Photography 101

Part 2: Focus and depthChapter 8

Tools

Contents

Part 1: Exposure

  1. 1 Mastering the Exposure TriangleBalance aperture, shutter speed and ISO for brightness, depth, motion and noise.
  2. 2 ISO and image noiseWhen to raise ISO, why grain appears, and how to trade it for a faster shutter.
  3. 3 MeteringWhat the meter aims for, how to read it, the four metering modes, and Sunny 16 when there's no meter.
  4. 4 Shooting modes and the histogramLet the camera set one setting, steer it with exposure compensation, and check the result with the histogram and zebras.
  5. 5 Dynamic rangeHow many stops a camera can hold at once, what to keep when a scene is wider, and measured figures for real cameras.
  6. 6 Shutter speed and motionCamera shake, freezing or blurring a moving subject, and ND filters for long exposures.
  7. Part 1 cheat sheetFormulas and starting settings from chapters 1–6.
  8. Part 1 quizCheck yourself on chapters 1–6.

Part 2: Focus and depth

  1. 7 Camera LensLens anatomy and how to choose wide, standard, telephoto, prime, zoom and specialist lenses for real subjects.
  2. 8 Aperture and depth of fieldWhy a wider aperture blurs the background, what else controls blur, and the shape of bokeh.
  3. 9 Sensor size and crop factorWhat a smaller sensor crops, equivalent focal length and aperture, and why phones fake the blur.
  4. 10 Hyperfocal distanceSharp from here to infinity: focusing landscapes with blur circles, focus stacking and a cheat card.
  5. 11 Diffraction and the sharpest apertureWhy f/22 looks soft, where your lens is sharpest, and how pixel size changes it.
  6. Part 2 cheat sheetFormulas and starting settings from chapters 7–11.
  7. Part 2 quizCheck yourself on chapters 7–11.

Part 3: Composition

  1. 12 The composition reading planAll 15 chapters of The Elements of Composition, with printed page numbers and a photography lesson for each.
  2. 13 Introduction to compositionComposition as visual communication: choose what to show, compare three frames, and see how viewers read them.
  3. 14 What is composition?Arrange subject, space and edges to change what a photograph says; compare photos and make three versions of one scene.
  4. 15 Focal pointsChoose where the eye lands first, use contrast and space to build a visual path, then test thirds and diagonals in a field exercise.
  5. 16 LinesFollow straight, curved and implied lines through a photograph, then make three frames of one subject from different positions.
  6. 17 ShapesFind shapes made by subjects and space, use a frame within the frame, and compare three ways to place one subject.
  7. 18 TexturesMake a surface feel tangible with light, detail and context, then photograph one material three ways.
  8. 19 PatternsUse repetition to build rhythm, break a pattern to draw attention, and make three versions of one repeated scene.
  9. 20 SpaceUse positive and negative space, fill the frame, and simplify without losing meaning.
  10. 21 DepthLayer foreground, middle ground and background, and use perspective or deep space to show distance.
  11. 22 BalanceWeigh elements with symmetry, asymmetry and radial arrangements; explore the golden ratio as an optional guide.
  12. 23 ContrastCreate meaning and focus through contrasting subjects, light, shadows and colour.
  13. 24 ColorChoose hue, saturation and brightness, then compare complementary, triadic, analogous and monochromatic palettes.
  14. 25 ToneRead shadow, midtone and highlight; shape global and local tone while keeping meaningful detail.
  15. 26 AngleChange camera height and tilt to compare level, straight up or down, high and low viewpoints.
  16. 27 Composition conclusionBring the elements together in a four-frame story and develop your own way of seeing.

Part 4: Color Theory

  1. 28 Color Theory in PhotographyUse HSL, warm and cool depth, four color harmonies, white balance and selective editing to guide attention.
  2. 29 White balanceRead the color of light, match Kelvin and tint, and choose what to preserve when lights mix.

