Why is ISO100 Bad? Understanding Its Limitations in Photography

Understanding Why ISO100 Might Not Always Be the Best Choice in Photography

As a photographer, I've spent countless hours wrestling with settings, particularly when the light gets tricky. One common piece of advice, often repeated, is to always shoot at the lowest possible ISO, usually ISO 100. This sounds like a great starting point, right? Who wouldn't want the cleanest, most noise-free image possible? However, in my experience, and as I've delved deeper into the nuances of photography, I've come to understand that while ISO 100 offers significant advantages, labeling it as universally "bad" is an oversimplification. Instead, it's more accurate to say that ISO 100 has specific limitations that can make it unsuitable or even detrimental in certain shooting scenarios. This article aims to explore why ISO 100, despite its reputation for image quality, isn't always the ideal setting and when photographers should consider pushing their ISO higher.

The Core Concept: What is ISO?

Before we dive into the "why," it's crucial to grasp what ISO actually represents in digital photography. In essence, ISO is a measure of your camera sensor's sensitivity to light. Think of it as an amplifier. When you set your ISO to a lower number, like ISO 100, you're telling your camera to require more actual light to produce a properly exposed image. Conversely, a higher ISO, say ISO 3200, means the sensor becomes more sensitive, requiring less light. This sensitivity can be achieved through various electronic amplification processes within the camera.

Historically, in film photography, ISO referred to the film's sensitivity to light, indicated by numbers like 100, 200, 400, and so on. Higher numbers meant faster film, capable of capturing images in lower light conditions but often at the expense of finer detail and increased grain. Digital cameras adopted this concept, but with a key difference: digital ISO is achieved through electronic amplification, which can introduce digital noise, often referred to as "grain" in digital terms, especially at higher settings.

The Perceived Advantage of ISO 100

The primary reason photographers are often encouraged to use ISO 100 is its association with the "base ISO" of most digital cameras. At its base ISO, the camera sensor is functioning with the least amount of electronic amplification. This typically results in:

The cleanest image possible: Less amplification means less introduction of digital noise. Noise manifests as random speckles or "grain" that can degrade image quality, making details appear muddy and colors less vibrant.
The widest dynamic range: Dynamic range refers to the camera's ability to capture detail in both the brightest highlights and the darkest shadows of a scene. At base ISO, sensors generally perform at their peak for dynamic range, allowing for more flexibility in post-processing to recover details in extreme tones.
The finest detail and sharpness: With minimal amplification, the sensor's native resolution and the camera's image processing can render the finest details with the highest clarity.

For many years, this was gospel. If you wanted the best possible image quality, you shot at ISO 100. This often meant using a tripod, wider apertures, or slower shutter speeds to compensate for the reduced light sensitivity. And for many situations, this approach is perfectly valid and yields stunning results. Think of studio photography, bright sunny days outdoors, or any scenario where you have ample light and the subject is stationary.

When ISO 100 Becomes a Hindrance: The Core of the "Bad" Argument

So, if ISO 100 provides the best image quality, why would anyone consider it "bad"? The answer lies in the trade-offs and the practicalities of real-world photography. The "badness" of ISO 100 isn't inherent to the setting itself, but rather its consequences when light is limited, or when specific shooting conditions demand a different approach. Let's break down the scenarios where ISO 100 becomes problematic:

1. Insufficient Light and the Need for Faster Shutter Speeds

This is arguably the most common reason why ISO 100 can be considered "bad." In low-light conditions, such as during a sunset, indoors without sufficient artificial lighting, or at night, ISO 100 requires a significant amount of light to achieve a correct exposure. If you're shooting at ISO 100 in these situations, you'll inevitably face a dilemma:

Longer Shutter Speeds: To gather enough light with ISO 100, you'll need to use a slower shutter speed. For example, if a scene requires f/5.6 at 1/125 second at ISO 400, you'd need approximately 1/15 second at ISO 100. This slow shutter speed is almost guaranteed to result in motion blur if your subject is moving, or even camera shake if you're not using a tripod. This is particularly problematic for photographing people, pets, or any dynamic subjects.
Wide Apertures: Alternatively, you might try to compensate by opening your aperture to its widest setting (e.g., f/1.4, f/1.8). While this lets in more light, it also significantly narrows your depth of field. This can be desirable for isolating a subject, but it can also make it incredibly difficult to keep your entire subject in focus, especially if they are moving or if you're shooting a group.

