This page is from a Physics teacher who is interested in Photography and is trying to consolidate here some of the science and theory behind capturing images on a camera.
This page is still a work-in-progress.
Parts of a Camera
(INSERT PICTURE OF CAMERA)
SLR, DSLR & Mirrorless
Old film cameras were usually SLR (Single-Lens Reflex) cameras.
In this type of camera:
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“Single-lens” → The same lens is used both to preview the scene and to take the photo.
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“Reflex” → A mirror reflects the light from the lens up into an optical viewfinder, so you’re seeing exactly what the lens sees.
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Mirrorless Cameras
A mirrorless camera (most modern digital cameras) removes the mirror entirely.
Light passes straight from the lens → to the image sensor.
The sensor’s signal is shown on an electronic viewfinder (EVF) or the rear screen.
So you’re seeing a live digital preview, not an optical image.
| Feature | SLR (DSLR) | Mirrorless |
|---|---|---|
| Viewfinder | Optical (via mirror and prism) | Electronic (via sensor) |
| Size & Weight | Bulkier (mirror + prism) | Smaller & lighter |
| Shutter Lag | Slight delay (mirror flip) | Faster, no mirror movement |
| Noise/Vibration | Mirror slap causes vibration | Quieter, no moving mirror |
| Autofocus (Older models) | Phase-detection via mirror module | On-sensor (contrast or hybrid) |
| Battery life | Usually longer | Shorter (EVF uses power) |
| Durability (mechanical) | More moving parts | Fewer moving parts |
| Real-time exposure preview | No (optical only) | Yes (what-you-see-is-what-you-get) |
Aperture
In photography, the aperture is the opening in a camera lens that controls how much light enters the camera and reaches the sensor (or film). It’s like the pupil of your eye — it can get wider to let in more light or narrower to let in less.
Aperture Controls Exposure
A larger aperture (wider opening) lets in more light. This will resut in a brighter image.
A smaller aperture (narrower opening) will let in less light. This will result in a darker image.
Aperture Controls Depth of Field (DoF)
Large aperture (small ƒ-number) → shallow depth of field → subject in focus, background blurry.
Small aperture (large ƒ-number) → deep depth of field → most of the scene in focus.
ƒ-stop
The “size” of the aperture in photography is measured by the ƒ-stop (ƒ-number). You have likely seen values such as ƒ/2.8 or ƒ/8 on a lens. These are telling you the “size” of the aperture of the lens.
| ƒ/4 should be read as ef-four or ef-stop-four. |
A smaller number like ƒ/2.8 or ƒ/4 corresponds to a LARGE aperture, and thus lets in more light.
A bigger number like ƒ/16 or ƒ/22 corresponds to a SMALL aperture, and thus lets in very little light.
The ƒ-stop is calculated by the relationship:
ƒ-stop=ƒ / D
where
ƒ = focal length of the lens
D = diameter of the aperture
The light intensity (amount of light) entering the lens is proportional to the area of the aperture, which goes as D2.
So, to double the light, the diameter must increase by a factor of √2, and to halve the light, it must decrease by √2. A change like this is referred to as a change of one stop.
| ƒ-NUMBER | Relative Light | Change |
|---|---|---|
| ƒ/1.4 | 1× (baseline) | — |
| ƒ/2 | 1/2× | ↓ one stop |
| ƒ/2.8 | 1/4× | ↓ one stop |
| ƒ/4 | 1/8× | ↓ one stop |
| ƒ/5.6 | 1/16× | ↓ one stop |
| ƒ/8 | 1/32× | ↓ one stop |
| ƒ/11 | 1/64× | ↓ one stop |
| ƒ/16 | 1/128× | ↓ one stop |
| ƒ/22 | 1/256× | ↓ one stop |
The above table lists the ƒ-stop values we will commonly find on a camera in order from largest aperture, ƒ/1.4, to smallest aperture, ƒ/22. As we move each step (1 stop) down the table we are halving the amount of light that the lens lets in.
If you change your aperture from f/4 → f/2.8,
you’re opening up by one stop → letting in twice as much light.
If you change from f/4 → f/5.6,
you’re closing down by one stop → letting in half as much light.
Most cameras (lenses) have ƒ-numbers ₋divided into smaller changes – typically ⅓-stop intervals such as this:
ƒ/1.4, ƒ/1.6, ƒ/1.8,
ƒ/2, ƒ/2.2, ƒ/2.5,
ƒ/2.8, ƒ/3.2, ƒ/3.5,
ƒ/4, ƒ/4.5, ƒ/5.0,
ƒ/5.6, ƒ/6.3, ƒ/7.1,
ƒ/8, ƒ/9, ƒ/10,
ƒ/11, ƒ/13, ƒ/14,
ƒ/16
So to change by one stop we would need to adjust the lens through 3 clicks – e.g. ƒ/2.8 → ƒ/3.2 → ƒ/3.5 → ƒ/4 would be a closing down one stop.
