What Is ISO? A Simple Guide to ISO in Photography

What is ISO? Why does ISO matter? And most importantly, when should you use a high ISO vs a low ISO for the best image quality?

In this article, I’m going to answer all of these questions – and more. ISO might seem like a complex topic, but by the time you’ve finished, you’ll be an absolute master (and you’ll be able to confidently choose the perfect ISO for every shooting situation).

Sound good? Let’s get started.

What is ISO in photography?

ISO refers to your camera’s sensitivity to light. The higher the ISO, the more sensitive your camera sensor becomes, and the brighter your photos appear.

ISO is measured in numbers. Here are a few standard ISO values: 100, 200, 400, 800, 1600, 3200.

That said, pretty much every camera offers intermediate ISO values (for instance, ISO 125 and ISO 160 between ISO 100 and ISO 200). And most cameras these days include additional ISOs on the high end of the range, such as ISO 6400, ISO 12800, ISO 25600, and beyond.

Note that, while ISO is mostly discussed in a digital context, film cameras use ISO, as well – every roll of film has a particular ISO, or sensitivity, that contributes to the image brightness.

What does “ISO” stand for?

ISO refers to the “International Organization for Standardization.”

Technically, it’s not an acronym – the International Organization for Standardization has different names in different languages, so to make things easier, they adopted the shortened “ISO” moniker, designed to be used across all languages.

For the purposes of photography, the name isn’t important. Just think of ISO as your camera’s sensitivity to light, and you’ll do just fine!

ISO and exposure: why ISO matters

By increasing the ISO, you make your photos brighter.

That’s why ISO is important.

In other words, ISO works alongside the other two exposure variables – aperture and shutter speed – to determine the overall brightness level of an image.

Dial in an ISO of 100, and your image might look dark. Boost that ISO to 200, and your image will become brighter. Boost it to 400, and your image will become brighter still.

Your ISO setting could be the difference between a dark image like this:

And a much brighter image like this:

Can you see why this might be useful? If you’re shooting a scene in low light – outside at night or at an indoor event – your photos might keep turning out dark. But boost the ISO, and your shots will brighten right up.

Even in decent light, boosting the ISO can be beneficial. You might need an ultra-fast shutter speed to capture a moving race car, yet raising the shutter speed lowers the exposure and can create a too-dark image. So instead of raising just the shutter speed, you increase the shutter speed (causing your image to darken) but also increase the ISO (counteracting the darkness by brightening the image).

a fast-moving racecar — When shooting race cars, the light might be good – but it often pays to increase the ISO, regardless.

But ISO comes with one major drawback, which I’ll address in the next section:

The problem with high ISOs: noise

Boosting the ISO is insanely useful. But it also comes at a serious cost:

The higher your ISO, the more noise or grain that will appear in your photos, which looks like speckles of color and light randomly strewn across your image.

I’ll illustrate this below with two enlargements of a flower photo. The image on the left was taken at ISO 100, and the image on the right was taken at ISO 3200.

A low ISO and a high ISO comparison — The image on the left was shot at ISO 100, and it’s noise-free. The image on the right was shot at ISO 3200, and it’s plagued by unwanted noise. Note that the exposures are identical (when I boosted the ISO, I raised the shutter speed to keep the brightness levels consistent).

Can you see the difference? Look at the middle few petals. The high-ISO photo (right) is full of unpleasant noise, whereas the low-ISO photo (left) is completely clean.

So raising the ISO, while useful, is part of a tradeoff. Yes, you get a brighter image, but you also get increased noise.

It’s the reason you can’t just shoot with a high ISO all the time. Instead, you keep the ISO low when you can, and you increase the ISO when you must.

That said, camera sensor technology is always improving. A decade ago, ISO 800 may have resulted in huge swathes of noise across your images (depending on your camera). But in 2021, you can shoot at ISO 1600 or 3200 and come away with nearly noise-free files, assuming you’re using a full-frame camera with the latest sensor technology, and that you used good exposure technique.

How to use ISO for the best results

Boosting your ISO gives brighter images as well as noise. Keeping your ISO low maintains image quality but may result in an underexposed or blurry shot. So what do you do?

Really, it all depends on the situation. I recommend leaving your ISO at its base value (probably ISO 50 or 100), except in three situations:

The light is low and you’re struggling to get a well-exposed photo.
You need to freeze motion and/or you’re struggling to get a sharp photo.
You’re struggling to get a photo with adequate depth of field.