My own early days of wildlife photography were a testament to this. I'd often find myself at a beautiful dawn or dusk, with stunning light, but my subject, a bird or a deer, would be a blurry mess because I was stubbornly sticking to ISO 100, resulting in shutter speeds too slow to freeze their movement. The "perfectly clean" image was useless if it was a blur.

2. The Importance of Freezing Motion

Photography is often about capturing a moment, and sometimes that moment is fleeting and dynamic. Whether it's a child running, a splash of water, a bird in flight, or a fast-paced sporting event, freezing motion is paramount. To freeze motion effectively, you need a fast shutter speed – typically 1/500 second or faster, depending on the subject's speed and distance.

If you're in an environment that doesn't offer abundant light, trying to achieve a fast shutter speed at ISO 100 is simply not feasible without resorting to extreme aperture settings or unacceptable levels of ambient light. This forces you to raise your ISO. In these instances, ISO 100 isn't "bad" because of its inherent quality, but because its lack of light sensitivity directly prevents you from achieving a technically sound and artistically successful photograph that captures the essence of the moment.

3. Dealing with Specific Lighting Conditions

Certain lighting conditions inherently make ISO 100 a less-than-ideal choice. Consider:

Indoor Events: Weddings, concerts, and recitals often take place in dimly lit venues. While some might have spotlights, the overall ambient light can be quite low. Trying to capture candid moments at ISO 100 would result in unusable shutter speeds or require a flash that might disrupt the atmosphere.
Night Photography (Without Tripod): While long exposures on a tripod at ISO 100 can yield spectacular results for astrophotography or cityscapes, attempting to handhold a shot at night at ISO 100 is impossible. The shutter speeds required would be seconds long, leading to extreme blur.
Candids and Documentary Photography: In situations where you need to be unobtrusive and react quickly to unfolding events, setting up elaborate lighting or using a tripod isn't always an option. You need to be able to adjust your settings rapidly to capture the scene as it is, and this often means raising the ISO.

4. The Myth of "Pure" Image Quality

While ISO 100 provides the least amplification and therefore the theoretical cleanest image, modern digital sensors and image processing engines have become incredibly sophisticated. The difference in noise between ISO 100 and, say, ISO 400 or even ISO 800 on many current cameras is often imperceptible to the naked eye, especially when viewed at typical print or web sizes. The fear of noise has, in some ways, become a more significant barrier than noise itself.

What often degrades image quality more than a moderate increase in ISO is:

Underexposure and Overcorrection: Shooting at ISO 100 in low light and severely underexposing, then trying to "lift" the shadows in post-processing, will often introduce more prominent noise and color artifacts than shooting at a higher ISO in the first place and exposing correctly. The camera's native dynamic range is often better utilized with proper exposure at a slightly higher ISO than forcing a dark scene to look bright.
Aggressive Sharpening: Over-sharpening in post-processing can introduce halos and artifacts that are far more detrimental to perceived image quality than a little bit of noise.
Low-Quality Lenses: A poor-quality lens will introduce softness, chromatic aberration, and distortion that will be more noticeable than a moderate ISO increase.

In my own work, I've found that a well-exposed image at ISO 800 from a good camera often looks significantly better and is more usable than an underexposed image at ISO 100 that has been pushed to balance exposure in editing. The noise at ISO 800 is typically fine-grained and can be easily managed or even embraced as a pleasant texture, whereas the artifacts from extreme shadow recovery can be ugly and unfixable.