Aperture to Describe a Lens
In general, the more light a lens can let in, the better the lens. So better lenses tend to have smaller ƒ-numbers for their maximum aperture. A large aperture allows the lens to be used in darker conditions and produces more bokeh if desired.
So a 50mm ƒ/2 lens would usually be more expensive and would offer more flexibility in terms of the photography options than a 50mm ƒ/4 lens. The ƒ/2 lens being 2 stops faster than the ƒ/4 lens can actually let in 4 times as much light.
Film and Sensor Sizes
The film sensor is the part of the camera that is sensitive to light and captures the image when activated. The sensor replaces film from old film cameras.
Film cameras took many different sizes and formats of film over the years. However, by far the most common for more than 50 years has been the 35 mm format. 35 mm film is so called because the width of the film is 35 mm. Due to the sprockets (holes) on the edge of the film that are used to pull the film through the camera, the actual image the gets captured on the film is 24 mm by 36 mm. This size is nowadays referred to as full-frame.
You can see that the area of this will be 24 x 36 = 864 mm²
Note that the aspect ratio of this 36:24 = 3:2 is the common aspect ratio for many modern cameras. If an image is taken with a different aspect ratio then it will not be using 100% of the sensor and so will show show loss of resolution or quality.
Many high-end modern cameras tend to use film sensors of this same size. They tend to refer to themselves as full-frame cameras.
Cheaper cameras will use smaller sensors as the cost savings can be dramatic.
Common smaller sensor sizes are:
APS-C (which used in many mid-range DSLR and mirrorless cameras) has a 24 mm by 16 mm giving a sensor area of 384 mm².
Micro 4/3 (which used in many entry level mirrorless cameras) has a 17 mm by 13 mm giving a sensor area of ~220 mm².
| Micro 4/3 should be read as micro-four-thirds |
Notice these all still have the same 3:2 aspect ratio for the frame.
Crop Factor
Using the same focal length lens on a smaller-sensor camera means less of the scene gets captured — the sensor “crops” the image more, so the image looks more “zoomed in” (narrower field of view) compared to a larger sensor.
Lenses are compared against the image on a full-frame camera.
A 50 mm lens on an APS-C camera body will generate an image similar, in terms of field-of-view, to a 75 mm lens on a full-frame camera. Thus the APS-C camera is said to have a crop factor of 1.5 ( 75÷50=1.5).
When people say “on this crop-sensor body the 50 mm acts like a 75 mm”, what they mean is for the field of view, not that the focal length physically changes. The lens is still 50 mm.
| Sensor | Crop Factor | FoV EFFECT |
|---|---|---|
| Full-Frame | 1.0x | Reference |
| APS-C | ~1.5x | Narrower FoV |
| Micro 4/3 | ~2.0x | Even narrower FoV |
Field of View
Field of View (FoV) in cameras describes how much of a scene the camera can see. It depends mainly on:
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- Sensor size
- Lens focal length
The FoV is measured in degrees.
Some typical examples for different lenses are shown below:
| Lens | FoV (FULL-FRAME) | FoV (APS-C) |
|---|---|---|
| 16 mm | ~100° | ~78° |
| 24 mm | ~74° | ~54° |
| 35 mm | ~54° | ~38° |
| 50 mm | ~40° | ~27° |
| 85 mm | ~24° | ~16° |
| 200 mm | ~10° | ~7° |
Typical Focal Lengths Used in Different Types of Photography
| Situation | Ideal FoV |
|---|---|
| Landscape | Wide (12–35mm) |
| Street / general | Normal (35–85mm) |
| Portrait | Narrow (85–135mm) |
| Wildlife / sports | Telephoto (200mm+) |
Shutter Speed
Shutter speed is the duration (time) for which the camera shutter remains open to allow light to hit the sensor.
- It controls exposure (amount of light captured)
- It influences motion blur (how movement appears in the image)
Shutter speed is measured in seconds or fractions of a second:
Common examples:
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Fast:
1/4000 s,1/1000 s,1/250 s -
Medium:
1/60 s,1/30 s -
Slow:
1 s,5 s,30 s
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Cameras will typically have the following shutter speeds:
4 s, 2 s, 1 s, 0.5 s, 1/4 s, 1/8 s, 1/15 s, 1/30 s, 1/60 s, 1/125 s, 1/250 s, 1/500 s, 1/1000 s, 1/2000 s, 1/4000 s, 1/8000 s
As we move down the list, each shutter speed is halving the previous value, and so allowing half of the light to get through. Each jump here is thus decreasing the exposure by one stop.
