Photography 101

Part 2: Focus and depthChapter 9

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 9Part 2: Focus and depth

Sensor size and
crop factor

A smaller sensor records only the middle of the picture the lens projects. See how much each sensor crops, what that does to framing and background blur, and why phones need software to blur the background.

What the lens projects

The photo you get

Sensor
Crop factor
Frames like, on full frame
Background blur like
Exposure at this f-number

Sensors drawn to scale. The highlighted one is the one you picked.

What crop factor means

A lens throws a circle of light at the back of the camera. A full-frame sensor, 36 × 24 mm like a frame of 35 mm film, records a rectangle of it. A smaller sensor records a smaller rectangle from the middle. Crop factor is how many times smaller its diagonal is.

SensorSizeCrop factorA 50 mm lens frames likeSharpness limit c
Full frame36 × 24 mm1×50 mm0.030 mm
APS-C (Nikon, Sony, Fujifilm)23.5 × 15.6 mm1.5×75 mm0.020 mm
APS-C (Canon)22.3 × 14.9 mm1.6×80 mm0.019 mm
Micro Four Thirds17.3 × 13 mm2×100 mm0.015 mm
1-inch (compacts, drones)13.2 × 8.8 mm2.7×135 mm0.011 mm
Phone main camera (1/1.3-inch)9.8 × 7.4 mm3.5×175 mm0.009 mm

The lens doesn't change

A 50 mm lens is 50 mm on any camera. The smaller sensor just crops its picture, so it looks like a longer lens. That's why a 50 mm lens on APS-C is a good portrait lens, and why wide angles are hard to find for small sensors.

Why the sharpness limit shrinks

A photo from a smaller sensor is enlarged more to reach the same print or screen size, so its blur is enlarged too. To look equally sharp, the blur on the sensor must be smaller: about 0.03 mm ÷ crop factor. The hyperfocal chapter uses these values.

Equivalent focal length and aperture

To get the same photo on two sensors, the same framing and the same background blur from the same spot, multiply both the focal length and the f-number by the crop factor.

33 mm f/1.8 on APS-C ≈ 50 mm f/2.7 on full frame

33 × 1.5 ≈ 50, and 1.8 × 1.5 = 2.7. Both give the same view and the same amount of background blur.

So to match the look of a full-frame f/2.8 portrait, an APS-C camera needs about f/1.8, and Micro Four Thirds about f/1.4.

What doesn't change

Brightness at the same f-number. At f/1.8 on either camera, the same shutter speed and ISO give roughly the same image brightness. But the equivalent pair above uses f/1.8 on APS-C and f/2.7 on full frame. To keep the same shutter speed and brightness at f/2.7, the full-frame camera needs about 2.25× the ISO. Aperture equivalence here describes framing and depth of field; it does not mean the f-numbers give identical exposure settings.

At the same f-number and shutter speed, a smaller sensor collects less light across the whole image. At the same output size this can mean more visible noise, though sensor design and processing matter too. See Cambridge in Colour's sensor-size comparison.

Why phones need portrait mode

A typical phone main camera has a 6.9 mm lens at f/1.8 on a sensor with a crop factor of about 3.5. That frames like 24 mm and blurs like f/6.3 on full frame. At normal distances almost everything is sharp, however bright the f-number sounds.

Portrait mode fakes it. The phone estimates how far away each part of the picture is, using two cameras, a depth sensor or machine learning, then blurs the background in software.

Spot the fake

  • Hair, glasses and the gaps between fingers are hard to separate, so edges look cut out or smudged.
  • The ground under the subject should blur gradually with distance. Fake blur often jumps from sharp to soft.
  • Out-of-focus lights may lack the shapes you saw in the bokeh section.
  • Get close to something small: the real optical blur of a phone is strongest in close-ups.