5. When the Dynamic Range of ISO 100 Isn't Enough

While ISO 100 typically offers the best dynamic range, there are high-contrast scenes where even this might not be sufficient to capture detail in both the extreme highlights and shadows without compromise. In such situations, photographers might employ techniques like High Dynamic Range (HDR) imaging, which often involves shooting multiple exposures at different ISOs or apertures. However, if you're forced to underexpose to protect highlights at ISO 100, you might end up with very dark shadows that are difficult to recover without introducing noise. In some very specific scenarios, a slightly higher ISO might allow for a better-exposed shadow area, which can then be merged with a properly exposed highlight image in a way that preserves more detail and less noise.

Understanding the "Extended" ISO Settings

Many cameras offer "extended" ISO settings beyond their native base ISO. These are often labeled as "Lo" or "Hi" settings (e.g., ISO 50, ISO 64, ISO 12800, ISO 25600). These extended ISOs are achieved through in-camera processing that essentially pushes or pulls the signal from the sensor, rather than the sensor's native sensitivity.

Low Extended ISOs (e.g., ISO 50, 64): These are essentially achieved by reducing the signal from the sensor at its base ISO. They can offer a slight benefit in dynamic range in the highlights but often come at the cost of reduced dynamic range in the shadows and can sometimes introduce banding or posterization in gradients. They are less about cleaner images and more about achieving specific creative effects, like using a very slow shutter speed in bright daylight without neutral density filters.
High Extended ISOs (e.g., ISO 12800, 25600): These are achieved by further amplifying the signal beyond the camera's native high ISO. The image quality at these settings is typically significantly degraded, with substantial noise, loss of detail, and color shifts. They are a last resort for capturing an image when no other option is available.

Therefore, while ISO 100 is the *native* base ISO, shooting at an extended low ISO like 50 or 64 is not the same as shooting at native ISO 100 and can introduce its own set of problems, often related to reduced shadow detail and potential artifacts.

The Modern Photographer's Approach to ISO

The notion that ISO 100 is always "bad" is as misguided as the idea that it's always "good." The modern approach to ISO is about understanding the exposure triangle (aperture, shutter speed, and ISO) as a dynamic interplay, and choosing the setting that best serves the artistic and technical requirements of the shot.

1. Prioritize Correct Exposure

The most crucial element for good image quality, regardless of ISO, is correct exposure. An underexposed image, even at ISO 100, will suffer from increased noise and color issues when you try to "fix" it in post-processing. Similarly, an overexposed image will lose highlight detail.

Checklist for Correct Exposure:

Understand your camera's meter: Learn how your camera's metering modes (evaluative, center-weighted, spot) work and how they interpret light.
Expose to the Right (ETTR): This is a technique where you aim to expose your image as brightly as possible without clipping the highlights (losing detail in the brightest parts of the image). This is often best achieved by looking at your camera's histogram. A well-exposed image with minimal shadow clipping at a slightly higher ISO will often yield better results than a significantly underexposed image at ISO 100.
Use your histogram: The histogram is your best friend for judging exposure. It graphically represents the distribution of tones in your image. Aim to have the histogram pushed towards the right side without touching the far-right edge (which indicates clipped highlights).

2. Embrace the "Sweet Spot"

For most modern digital cameras, there's a "sweet spot" of ISO settings where image quality is excellent, and the trade-offs are minimal. This sweet spot is typically between the base ISO (ISO 100) and a moderate higher ISO, often ranging from ISO 200 to ISO 800, and sometimes even up to ISO 1600 or 3200 on professional-grade cameras. Within this range, the increase in noise is often negligible or easily manageable.

My Personal "Sweet Spot" Strategy:

I often find myself operating comfortably between ISO 400 and ISO 1600 for general-purpose shooting. If the light is abundant, I'll happily use ISO 100 or 200. However, if I'm shooting indoors at a family gathering or in a park during cloudy weather, I'll set my ISO to 800 or 1600 to ensure I can maintain a shutter speed fast enough to avoid blur and an aperture that gives me a reasonable depth of field. The resulting images are typically very clean and more than suitable for sharing online or printing.

