — only the blur changes.
Chapter 6
Buckets of light, and turning up the volume.
A sensor is a grid of photosites — tiny wells that collect photons during the exposure. More light → fuller wells → brighter pixels.
Each well only measures how much light, not colour. A Bayer filter of red, green and blue squares lets each well see one colour; the camera fills in the rest. Microlenses funnel light into each well.
ISO doesn't catch more light. It amplifies what was caught (gain). In dim light a few random photons make a big difference, and amplifying them makes that randomness visible as noise.
Chapter 7
Fill the bucket. Choose your side effects.
Exposure is filling a bucket with light. The tap width is the aperture, how long it runs is the shutter speed, and the bucket size is ISO — higher ISO means a smaller bucket, filled by less light.
Many combinations fill the bucket exactly to the line. f/4 at 1/250 lets in the same light as f/5.6 at 1/125: one stop narrower, one stop longer. Those are equal EVs.
Chapter 8
Wide-angle to telephoto: how much of the world fits.
Focal length (in mm) is how strongly a lens magnifies. Short focal lengths squeeze a wide slice of the world onto the sensor; long ones magnify a narrow slice. The orange pyramid in the 3D view is the camera's field of view.
Roughly: 14–35 mm is wide-angle, around 50 mm is "normal" (similar to your eye's central view), 85 mm and beyond is telephoto.
Try “Focal Length & Field of View” in the interactive model →
Chapter 9
How deep is the slice of sharpness?
Only the plane of focus (yellow) is perfectly sharp. But things a little in front and behind still look sharp, because their blur circles are too small to notice. That zone — green in the 3D view — is the depth of field.
Three things make it shallower: a wider aperture, a longer focal length, and focusing closer. Reverse them for deep focus.
Chapter 10
Full manual. Six briefs. Make it happen.
Everything is in your hands now: light, aperture, shutter, ISO, focal length and focus. Each assignment below is a real-world brief — read it, think about which side effects you want, set up the camera and press the red shutter button (or Space).
Real cameras also have white balance, RAW files and limited dynamic range — look them up in the glossary.
Test yourself
Light travels in straight lines, so rays from the top cross through the hole to the bottom. Rays from the top of the subject travel downward through the opening and land low on the sensor — and vice versa. The camera flips it back for you.
Brighter and blurrier. More light gets in, but each point of the scene now makes a bigger blob on the sensor.
A wide opening that is still sharp. A lens bends every ray from one point back to one point, so you can use a big, bright opening without blur.
Lens → aperture → shutter → sensor. Glass focuses it, the aperture limits it, the shutter times it, and the sensor records it.
Attaching interchangeable lenses at the exact right distance from the sensor. The mount fixes the lens-to-sensor distance precisely and carries autofocus/aperture signals.
You choose the aperture, the camera picks the shutter speed. Aperture priority: you control depth of field, the camera balances exposure with shutter speed.
A disc called the blur circle (circle of confusion). The cone of rays is cut by the sensor before or after it converges, leaving a disc.
Away from the sensor. 1/f = 1/u + 1/v: smaller u means bigger v, so the image forms further back and the lens must move forward.
The slice of the scene at the focus distance that is perfectly sharp. Only things at exactly the focus distance are perfectly sharp; depth of field extends a little either side.
f/2. f/2 is a much bigger opening — six stops (64×) more light than f/16.
Halves it. Each full stop along the f-number sequence halves the light.
Use a wide aperture (f/1.8). Wide apertures give shallow depth of field, so the background falls out of focus.
1/1000. 1/1000 s is so short that the dog barely moves while the shutter is open.
Doubles it. The shutter stays open twice as long, so twice the light arrives — one stop brighter.
1/200. The reciprocal rule: 1 ÷ focal length. Longer lenses magnify shake.
Amplifies the signal the sensor already caught. ISO is gain. The amount of light collected is set by aperture and shutter speed.
Few photons means big random variation, and amplifying it makes it visible. Photons arrive randomly. With little light the randomness is large relative to the signal, and gain magnifies it (plus the electronics' own noise).
One stop brighter. Doubling ISO doubles the amplification: one stop.
f/5.6 at 1/125. f/5.6 is one stop less light; 1/125 is one stop more. They cancel out.
1/250 → 1/125. Doubling the shutter time doubles the light: one stop brighter.
More noise, unless you compensate. Less time means less light; to compensate you open the aperture (shallower DOF) or raise ISO (more noise).
16 mm. Shorter focal length = wider field of view.
75 mm on full frame. The smaller sensor crops the middle of the image: 50 × 1.5 = 75 mm equivalent.
Standing far away — perspective depends on camera position. Perspective only depends on where the camera is. A telephoto just lets you frame tightly from far away.
85 mm at f/1.8, focused at 2 m. Long focal length + wide aperture + close focus all shrink depth of field.
Half the hyperfocal distance to infinity. That is the definition — and why landscape photographers love it.
Deeper behind the focus point than in front. Roughly a third in front and two-thirds behind at normal distances — watch the green zone.
Raise ISO and open the aperture. Without a tripod you must keep the shutter fast, so get the light from a wider aperture and more gain.
f/11, focus at the hyperfocal distance. A mid-narrow aperture and hyperfocal focusing maximise depth of field.
Overexposed with clipped highlights. The right edge is pure white — detail there is lost.
Words worth knowing
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