Tools

  1. Color wheelExplore hues, adjust saturation and luminosity, enter RGB or hex values, and preview the color live.
  2. Photo editorTry light, colour, detail and crop adjustments on a sample or your own photo, then save the result.
  3. Golden hour plannerSunrise, sunset, golden hour and blue hour for where you are, and where the sun rises and sets.
  4. ND filter calculatorYour shutter speed with an ND filter on, and a countdown timer for bulb mode.
  5. Depth of field calculatorHow much is sharp for your lens, aperture and focus distance, and the hyperfocal distance.
  6. Photo info and location removerSee a photo's camera settings and where it was taken, then remove the location before you share it.

Chapter 8Part 2: Focus and depth

Why a wider aperture
blurs the background

Change the aperture and watch the orange rays. The lens focuses the green subject point on the sensor, but light from the background lands as a disk. A wider opening makes that disk bigger.

Lens lab

1/3 stop per step. Lower is wider.

Across a 24–70 mm zoom.

Where the rays land

Opening

Scene Lens and aperture Sensor Subject Background

What the photo looks like

How much looks sharp

Sharp range

Near limit
Far limit

Sensor close-up

Blur disk

Subject point Background point

Dashed ring: 0.03 mm, the usual limit for looking sharp.

What you're seeing

Rays

Each line is one ray from the same tiny point. Watch where the orange rays cross.

Sensor

Sharp range

Want the far limit to reach infinity? That is the hyperfocal distance, covered in its own chapter.

The science in three steps

  1. Focus is a precise plane. Rays from the subject meet on the sensor. Rays from an object at another distance would meet at a different position.
  2. Defocus makes a disk. At the sensor, out-of-focus rays are still spread apart. That spread is the circle of confusion.
  3. The opening controls the spread. A wide aperture accepts rays at steeper angles, so the disk grows. A small aperture blocks those outer rays and shrinks the disk. More distances then appear sharp, even though only one plane is perfectly focused.

The diagrams are schematic: distances and ray angles are enlarged for teaching. The blur numbers use a thin-lens approximation and ignore lens aberrations and diffraction. The photo is a stylised preview whose blur follows the calculation. The background distance must be farther than the focus distance.

What blurs the background most

Aperture is only one of four things. Start from a portrait at 50 mm and f/4, with the person 2 m away and a hedge 2 m behind them, and change one thing at a time. The dashed ring is the starting blur disk.

×2.7Get closer: 2 m to 1 mThe biggest change, though the person also fills more of the frame.
×2.0Open up: f/4 to f/2Twice the opening, twice the blur disk. Simple and reliable.
×1.8Background farther: 2 m to 20 m behindPull your subject away from walls and hedges. It helps less once the background is already far.
×1.3Zoom to 100 mm and step backSame framing, a little more blur. Longer lenses also show less background, so it looks simpler.

Blur disk sizes from the same thin-lens calculation as the lab, on full frame. Try each change in the lab above and watch the blur disk.

Bokeh shape

Every point of light in the background spreads into a blur disk, and that disk takes the shape of the opening in the lens. Wide open, the opening is round. Stop down and the aperture blades close in and make a polygon. Near the edges of the frame, the lens barrel also cuts the disk into a "cat's eye".

Aperture
Aperture blades

Lens specs list the blade count. Rounded blades keep the disks round when you stop down a little.

In the field

Group photos

A group has depth: the back row may be a metre behind the front. With a 35 mm lens focused on a group 4 m away:

ApertureSharp fromto
f/2.83.1 m5.5 m
f/5.62.6 m8.8 m
f/82.3 m17.9 m

Focus on a middle row, then check the nearest and farthest faces. Depth of field is not split in a fixed ratio: in this example it stretches much farther behind the focus point at f/8 than at f/2.8. Stop down if either row looks soft; f/5.6 to f/8 is a useful starting range for many groups. See the depth-of-field explanation at Cambridge in Colour.

Focus and recompose

A common habit: put the centre focus point on the eye, half-press, then turn the camera to frame the shot. The plane of focus turns with the camera, so it ends up behind the eye.

Focused on the eye After turning Camera

At 50 mm, f/1.4 and 1 m, only about 3 cm is sharp. Turning 15° leaves the eye about 3.4 cm in front of the new plane, outside the sharp zone. (The drawing exaggerates the angle.) At wide apertures, move the focus point onto the eye instead, or use eye detection.