3. Understand Your Camera's Capabilities

Not all cameras are created equal. A brand-new, high-end full-frame camera will handle high ISOs far better than an entry-level crop-sensor camera from several years ago. It's essential to know your camera's limitations and strengths.

How to Assess Your Camera's High ISO Performance:

Test Shots: Take a series of test shots of a static scene (e.g., a landscape with a range of tones, or a detailed object) at various ISOs, from your base ISO up to your camera's maximum.
Review at 100% Zoom: Examine these images on your computer at 100% magnification. Look for the onset of noticeable noise, color shifts, and loss of fine detail.
Compare Dynamic Range: Also, test how well you can recover shadows and highlights at different ISOs. This will give you a practical understanding of your camera's usable ISO range.

4. Embrace Post-Processing Tools

Modern noise reduction software (like Adobe Lightroom's Denoise AI, Topaz DeNoise AI, or DxO PureRAW) is remarkably effective. These tools can significantly reduce or even eliminate noise from images shot at moderately high ISOs, often with minimal impact on detail.

This means that the noise penalty for shooting at, say, ISO 1600 or even 3200, is far less severe today than it was a decade ago. The ability to salvage a slightly noisy but well-exposed image is a powerful advantage.

5. Consider the Creative Intent

Sometimes, a bit of grain or noise can even add to the aesthetic of an image. Think of certain cinematic styles or black-and-white photography where a touch of grain can enhance mood and texture. While ISO 100 is designed to be pristine, there are creative reasons to embrace a higher ISO.

Case Studies: When ISO 100 Fails and Higher ISOs Shine

Let's illustrate with some real-world scenarios:

Scenario 1: A Child's Birthday Party Indoors

Imagine you're at your nephew's birthday party. The ambient light is coming from a few lamps and the natural light from a window that's fading as the sun sets. The kids are running around, blowing out candles, and you want to capture these energetic moments.

Attempting ISO 100: If you try to shoot at ISO 100, you'll need a very slow shutter speed (likely seconds long) to get a proper exposure without a flash. This means every movement will be a blur. Opening your aperture wide might help slightly, but you'll struggle with focus.
Using a Higher ISO (e.g., ISO 1600-6400): By raising your ISO to 1600 or 6400, you can achieve a shutter speed of, say, 1/125 second or faster. This will freeze the motion of the children. Even if the image has a bit more noise, it will be sharp, in focus, and capture the moment. Modern noise reduction can clean this up considerably, or the noise might be subtle enough to be unnoticeable in typical viewing conditions. The "badness" of ISO 100 here is its inability to allow you to freeze the action.

Scenario 2: A Cityscape at Twilight

You're out at twilight, the city lights are starting to twinkle, and the sky has a beautiful, deep blue hue. You want to capture the blend of natural and artificial light.

Attempting ISO 100: To capture the subtle sky colors and the city lights, you'll need a relatively long exposure, perhaps several seconds. If you use ISO 100, this might be achievable on a sturdy tripod. However, if there's any significant movement in the scene (cars, people), they will be streaked. Also, longer exposures can sometimes lead to "noise" from heat generated by the sensor, even at base ISO on some cameras.
Using a Moderate ISO (e.g., ISO 400-800) with a Shorter Exposure: If you opt for ISO 400 or 800, you can reduce your exposure time to, say, 1-2 seconds. This still allows for beautiful light capture but can freeze some movement more effectively. Furthermore, on many cameras, the noise characteristics at ISO 400 or 800 are very manageable, and the dynamic range is still excellent. The "badness" of ISO 100 here is not about image quality degradation but about potential compromises in capturing movement or managing the overall exposure balance effectively without extremely long exposures.

Scenario 3: Wildlife Photography in the Early Morning

You've woken up before dawn to photograph a deer in the misty forest. The light is soft and ethereal, but definitely low.

