Day Four

Your Camera’s Sensitivity, and the Secret to Shooting in Low Light

ISO is the quiet third piece of exposure — the one that helps you keep shooting when the light gets low.

If aperture is your window
and shutter speed is how long the window stays open,

ISO is your ability to “hear” light when it’s faint.

It makes your camera more sensitive so you can brighten a dark scene without slowing your shutter or opening your aperture more than you want to.

ISO isn’t scary.
It’s actually simple once you understand the trade-off.

Today, you’ll learn exactly how ISO behaves and when to use it — calmly and without overthinking.

ISO in Real Life Terms

ISO controls how sensitive your camera is to light.

Think of it like listening in different environments:

Low ISO (100–400)

• less sensitive
• darker image
• very clean
• best for bright light

It’s like talking in a quiet room — you don’t need to turn the volume up.

High ISO (800–3200+)

• more sensitive
• brighter image
• adds grain/noise
• used in low light

This is like turning up the volume to hear a whisper — you hear more, but you hear more static too.

Neither is wrong.
You just choose based on the light you have.

Where ISO Lives on Your Camera

Most cameras keep ISO in one of these places:

• a button labeled ISO
• a quick menu (Q or i button)
• a setting in your shooting or exposure menu

In Aperture Priority (A/Av) you’re still free to change ISO manually at any time.

If you see numbers like 100, 200, 400, 1600 — that’s it.
Those are your ISO settings.

ISO on an iPhone

Your iPhone controls ISO automatically.
You don’t need to set it yourself.

But you’ll see the effect when you brighten a dark scene — the image gets grainier, especially at night.

That’s ISO working behind the scenes.

How ISO Works With Everything Else

This week you’ve learned:

Aperture
big opening = bright
small opening = dark

Shutter speed
fast = dark
slow = bright

ISO
low number = dark + clean
high number = bright + grainy

ISO is what you reach for when you’re out of room with the other two.

Example:

If your lens is already at its lowest f-number
and your shutter is already slow enough
but the photo is still too dark…

ISO is your helper.

It brightens the scene without changing your composition or your motion blur.

Today’s Practice: The ISO Test

Choose one simple subject.
Place it in a dimly lit space — not pitch black, just a little darker than your usual shooting area.

Take three sets of photos:

ISO 100 (or lowest your camera allows)

Take 2–3 photos.
Expect them to look dark.

ISO 800

Take 2–3 photos.
They should look brighter — close to “correct.”

ISO 1600 or 3200

Take 2–3 photos.
They will be brighter still, and you’ll start to see grain or noise.

Now compare the three sets.

Same subject.
Same light.
Same aperture.
Same shutter behavior.

Only the ISO changed — and the mood changed with it.

Does time of day matter today?

No.
ISO doesn’t care what time it is — it only responds to how bright your environment is.

Morning, afternoon, nighttime indoors…
ISO behaves the same way.

Why This Matters

ISO is your safety net.
It lets you shoot in low light without panic and without letting your shutter drop so slow that everything turns blurry.

Once you understand ISO, you stop feeling helpless in dark rooms or evening light.

You know exactly what to do.

Looking Ahead

Tomorrow is Day Five — the day everything comes together.
Aperture, shutter speed, ISO.
All three, working as one system.

It’s the moment exposure finally clicks.