How Aperture, Shutter Speed and ISO Work Together
There’s a point in every photographer’s journey where they stop guessing and start making decisions. That shift usually happens when the exposure triangle properly clicks into place. Not as a concept you’ve read about, but as something you actually feel and understand while you’re standing in a field watching the light change.
This post is aimed at photographers who are already out there shooting but haven’t quite got their head around why the three settings work the way they do. If that’s you, stick with me.
So what is the exposure triangle?
The exposure triangle is the relationship between three camera settings; aperture, shutter speed, and ISO. Together they determine how much light reaches your sensor and how your final image looks. Change any one of them and the other two are affected. Understanding how they interact is the foundation of taking a photograph.
Three settings, one result
Every photograph is the result of three settings working together: aperture, shutter speed, and ISO. They each control how much light reaches your sensor, and they each carry a side effect that goes beyond exposure. Change one and you affect the others. That’s the triangle. Not a complicated idea, just a reminder that nothing works in isolation.
Aperture: your creative starting point
Aperture is the opening inside your lens that controls how much light passes through. A wide aperture lets in more light. A narrow one lets in less.
But the more important side effect is depth of field. A wide aperture (small number, e.g. f/1.8) gives you a shallow depth of field, meaning only a thin slice of the scene is sharp and everything else falls away into blur. That’s the look you see in portraits where the subject is crisp and the background goes soft.
A narrow aperture (big number, e.g. f/11) keeps more of the scene sharp from front to back, which is what most landscape photographers are after.
Aperture is measured in f-stops: f/1.8, f/2.8, f/4, f/8, f/11 and so on. The bit that trips people up every time is that the smaller the number, the wider the opening. f/1.8 is wide open. f/16 is closed right down.
For a typical landscape on a tripod, I’ll drop straight to f/9 or f/11 as my starting point. I want everything sharp from the foreground rock to the horizon, and that range gives me the depth of field I need.
Aperture is also the setting I reach for first in most situations. Before I think about light levels or motion, I’m making a creative decision about how I want the image to look. That decision shapes everything that follows. The one exception is when motion is the whole point of the shot, a waterfall I want silky, a bird in flight I want frozen. In those cases, shutter speed has to come first. But most of the time, aperture leads.
Shutter speed: freezing or showing movement
Shutter speed controls how long your camera’s sensor is exposed to light. A slow shutter keeps the sensor open longer, letting in more light but introducing motion blur if anything in the scene is moving, or if the camera itself moves. A fast shutter freezes everything but cuts the exposure time right down.
The practical question is always: do I want to freeze this, or do I want to show movement? A waterfall at 1/1000th of a second looks like a frozen sheet of glass. At a quarter of a second on a tripod, it becomes that silky flowing effect you see in travel photography. Neither is wrong. It depends entirely on what you’re going for.
Take shooting the tide coming in and out at the beach. I know from experience that around half a second gives me that motion effect I’m after while still retaining detail in the water. So in that situation, shutter speed is actually the first decision I make. Half a second on a tripod, which then means base ISO of 160 on my X-T5, and f/9 for depth of field.
If the image is too bright with those settings, that’s where an ND filter comes in. Think of it as sunglasses for your lens. It cuts the amount of light hitting the sensor without affecting anything else, letting you use a slow shutter speed in bright conditions. A 3 stop ND makes a modest difference, a 6 stop is more significant, and a 10 stop can turn a bright day into something your sensor thinks is near darkness. Which one you reach for depends on how much light you’re dealing with.
If the image is underexposed at those settings, I’ve got two options. I could open the aperture slightly to let more light in, but that costs me depth of field. Or I could bump ISO up to 400, and accept that I’m introducing a little noise into the image. In that situation I’ll take the noise every time. A slightly grainier image with the exposure and depth of field I wanted beats a sharp but flat one.