Attempting ISO 100: The light levels are so low that even with your widest aperture (say, f/4) and a steady hand, you might need a shutter speed of 1/30th of a second or slower. This will almost certainly result in a blurry deer, especially if it moves its head or takes a step.
Using a Higher ISO (e.g., ISO 1600-3200): By increasing your ISO to 1600 or 3200, you can achieve a shutter speed of 1/250th second or faster. This is usually sufficient to freeze the movement of a deer's head and body. The resulting image might have some noise, but it will be sharp and capture the intended subject. The "badness" of ISO 100 is its inability to allow for a fast enough shutter speed to freeze the subject.

Frequently Asked Questions About ISO 100

Why is ISO 100 generally recommended for the cleanest image?

ISO 100 is generally recommended for the cleanest image because it represents the "base ISO" of a digital camera's sensor. At this setting, the sensor requires the most actual light to achieve a proper exposure. This means the camera's internal electronic amplifiers are working at their minimum level, or not at all. Electronic amplification is what boosts the signal from the sensor to create an image, but it can also amplify unwanted electronic interference, which we perceive as digital noise or grain. By using the least amplification, ISO 100 minimizes the introduction of this noise, leading to images with the smoothest tones, finest detail, and least apparent grain. It also typically provides the camera's maximum dynamic range, meaning it can capture the most detail in both the brightest highlights and the darkest shadows simultaneously, giving photographers the most flexibility in post-processing.

Furthermore, at base ISO, the sensor's resolution and color rendition are often at their purest. There's no digital manipulation or enhancement applied to boost sensitivity. This purity is why professionals often strive to shoot at their camera's base ISO when conditions permit, especially in controlled environments like studios where ample lighting is available and subject movement is not a concern. The goal is to capture the most "raw" and unadulterated data from the sensor, which then can be manipulated with more control and fidelity later.

What are the biggest drawbacks of shooting exclusively at ISO 100?

The biggest drawbacks of shooting exclusively at ISO 100 stem from its requirement for more light. In situations where light is scarce, such as indoors, at dawn, dusk, or during the night, you will be forced to compensate for the lack of light sensitivity. This compensation typically takes one of two forms, both of which have significant drawbacks:

First, you'll need to use slower shutter speeds. For example, if a scene requires a shutter speed of 1/125 second at ISO 400, you might need a shutter speed of 1/15 second or even slower at ISO 100 to achieve the same exposure. This very slow shutter speed is highly susceptible to motion blur. If your subject is moving, even slightly, they will appear blurred. If you're hand-holding the camera, camera shake will introduce blur throughout the image, rendering it unusable unless you're intentionally going for a "ghosting" effect. This makes ISO 100 impractical for capturing fast-moving subjects, children playing, or even just candid moments where you need to react quickly.

Second, you might resort to opening your lens aperture to its widest setting (e.g., f/1.4, f/1.8, f/2.8). While this lets in more light, it significantly reduces the depth of field. This means that only a very narrow plane of the image will be in sharp focus. For portraits, this can be a desirable effect, but for landscapes or group photos, it can be incredibly challenging to keep everything you want in focus. You might end up with eyes that are sharp but noses that are soft, or the person in front is sharp but the person behind them is out of focus.

Therefore, the primary drawback is the limitation it places on your ability to use faster shutter speeds and control your depth of field, directly impacting your creative options and the ability to capture sharp images in less-than-ideal lighting conditions.

When would shooting at ISO 100 be genuinely detrimental to a photograph?

Shooting at ISO 100 would be genuinely detrimental to a photograph in any situation where capturing a sharp image requires a shutter speed faster than what ISO 100 can accommodate, or where controlling depth of field is crucial and a wide-open aperture is not feasible. This includes:

Capturing Fast Action: Sports photography, wildlife photography (birds in flight, running animals), or even capturing a child's spontaneous movement – all require shutter speeds of 1/500 second or faster to freeze motion. Trying to achieve these speeds at ISO 100 in anything less than bright, direct sunlight is virtually impossible without resorting to extremely impractical apertures or significantly underexposing the image.
Low Light Events: Concerts, theatrical performances, indoor parties, or dimly lit restaurants are classic examples. The ambient light is simply not enough to allow for a fast enough shutter speed at ISO 100 without a very bright flash, which can ruin the atmosphere.
Handheld Shots in Less Than Ideal Light: Even if your subject is stationary, if the light is dim, hand-holding a camera at ISO 100 will necessitate shutter speeds so slow that camera shake becomes inevitable, leading to a blurry, unusable image.
Situations Requiring Specific Depth of Field: Imagine a photographer trying to capture a sharp image of a person standing in front of a busy background. If they are at ISO 100, they might be forced to use a wide aperture like f/1.4 to get enough light. This creates a very shallow depth of field, making it difficult to keep the entire subject in focus, especially if they are not perfectly still. If they could use a slightly higher ISO (e.g., ISO 400), they might be able to use an aperture like f/5.6, which offers a much more forgiving depth of field while still allowing for a manageable shutter speed.

In these scenarios, sticking to ISO 100 leads to either a blurry image due to slow shutter speeds or an image with unacceptable focus issues due to narrow depth of field. The "detriment" comes from the fundamental inability of ISO 100 to meet the shooting requirements of the scene, forcing compromises that severely degrade the final photograph's technical quality and artistic intent.

Can shooting at a higher ISO (e.g., ISO 800 or 1600) introduce more problems than ISO 100?

Yes, shooting at a higher ISO can introduce more problems than ISO 100, but the severity of these problems has drastically decreased with modern camera technology. The primary problem introduced by higher ISO settings is digital noise. This noise appears as random speckles of color and brightness that can degrade image quality, making textures appear muddy, colors less vibrant, and fine details less distinct. At very high ISOs, this noise can become so prominent that it significantly compromises the photograph's aesthetic and usability.

Additionally, as ISO increases, the dynamic range of the sensor typically decreases. This means the camera becomes less capable of capturing detail in both the extreme highlights and shadows simultaneously. You might lose detail in the brightest areas or find that the shadows become unnaturally dark and difficult to recover without introducing even more noise or color banding. Color accuracy can also be affected at higher ISOs, with colors sometimes appearing less saturated or shifting in hue.

However, the "problem" is relative. For a long time, any noticeable noise was considered unacceptable. Today, with advanced noise reduction algorithms in cameras and sophisticated post-processing software, images shot at ISO 800 or even 1600 on many modern cameras can be exceptionally clean. The noise is often fine-grained and can be easily managed. The crucial point is that the *trade-off* for using a higher ISO is a potential decrease in image quality (noise, reduced dynamic range), but this trade-off is often well worth it if it allows you to achieve correct exposure, freeze motion, or use a desirable aperture that you couldn't achieve at ISO 100.

Therefore, while higher ISOs *can* introduce problems, the question is whether those problems are more detrimental than the problems caused by *not* being able to use adequate shutter speeds or apertures at ISO 100. For most dynamic or low-light situations, the problems introduced by higher ISOs are far less detrimental than the unusable, blurry, or unfocused images that would result from sticking to ISO 100.

What is the "exposure triangle" and how does ISO fit into it?

The "exposure triangle" is a fundamental concept in photography that describes the relationship between three key settings that determine the overall exposure (brightness) of an image: Aperture, Shutter Speed, and ISO. Each of these settings affects how much light reaches the camera's sensor, and they are interconnected, meaning changing one will often require adjusting another to maintain the desired exposure.

Aperture: This refers to the size of the opening in the lens that allows light to pass through to the sensor. It's measured in f-stops (e.g., f/1.4, f/2.8, f/5.6, f/11). A wider aperture (smaller f-number like f/1.4) lets in more light and creates a shallow depth of field (blurry background). A narrower aperture (larger f-number like f/11) lets in less light and creates a greater depth of field (more of the scene in focus).
Shutter Speed: This is the duration for which the camera's sensor is exposed to light. It's measured in seconds or fractions of a second (e.g., 1/1000s, 1/60s, 1 second, 30 seconds). A faster shutter speed lets in less light but is effective at freezing motion. A slower shutter speed lets in more light but can result in motion blur or camera shake.
ISO: As discussed, this is the sensitivity of the camera's sensor to light. A lower ISO (like ISO 100) means the sensor is less sensitive and requires more light. A higher ISO means the sensor is more sensitive and requires less light.

