The Machine: how a DSLR really works
Before you touch a single setting, it helps to understand what happens inside the camera the instant you press the button. Once you can picture the light bouncing around in there, every setting later will make sense instead of feeling like magic.
What does “DSLR” even mean?
DSLR = Digital Single Lens Reflex. The clever part is “reflex,” which just means mirror. A mirror inside the camera bounces the light coming through the lens up to your eye, so what you see in the viewfinder is the exact view the lens sees. Press the button and that mirror flips up out of the way so the light can hit the sensor and record the photo.
The 6 parts that matter
- 1LensGathers light and focuses it. It also holds the aperture — the adjustable hole that controls how much light gets in.
- 2MirrorSits at 45° and bounces the view up to the viewfinder. Flips up the moment you shoot.
- 3PentaprismA five-sided glass block that flips the image the right way up so the viewfinder looks correct.
- 4ShutterA tiny set of curtains in front of the sensor. How long they stay open is your shutter speed.
- 5SensorThe digital “film.” It reads the light and turns it into your photo. Its sensitivity is the ISO.
- 6ProcessorThe camera’s brain — turns the sensor’s data into the JPEG or RAW file on your card.
Your D90 control map
On the D90 you have two command dials: the rear dial sets shutter speed, and holding a button while spinning the front dial sets aperture. You’ll use both constantly in Module 3.
Meet your lens: the Nikon 18–55mm
Your D90 wears the AF-S DX Nikkor 18–55mm — the perfect learning lens. Two numbers describe it, and knowing them saves a ton of confusion:
- ①18–55mm = the zoom range18mm is wide (fits a lot in the frame); 55mm zooms in closer. On the D90’s sensor these “feel” like about 27–82mm.
- ②f/3.5–5.6 = a changing max apertureZoomed out to 18mm you can open as wide as f/3.5. Zoom in to 55mm and the widest you get is f/5.6. That’s normal for a kit lens — the opening physically can’t stay as wide when zoomed.
- ③The focus switchOn the lens barrel is an A – M switch. Leave it on A (autofocus) for now. This lens has no stabilizer, so keep your shutter speed up when shooting handheld (next tip).
▸ Watch: get to know the camera
The Language of Light
Here’s the big secret: photography is really just collecting light. Every photo needs the right amount of light — not too dark, not too bright. Three controls decide how much light you collect, and each one has a magical side effect that changes how the photo looks. Master these three and you’ve mastered photography.
The Exposure Triangle
Aperture, shutter speed, and ISO are the three sides. Together they set how bright your photo is. Change one and you usually have to change another to keep the balance — like a see-saw.
Photographers measure changes in “stops.” One stop = double or half the light. That single idea ties all three controls together.
① Aperture — the size of the opening
Aperture is how wide the lens opening is. It’s written as an f-number. On your 18–55mm lens the range runs from f/3.5 (widest) down to f/22 (tiniest). Here’s the part that trips everyone up: a small f-number means a big opening (lots of light), and a big f-number means a tiny opening (little light).
The magical side effect: depth of field. The widest your lens opens (f/3.5 at 18mm, or f/5.6 zoomed to 55mm) softens the background — nice for portraits. A narrow aperture (f/11–f/16) keeps everything sharp front-to-back — perfect for landscapes.
② Shutter Speed — how long the light comes in
How long the shutter stays open, in seconds. 1/1000 is super fast; 1/30 is slow. Longer = more light.
Rule of thumb: your lens has no stabilizer, so to avoid blur from shaky hands keep the shutter faster than 1/60s — and faster still, around 1/125s, when zoomed in to 55mm.
③ ISO — the sensor’s sensitivity
ISO makes the sensor more sensitive to light. Low ISO (100) needs lots of light but is super clean. High ISO (3200+) works in the dark but adds grainy “noise.”
Golden rule: use the lowest ISO you can get away with. Only raise it when there isn’t enough light.
The trade-off, at a glance
Every control does two jobs — brightness and a look. This is the whole language of photography on one table:
| Control | More light when… | Less light when… | The “look” it controls |
|---|---|---|---|
| Aperture | f-number ↓ (f/3.5) | f-number ↑ (f/22) | Background blur (depth of field) |
| Shutter | slower (1/15) | faster (1/1000) | Frozen vs. blurred motion |
| ISO | higher (3200) | lower (100) | Clean vs. grainy noise |
Bonus: making it look good (composition)
Exposure gets a correct photo. Composition gets a beautiful one. Three quick habits:
Rule of thirds: imagine the frame split into 9 boxes. Place your subject where the lines cross, not dead center.
Fill the frame: get closer than feels natural. Cut out the boring empty space.
