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The Complete Beginner’s Guide to ISO

The ISO is part of the holy trinity of photography settings, along with aperture and shutter speed. Knowing the function of ISO and how to change it appropriately helps ensure that you get the best image quality. By the end of this article, you’ll understand the fundamentals and effects of ISO on your photos.

Introduction to ISO

If you’ve seen camera film boxes or cartridges, you’ll notice that they’re labelled along with a number—e.g., Kodak Ektar 100, Fujifilm Fujicolor 400. That number indicates the ISO value of that film. There used to be no way of changing the inherent ISO of the film, but with modern digital cameras, the digital sensor is now capable of adjusting its ISO.

ISO is a measure of how bright or dark the camera makes the light the sensor receives, thus affecting how bright or dark your image is. A higher ISO value means that your camera is more sensitive to light, allowing it to capture more light and make your image brighter. A lower ISO means less sensitivity to light, which makes the image darker. Of course, image brightness still depends on the entire holy trinity, so shutter speed and aperture can compensate for ISO sensitivity.

It seems like ISO is related to exposure, but not as directly as you think. What comprises exposure is the shutter speed and aperture because they are mechanisms that allow you to physically capture either more or less light on the exposure. The sensor or film receives the provided light, and the camera ISO processes the light to appear brighter.

However, based on the image output, ISO changes the brightness and darkness of the image, so photographers refer to it as part of the exposure settings anyway. We can also call ISO value “ISO speed.”

Adjusting the ISO has another significant effect on your photo: increasing or decreasing image noise/grain. Noise is a massive concern for photographers since image noise has a substantial impact on the overall technical quality of the photo. I’ll give an in-depth explanation of how noise and ISO work in a section below.

Exposure and ISO

As stated above, a low ISO makes the camera less sensitive to light, which translates to a darker image; a high ISO means more light sensitivity and a brighter image. Low ISO values typically mean 100-400, while high ISO values are at 1600-6400. Take a look at the ISO comparison image below:

Exposure and ISO

As the ISO increases, the image brightens up as well

ISO is conventionally measured in double increments. The progression is as follows: 100, 200, 400, 800, 1600, 3200, 6400, and so on. Every time the ISO speed doubles in value, the sensor becomes twice as sensitive to light; when the ISO halves, the camera is half less sensitive to light. There are ISO speeds lower than 100 and higher than 6400, but the typically usable range of ISO falls under 100 - 6400.

Setting Your ISO

The camera’s ISO settings can be adjusted in various ways.

If you’re using a film camera, the only way to change your film’s ISO is to change the film itself. For instance, if the photographer often shoots in low-light conditions, they’ll load the camera with an 800 or 1600 ISO film. The film camera has an ISO dial that you then turn to match the ISO loaded into the camera. (There’s a technique called “pushing the ISO” in film photography, but that’s a needlessly complicated discussion for this article.)

Digital sensors, on the other hand, can adjust their ISO speed based on the built-in range provided by their manufacturer. All it takes is the scrolling of a dial to move from the camera’s lowest to highest ISO. The camera has a “native” ISO range that manufacturers intend you to use.

Your camera needs to be in a non-auto mode to enable manual adjustments. Then, go into your camera’s quick settings menu, select the ISO adjustment option, and adjust it by scrolling your camera’s dial.

Some cameras use a dedicated button or dial for the ISO. If it’s a button, you just have to press the button and turn the main dial to adjust the ISO. Dedicated ISO dials provide a more “film-like” experience by using a dial with ISO value markings on it (Fujifilm’s high-end mirrorless cameras are known for having this feature). Turning the dial adjusts the camera ISO accordingly.

Of course, you can always leave the camera on auto-ISO. Still, as you progress to more advance knowledge in photography and post-processing, you’ll understand the importance of being conscious about your ISO speed.

Base ISO

The base ISO is the lowest ISO setting that’s native to your camera. It’s considered the best ISO speed to shoot at since your photos will have barely any or no noise. It also captures the best dynamic range for your image, which is extremely helpful when you’re post-processing your images. The base ISO can be achieved by shooting in bright enough conditions or by adjusting your shutter speed or aperture to compensate for the low ISO speed.

Digital cameras will almost always have a base ISO of 100 or 200. As a rule of thumb, shoot at the lowest ISO if you can afford to (unless you deliberately want to insert some noise in your image).

There are cameras where ISO can be extended lower than the base ISO or higher than its maximum native range. This is indicated as “Lo” and Hi” ISO values. However, those settings are accomplished by using some software trickery. Lowering the ISO below the base ISO has negative impacts on the image quality. I don’t shoot in these extended ISO settings and just stick to the native range of my camera to get optimal quality out of my photos.

Noise and ISO

I used to complain about how my photos came out “noisy” or “grainy,” especially during low-light conditions. I know you wouldn’t want to post an overly noisy photo on social media either. After studying, I found out that those photos were noisy because I shot them at a high ISO.

Take a look at the comparison photos below to see how low ISO produces overall cleaner images while high ISO creates more noise.

Noise and ISO

The photo on the left was shot at base ISO, while the photo on the right used a very high ISO. The significant jump in ISO speed better illustrates the difference in noise levels

You might be wondering why that’s the case, and the explanation is relatively simple. We can think of higher ISO as amplifying the light signals received by the sensor, which causes the brighter output. These light signals contain unnecessary or extra information that wouldn’t be needed to produce a clean image. Since higher ISO means more sensitivity, it reveals the additional light information to create a brighter image at the expense of more noise detail. You can also refer to this as the signal-to-noise ratio—the more the camera boosts the light signals, the more noise appears in the photo.