How they work together:

Imagine you want to achieve a specific exposure. You can get there in multiple ways by balancing these three elements. For example:

Scenario A: Bright light, need fast shutter. You're in bright daylight and want to freeze a running dog. You'll likely use a wide aperture (e.g., f/4) and a fast shutter speed (e.g., 1/1000s). To get enough light with these settings, you'll use your base ISO (ISO 100).
Scenario B: Low light, need to freeze motion. You're indoors, and the dog is still running. You still need 1/1000s to freeze it, and you might still want a decent depth of field (e.g., f/5.6). To get the necessary light with these more demanding settings, you'll have to raise your ISO significantly (e.g., to ISO 6400 or higher).
Scenario C: Low light, no need to freeze motion. You're indoors, and the dog is sleeping. You still want a deep depth of field (f/8). You can use a slow shutter speed (e.g., 1/30s) and a lower ISO (e.g., ISO 400).

Understanding the exposure triangle is crucial for deciding when to prioritize one setting over another. If freezing motion is paramount, you'll prioritize shutter speed and adjust ISO and aperture accordingly. If depth of field is critical, you'll prioritize aperture and adjust shutter speed and ISO. If image cleanliness is the absolute priority and light is abundant, you'll prioritize ISO 100 and adjust aperture and shutter speed.

Is there a specific "best" ISO setting for all situations?

No, there is no single "best" ISO setting for all situations. The optimal ISO setting is entirely dependent on the specific shooting conditions and your creative intent. As the exposure triangle illustrates, the ideal ISO is a balance struck between aperture, shutter speed, and the available light. What is "best" is the ISO that allows you to achieve your desired aperture for depth of field and your desired shutter speed to freeze motion or prevent blur, all while resulting in a correctly exposed image.

For example:

Bright Sunny Day Outdoors: ISO 100 is often ideal, providing the cleanest image and maximum dynamic range, allowing you to use any aperture or shutter speed you need without exposure concerns.
Indoor Portraits: You might aim for an aperture like f/2.8 to blur the background. If the available light is low, you might need ISO 800 or 1600 to achieve a shutter speed that prevents subject motion blur (e.g., 1/125 second).
Action Photography: In sports or wildlife, where freezing fast motion is critical (e.g., 1/1000s or faster) and you might need a reasonable depth of field (e.g., f/5.6), you'll almost certainly need to use a significantly higher ISO, perhaps 3200, 6400, or even higher depending on the camera and light.
Night Landscapes (with tripod): You can use very long exposures (seconds or minutes) at ISO 100 to capture the scene with minimal noise.

The "best" ISO is the one that serves the photograph you are trying to create. Forcing yourself to always use ISO 100 will often lead to compromises that are far worse for image quality than a moderate increase in ISO.

Conclusion: Moving Beyond the "Bad" Label

The idea that "ISO 100 is bad" is a misunderstanding. It's not inherently bad; rather, it's a setting with specific characteristics that make it unsuitable for many real-world shooting scenarios. Its primary limitation is its low light sensitivity, which demands more light and consequently forces compromises in shutter speed and aperture, leading to motion blur, camera shake, or an unacceptably shallow depth of field when light is scarce.

The true skill of a photographer lies not in blindly adhering to a single setting like ISO 100, but in understanding the interplay of the exposure triangle and making informed decisions. Modern cameras and software have significantly blurred the lines of what constitutes "acceptable" noise. Embracing moderately higher ISOs when necessary allows you to freeze action, control depth of field, and achieve proper exposure, often resulting in a far more successful and compelling photograph than a technically "clean" but blurry or out-of-focus image shot at ISO 100.

So, is ISO 100 bad? No. Is it always the best choice? Absolutely not. Learn its strengths, understand its weaknesses, and most importantly, know when to push it higher to capture the image you envision.