Watch the light: soft light near sunrise/sunset (“golden hour”) flatters almost everything.
▸ Watch: the exposure triangle
The Exposure Lab
This is where it clicks. Drag the three sliders and watch the photo react — brightness, background blur, motion blur, and grain all change at once, just like a real camera. Your goal: keep the light meter on 0 while choosing the look you want. Try to make the exact same brightness three different ways!
Manual Mode: you’re in control
This is the finish line. In Manual (M) mode you set all three controls yourself. It sounds scary but the camera gives you a built-in cheat: the light meter. Learn to read it and manual mode becomes easy.
The light meter — your built-in guide
Look in the viewfinder or top screen and you’ll see a little scale from -2 to +2 with a marker. It’s the camera telling you if your current settings will be too dark, too bright, or just right. Your job in manual mode: adjust the three controls until the marker sits on 0.
Set up your D90 for Manual — step by step
- Turn the mode dial to M. That top-left dial — rotate it until the white line points at the M.
- Set your ISO first. Press the ISO button and spin the rear dial. Bright day outside? ISO 100–200. Indoors? ISO 400–800. Dim? 1600+.
- Set shutter speed with the rear command dial. Start around 1/125 for everyday handheld shots.
- Set aperture: hold the +/- (exposure) button and spin the front dial. Try f/5.6 to start. (This button-hold is the D90 “two-button” trick from Module 1.)
- Half-press the shutter and read the meter. Marker left of 0 = too dark; right of 0 = too bright.
- Adjust to reach 0. Too dark? Slower shutter, or bigger aperture (smaller f-number), or higher ISO. Too bright? Do the opposite. Then shoot.
“Which setting do I change?” — a simple order
When the meter isn’t at 0, don’t panic. Change things in this priority:
- 1Aperture — set it for the look firstWant blurry background? Wide (small f-number). Want everything sharp? Narrow (big f-number). Decide this based on your subject.
- 2Shutter — set it fast enough to be sharpHandheld: 1/125 or faster. Fast action: 1/500+. This stops blur from shaky hands.
- 3ISO — use it to finish reaching 0If aperture and shutter are set for the look you want but it’s still too dark, raise ISO until the meter hits 0. Keep it as low as possible.
▸ Watch: shooting manual
Quick check
Six questions. Tap an answer to see if you’re right — you’ll get an explanation either way. No pressure, this is just to see what stuck.
Starting-point recipes
Not sure where to begin in a scene? Dial in one of these, check the meter, and adjust to 0. These are training-wheels starting points — trust the meter over the numbers.
Portrait of a person
- Zoom
- 55mm
- Aperture
- f/5.6
- Shutter
- 1/200
- ISO
- 200 – 400
Zoom in, open up, stand close — softens the background so the face pops.
Sports / running pet
- Zoom
- any
- Aperture
- f/5.6
- Shutter
- 1/1000
- ISO
- 400 – 800
Fast shutter freezes the action mid-move.
Landscape / outdoors
- Zoom
- 18mm
- Aperture
- f/8 – f/11
- Shutter
- 1/125
- ISO
- 100 – 200
Wide angle + narrow aperture keeps everything sharp, near to far.
Indoors / low light
- Zoom
- 18mm
- Aperture
- f/3.5 (widest)
- Shutter
- 1/100
- ISO
- 800 – 1600
Zoom out for the widest f/3.5 and raise ISO to grab scarce light. Keep the shutter ≥ 1/100 to stay sharp.
Photographer’s glossary
Every craft has its own language. Here are the words a real photographer uses — flip back anytime you forget one.
- Aperture
- The adjustable opening in the lens that controls how much light enters. Written as an f-number.
- f-number (f-stop)
- How wide the aperture is. Small number = big opening = more light + blurrier background.
- Shutter Speed
- How long the shutter stays open, in fractions of a second. Controls motion blur vs. freeze.
- ISO
- How sensitive the sensor is to light. Higher = brighter in the dark, but grainier.
- Exposure
- The overall brightness of a photo — the result of aperture, shutter, and ISO working together.
- Stop
- A unit of light. One stop = double or half the light. The idea that links all three controls.
- Depth of Field
- How much of the photo is in sharp focus, front to back. Wide aperture = shallow (blurry background).
- Light Meter
- The –2 to +2 scale in the camera that tells you if your exposure is dark, bright, or balanced (0).
- DSLR
- Digital Single Lens Reflex — a camera with a mirror that shows you the lens’s exact view.
- Bokeh
- The soft, creamy look of an out-of-focus background. Those glowing blurry circles.
- Pentaprism
- The five-sided glass block that flips the image upright for the viewfinder.
- Noise
- The grainy speckles that appear at high ISO. The digital version of film “grain.”