Recommended ISO Settings

There are appropriate ISOs to use for various scenarios. Knowing when to use high or low ISOs makes you adaptable to all kinds of shooting conditions.

Using Low ISO

If you want the best image quality with the least amount of noise, you should always stick to the base native ISO of your camera. Even at around ISO 400-800, the amount of noise is so minimal that it’s almost invisible unless you look up-close. Shooting a scene with a lot of light will allow you to shoot at low ISO speeds with ease.

It’s still possible to shoot low ISO at darker scenes. For this, you’ll need to use a wide aperture or slow shutter speed to allow more light in, thus compensating for the decreased light sensitivity. Often, your camera needs to be stable on a flat surface or tripod to avoid camera shake at slow shutter speeds. If your camera or lens has built-in stabilization, you can probably get away with handheld shots.

Using High ISO

You don’t have to avoid high ISO speeds like you’re avoiding the plague. Sometimes, it’s unavoidable or more convenient to use higher ISO speeds in darker scenarios; the trade-off for having more noise is acceptable if it means getting a brighter image. A high ISO also lets you use faster shutter speeds, meaning you can avoid unwanted blur in your images, which is particularly useful in low-light sports or nature photography.

Here’s a sample photo that used a high ISO and fast shutter speed:

High ISO photography

I had to use a shutter speed of 1/2000 and an ISO of 1600 to get an entirely frozen image. If I lowered my ISO, then that would’ve meant using a slow shutter speed. Would I have been able to freeze the subject? Most likely not.

I always weigh the pros and cons of using a higher ISO. Are the benefits worth more than the increased image noise? Or do I want the best image detail and quality? Can I afford to shoot at a slower shutter speed or not? These are questions you should ask yourself, too.

Getting The Best Image Quality With ISO

It would be awesome if we could always shoot at the camera’s base ISO, but that’s impossible. There will be instances where you simply need to use a higher ISO setting to get an acceptable photo. To get the optimal ISO that is appropriate to the shooting conditions, you’ll need a grasp of how shutter speed and aperture work, too. If you know how to adjust the entire holy trinity of settings, then you can achieve the best possible image quality.

I’ve come up with easy step-by-step instructions that will guide you to the best ISO setting at any given scenario while incorporating your decisions on shutter speed and aperture. If you haven’t read my articles on either setting yet, I recommend you do so first.

  • Decide how much depth of field is needed in your photo. For a shallow depth of field, set a wide aperture; for a large depth of field, set a small aperture.
  • Decide how much motion blur you want. If you plan to freeze a moving subject, then select a fast shutter speed. If there aren’t any moving subjects or if you plan to introduce some motion blur, then you can select a slower shutter speed.
  • Now, set the ISO. If the image is too dark, raise the ISO speed; if it’s too light, lower the ISO. Adjust until you get the exposure levels you want.
  • Observe the amount of image noise. If it’s too noisy, then you need to lower the ISO. However, you need more light to compensate for the decreased light sensitivity. You can achieve that by doing two things: 1) make the aperture wider, or 2) use a slower shutter speed. So it’s up to you to decide what you’re willing to change: the amount of depth of field or motion blur.

It’s all a balancing act of finding the right combination of settings to get the best image quality. You can always set your camera to change these settings automatically. It’s more convenient and less brainpower is needed, and you’ll still get the image you want.

However, putting more thought into the shooting process ensures that you have complete control of how your image will come out. Don’t be afraid to go into manual settings—that’s how you become a better photographer, after all!

Frequently Asked Questions

What does ISO stand for?

ISO means “International Organization for Standardization.” Back then, film sensitivity used to be labelled as ISO, ASA, or DIN. The “ISO” label eventually won and became the standardized term we use today for both film and digital cameras.

What are the effects of ISO on my photo?

Adjusting the ISO affects how bright or dark your image is. It also dictates how much noise is introduced into the photo. At higher ISOs, the image is brighter at the expense of more noise; at lower ISOs, it’s darker but less noisy.

Is making the image brighter during post-processing like changing the camera ISO?

It looks like that at first glance, but the process is different. While the result of increasing image brightness on your computer or phone and increasing the in-camera ISO setting seems similar, it’s always better to do it natively on your camera before taking the shot. The initial processing of the image is done on the digital processor of the camera. After that, adjusting the image’s brightness in post-processing is like processing what’s already been processed.

You tend to lose fine details if you simply adjust image brightness, and you can’t push the image exposure in post-processing as well as changing the ISO. That’s why it’s always better to get the ISO setting as best as possible before taking the shot.

Is my camera’s base ISO the best ISO setting?

In a word: definitely. The base ISO yields very little to no noise in your image. If you want to simulate a noise effect for aesthetic purposes, you can always add it in post-processing.

For post-processing purposes, shooting at low ISOs is always better. At base ISO, your image has the best dynamic range. When you bump up the exposure afterwards or try to recover some details in the image, especially in the highlights or shadows, a low ISO retains more details, which provides more editing flexibility.

When should I shoot with high or low ISOs?

Generally speaking, high ISO speeds are preferable for low-light purposes, like astrophotography or indoor events, to create a brighter image. In times where there’s enough light, like in a studio where you can add as much light as needed, or during sunny days, you can safely shoot at a low ISO speed.


Knowing what ISO does and mastering it are valuable for any beginner photographer. After I learned how to select the ideal ISO for any scenario, I was able to adjust it to suit my aperture and shutter speed needs, and I could handle various lighting conditions with more ease.

The best way to experience the effects of ISO is to keep shooting and studying how your images look. Expose yourself to all kinds of lighting scenarios and learn to adapt. Mastering ISO, along with aperture and shutter speed, will give you the skills needed to take your photography to the next level!

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