You will realise that mathematically halfing the numbers would give the following sequence:
1/4 s, 1/8 s, 1/16 s, 1/32 s, 1/64 s, 1/128 s, 1/256 s, 1/512 s, 1/1024 s
The numbers in bold are rounded to whole numbers to make it easier to quickly identify doubling, quadrupling, halving etc….
As one-stop is quite a large usually the shutter speed will be divided into ⅓ -stop intervals. The following is the usual sequence indicated on the camera:
4 s, 3.2 s, 2.5 s,
2 s, 1.6 s, 1.3 s,
1 s, 0.8 s, 0.6 s,
0.5 s, 0.4 s, 0.3 s,
1/4 s, 1/5 s, 1/6 s,
1/8 s, 1/10 s, 1/13 s,
1/15 s,1/20 s, 1/25 s,
1/30 s, 1/40 s, 1/50 s,
1/60 s,1/80 s, 1/100 s,
1/125 s, 1/160 s, 1/200 s,
1/250 s, 1/320 s, 1/400 s,
1/500 s, 1/640 s, 1/800 s,
1/1000 s, 1/1250 s, 1/1600 s,
1/2000 s, 1/2500 s, 1/3200 s,
1/4000 s, 1/5000 s, 1/6400 s,
1/8000 s
Moving 3 steps forward or backward will result in an exposure change of one-stop.
Shutter Speed to Adjust Exposure
If the shutter is open for twice as long, then it will allow for twice as much light to pass through.
| Shutter Speed | Exposure Change |
|---|---|
| 1/1000 s → 1/500 s | +1 stop (2× light) |
| 1/250 s → 1/500 s | −1 stop (½ light) |
| 1/20 s → 1/80 s | –2 stops (¼× light) |
| 1/60 s → 1/15 s | +2 stops (4× light) |
Some Typical Shutter Speeds
| Situation | Typical Speed | Reason |
|---|---|---|
| Sports | 1/1000 s | Freeze fast motion |
| Portrait | 1/125 s | Freeze human motion |
| Night/Stars | 10–30 s | Gather lots of light |
| Waterfalls (silky effect) | 1/2 – 2 s | Intentional blur |
These are obviously guidelines and exact shutter speeds will vary depending on other factors such as aperture, focal length of lens, etc..
Pixels
ISO
In cameras (digital or film), ISO refers to the sensitivity of the film or image sensor to light.
ISO stands for the International Organisation for Standardisation.
They chose ISO from the Greek word isos meaning equal, to represent standardisation across languages.
It’s pronounced eye-so, but photographers often say I-S-O too..
A lower ISO number (e.g., ISO 100) = less sensitivity to light → you’ll need more light (or longer exposure) for a good image.
A higher ISO number (e.g., ISO 1600, ISO 3200) = more sensitivity to light → you can shoot in darker conditions or use faster shutter speeds.
Example: increasing ISO from 100 to 200 lets you halve the light needed (or double the shutter speed) for the same exposure.
While higher ISO makes shooting in low light easier, it comes at a cost: noise (digital grain) appears in the image.
Lower ISO usually yields cleaner images with less noise—but you must compensate by letting in more light (via slower shutter or wider aperture).
Sensor size and quality matter: a larger sensor or a newer high-end camera may handle high ISO better (less noise) than a small sensor budget camera
Focal Length
Recall that focal length of a lens is the distance between the optical centre of the lens and the image formed from parallel light rays (i.e. object is at a very far (infinite) distance away).
The focal length of a camera lens is the distance (in mm) between the lens’s optical centre and the camera sensor when the lens is focused at infinity.
The focal length of a lens has a big effect on the properties of an image captured by the camera:
| Property | Effect |
|---|---|
| Field of View (FoV) | Short focal length → wide view Long focal length → narrow view |
| Magnification | Longer focal length → higher magnification |
| Perspective | Longer focal length compresses distances Shorter focal length exaggerates depth |
| Depth of Field (DoF) | Short focal length has larger DoF Long focal length shallower DoF (for the same framing & aperture setting) |
| Lens type | Focal length | Visual effect |
|---|---|---|
| Ultra-wide | 10–24 mm | Very wide view, architecture, landscapes |
| Wide | 24–35 mm | Street, environmental portraits |
| Standard | 35–70 mm | Natural human perspective |
| Telephoto | 70–200 mm | Portraits, sports |
| Super-telephoto | 200 mm+ | Wildlife, distant subjects |
The above table is for full-frame cameras. For ASP-C cameras we would need to take the crop factor into account.
If a lens has only one focal length it is referred to as a prime lens.
If the lens allows the focal length to be changed the lens is referred to as a zoom lens.
Camera Sensors
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| Water Bucket Analogy | |||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Think of each pixel on a camera sensor as a tiny light bucket.
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Further Sections I may add at some point…
- polarisation
- motion blur
- image stabilisation
- rolling shutter
- white balance & colour temperature
- sensor science
- exposure triangle