Let’s take a closer look at each scenario:

1. The light is low and you’re struggling to get a well-exposed photo

This is the most common reason to raise your ISO. You need to increase your exposure, but you’re shooting indoors or at night.

a singer at a concert with a high ISO — A concert is a classic low-light scenario where you need to increase the ISO.

So you raise the ISO to brighten up your shots.

Of course, ISO is just one of three exposure variables. If your shot is looking too dark, you can always widen the aperture or decrease the shutter speed instead. (And indeed, I recommend considering whether you can make aperture or shutter speed adjustments before you think about boosting the ISO.) But this isn’t always feasible; widening the aperture will narrow the depth of field (see my discussion in the next two sections). And decreasing the shutter speed risks sacrificing sharpness unless you use a sturdy tripod and proper technique.

In the end, if you size up the situation and decide that you can’t widen your aperture or drop your shutter speed, then there’s no way around it: you should boost the ISO.

2. You need to freeze motion and/or you’re struggling to get a sharp photo

If you’re working with a fast-moving subject, you’ll need a correspondingly fast shutter speed.

But if the light is limited, or you need an extremely high shutter speed (e.g., 1/4000s), then you’ll often need to boost the ISO and raise the shutter speed together. (Why can’t you just boost the shutter speed? Because your shots will turn out underexposed!).

ISO in photography a bird with a high ISO — Unless the light is very powerful, you’ll often need to increase the ISO to photograph birds in flight.

As I mentioned in the previous section, widening the aperture is always an option. But again, it’s not always feasible. Sometimes, you’ll need to maintain a deep depth of field; other times, your aperture will already be at its widest.

Bottom line: A sharp shot is better than a blurry shot, even if you need a high ISO to make it happen.

3. You’re struggling to get a photo with adequate depth of field

If you’re shooting a landscape or an architectural scene, you’ll often aim for a deep depth of field – but depending on the situation, you may need an aperture of f/11, f/13, and beyond. In good light, you may struggle to capture a detailed exposure at f/11. In bad light, your shots will definitely turn out far, far too dark.

(Why? To produce a deep depth of field, you narrow the aperture. And narrowing the aperture darkens the exposure.)

That’s where raising the ISO comes in handy. Instead of shooting at ISO 100, you can switch to ISO 200, 400, or even higher while maintaining your f/11 aperture.

a landscape with a higher ISO — A shot like this needs a deep depth of field. To maintain a narrow aperture while handholding, you can dial in a high ISO.

To avoid noise, you might consider dropping the shutter speed instead of boosting the ISO. But if you do decide to go that route, make sure you get a tripod or use proper handholding technique. Otherwise, you’ll end up with a blurry image, which is counterproductive!

Make sense?

Setting your ISO: practical examples

In this next section, I’d like to share a few common photography scenarios when you’d need to raise or lower your ISO for the best photos.

When to raise the ISO

You should probably raise the ISO if:

You’re shooting at an indoor sports event, especially if your subject is moving fast
You’re shooting a landscape without a tripod and you need a deep depth of field
You’re shooting a landscape at night (or doing astrophotography) and you need a reasonable shutter speed to freeze the stars
You’re photographing portraits in a dark room or in the evening/night
You’re shooting an event indoors with limited window light (such as a party)
You’re photographing a dark concert
You’re photographing an art gallery, a church, or a building interior (you might also consider using a tripod, but this is against the rules in a lot of spaces)
You’re photographing wildlife in the early morning or evening (especially if you need a fast shutter speed)
You’re photographing fast-moving subjects and you need an ultra-fast shutter speed

a party at night with a high ISO in photography — Dark parties can’t be photographed without a high ISO.

When to keep the ISO low

Here are a few times when you should shoot at your camera’s base ISO:

You’re shooting motionless landscapes and your camera is mounted on a tripod
You’re photographing portraits in good light
You’re photographing an event, and you have plenty of window light or you’re using flash
You’re photographing products with a powerful artificial lighting setup

a portrait in good light with a low ISO — A portrait in good light? Stick to your camera’s base ISO!

ISO in photography: conclusion

Now that you’ve finished this article, you should be an ISO master.

So pick up your camera. Practice working in difficult situations. Consider when you should or shouldn’t boost the ISO.

Now over to you:

When do you raise your ISO? Do you struggle to determine when it’s better to keep the ISO low? Share your thoughts in the comments below!