If you’re handholding the camera, a rough rule is to keep your shutter speed at double your focal length. Shooting at 24mm, your minimum is 1/50th. At 50mm, 1/100th. At 100mm, 1/200th. It gives you a practical safety margin and accounts for the fact that longer focal lengths amplify any shake far more than wider ones. And yes, I know image stabilisation exists, and modern lenses and bodies can give you several extra stops of leeway, but if you’re still getting to grips with the basics, learn the rule first and understand your limits before relying on technology to cover for you. On my X100V I know through experience that I can handhold and get a sharp shot at 1/50th. I know how to hold the camera, I know my footing, and I’ll even hold my breath at the moment of pressing the shutter. It sounds like overkill but it makes a real difference. That floor is something you’ll find for yourself over time on your own camera.
ISO: sensitivity as a last resort
ISO controls how sensitive your camera’s sensor is to the light hitting it. A low ISO like 100 or 200 means the sensor needs plenty of light to record a proper exposure, but the image will be clean and detailed. A high ISO like 3200 or 6400 means the sensor is cranked up and can work in much darker conditions, but you’ll pay for it with noise, that grainy texture that degrades the finer details.
The general rule is to keep ISO as low as the situation allows. Get aperture and shutter speed where you need them first, then raise ISO only as much as you have to in order to get the exposure right.
On a tripod landscape in decent light, I’ll set my ISO to 160, which is the base ISO on my X-T5. That’s my anchor. From there, shutter speed does the work of balancing the exposure. If I’m out shooting handheld and the light drops, I’ll pull ISO up in small steps, always checking what I’m actually gaining against what I’m losing in image quality.
How they work together in practice
The triangle makes more sense when you see it applied to real situations rather than explained in the abstract.
On a tripod, shooting a landscape at dawn, I set f/9 or f/11 first. ISO goes to 160. Then I adjust the shutter speed until the exposure indicator is sitting at or near zero as a starting point, take the shot, and check the histogram. If I find it needs adjusting, I move from there.
Handheld and shooting on the move, say I’ve spotted something from the road and jumped out of the car, the process is faster and more instinctive. I’ll read the light straight away. If it’s overcast and flat, I might open the aperture to f/5.6 and start with ISO 400. If the shutter speed is running faster than I need, I’ll drop ISO back down. If it’s still too fast, I’ll narrow the aperture slightly. It all happens in a few seconds. The more you shoot, the more automatic those adjustments become.
The exposure indicator and the histogram
Your camera’s exposure indicator is making an educated guess. It analyses the scene, calculates the light, and tells you where it thinks the exposure should sit. Think of it like a sat nav. It’ll get you to roughly the right place, but it doesn’t know there’s a tricky junction ahead, or that you’d rather take the scenic route. You still have to use your own judgement.
The indicator is a starting point. The histogram is the truth.
The histogram shows you exactly what landed on the sensor: no guesswork, no assumptions. A graph pushed heavily to the right means overexposure. Pushed hard to the left means underexposure. From there you adjust and shoot again. If you’re not yet fully confident finding the histogram on your camera, I covered that in detail in my post on knowing your camera before you miss the shot.
Metering modes and why they matter here
Your metering mode affects what the exposure indicator is actually telling you, which is why it’s worth understanding even at a basic level.
Most cameras offer three main options.
Matrix metering (called evaluative on Canon cameras, multi on Fujifilm) reads the entire frame and makes a balanced decision across the whole scene. It’s the most reliable general-purpose mode and the one I’d recommend as a default, particularly for landscapes.
Centre-weighted metering reads the whole frame but gives more importance to whatever is in the middle. Useful when your subject is centred and you don’t want the background pulling the reading in a different direction.
Spot metering reads only a very small area, usually wherever your focus point is positioned. It’s precise, but if you’re not pointing it at the right part of the scene, the reading means very little.
The important thing to take from this is that the same number on the exposure indicator can mean different things depending on which mode you’re in. Know your mode before you trust the reading.
Putting it together
The exposure triangle isn’t a formula to memorise. It’s a relationship to understand. Aperture shapes the image. Shutter speed captures the moment. ISO fills the gap. The indicator gets you close. The histogram tells you the truth.
Once that relationship is clear, the technical side of photography starts to get out of the way. Your hands make the adjustments, your eye stays on the scene, and the camera becomes less of something you operate and more of something you think through.
That’s what you’re working towards.
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