So, you are standing in front of the film fridge at your local camera shop, staring at a stack of colorful cardboard boxes. You see numbers everywhere: 100, 200, 400, 800. If you are relatively new to the analog world, this can feel a bit like trying to decipher a secret code. Even if you have been shooting for a while, it is easy to just grab whatever is cheapest and hope for the best.

But those numbers—the ISO or film speed—are actually the most important decision you will make before you even press the shutter. Unlike digital cameras where you can just bump up the sensitivity with the flick of a dial, shooting on film means you are locked into that ISO for the next 24 to 36 shots. Pick the right speed, and you will get gorgeous, properly exposed photographs. Pick the wrong one, and you might end up with a roll of muddy, blurry, or underexposed memories.

I get asked about this a lot, so I want to break down exactly what film speed means, how it impacts the look of your photos, and how to pick the perfect ISO for whatever situation you happen to find yourself in.

The Basics: What Even is ISO?

Without getting overly scientific, ISO is quite simply a measurement of how sensitive your film is to light. The acronym stands for the International Organization for Standardization, but all you really need to know is how the numbers scale.

Lower numbers (like ISO 50 or 100) mean the film is less sensitive to light. We call these "slow" films. Because they are not very sensitive, they need a lot of light to properly capture an image. Higher numbers (like ISO 800 or 3200) mean the film is highly sensitive to light. We call these "fast" films because they can capture a well-exposed image in a fraction of a second, even in relatively dark environments.

On a physical level, this sensitivity comes down to the size of the silver halide crystals baked into the film emulsion. Slow films have very fine, small crystals, which take more time to absorb light. Fast films have larger crystals that catch light quickly. Those larger crystals are the reason why higher ISO films look "grainier" than lower ISO films. That trade-off between light sensitivity and grain is the heart of analog photography.

ISO 100 & 200: Sun-Drenched Days and Crisp Details

If you are heading out on a bright summer day, a beach holiday, or a hike with clear skies, lower-speed films like ISO 100 and 200 are your best friends. Think of legendary stocks like Kodak Ektar 100, Fuji Superia 200, or the beloved Kodak Gold 200.

Because the silver crystals in these films are incredibly fine, the resulting photos are sharp, clean, and practically grain-free. You will usually get the most vibrant, punchy colors and the highest level of contrast out of a slow film. It is perfect for sweeping landscapes, architecture, or bright, well-lit portraits where you want every eyelash and fabric texture to be crystal clear.

The catch? As soon as the sun dips behind a thick cloud, or you step inside a building, you are going to struggle. Slow film needs so much light that if you shoot it indoors, your camera will have to keep the shutter open longer to compensate, resulting in blurry photos from your hands shaking. Save your ISO 100 and 200 rolls for the great outdoors.

ISO 400: The Goldilocks Zone

If I was stranded on a desert island and could only shoot one film speed for the rest of my life, it would be ISO 400. It is the absolute sweet spot of the analog world. Film stocks like Kodak Portra 400, Kodak Ultramax, and Ilford HP5 Plus have built their legendary reputations on being incredibly versatile.

When I am testing out vintage 35mm film cameras for the shop, I almost exclusively load them with a 400-speed film. ISO 400 gives you a massive safety net. It is fast enough to shoot on relatively overcast days or in the shade, but still slow enough that you can use it on a sunny afternoon without overexposing everything.

With an ISO 400 film, you will notice a bit more grain than you would with a 100-speed film, but it is usually a very pleasing, cinematic texture. It just feels like "film." If you only have space for a few rolls in your bag and you don't know exactly what the weather is going to do, grab a 400.

ISO 800 and Beyond: Embracing the Dark (and the Grain)

Sometimes the most interesting moments happen after the sun goes down. Whether you are shooting at a dimly lit gig, hanging out around a campfire, or trying to capture the moody vibe of a late-night diner, you are going to need a fast film. This is where ISO 800, 1600, or even 3200 film enters the chat.

Stocks like Cinestill 800T, Kodak Portra 800, or Ilford Delta 3200 are designed with massive silver crystals to soak up whatever ambient light is available. In these low-light scenarios, you need all the light-gathering capability you can get. If you want to learn more about the specifics of low-light shooting, I wrote a whole guide on how to shoot film indoors without a flash, and stepping up to a fast film is entirely critical to pulling it off.

Keep in mind that when you shoot at 800 or above, grain becomes a prominent feature of your photo. The colors might also shift slightly or become a bit more muted compared to the highly saturated look of lower ISO films. But honestly? I love it. Plucky, gritty grain is a massive part of the analog aesthetic. It gives night shots a beautiful, rough-around-the-edges atmosphere that digital sensors just can't replicate.

How ISO Changes the Vibe: Grain, Contrast, and Color

To summarize the artistic side of picking your film speed, always remember these three rules of thumb:

• Grain: Lower ISO means finer grain and a smoother image. Higher ISO means chunkier grain and a grittier texture. Film grain gives a photograph soul, so don't be afraid of it.
• Contrast: Slower films generally provide higher contrast, with deep blacks and bright highlights. Faster films tend to have a flatter, softer contrast.
• Color: Fast films can sometimes look a bit desaturated, especially in shadows. Slower films usually pack a more vibrant, saturated punch when perfectly exposed in sunlight.

Gear Up for Your Next Roll

Choosing the right ISO is half the battle, but making sure your camera actually understands the light around you is just as important. If you are shooting outside of the "sunny 16" rule or dealing with tricky indoor lighting, having the right accessories makes a world of difference.

If you have an older camera where the built-in meter has died (or it never had one to begin with), treating yourself to a dedicated light meter will save you from wasting expensive film. You can see what I've currently got in the shop here: find a vintage light meter.

On the flip side, if you are absolutely determined to use a beautiful ISO 100 film but you want to shoot some portraits at a house party, you simply need more light. A reliable external flash is essential for those moments. You can browse my current stock and grab a flash to light up the night.

At the end of the day, film speed isn't a strict set of rules meant to limit you—it's a creative choice. It dictates the texture, the mood, and the capabilities of your camera on any given day. So next time you are staring at the film fridge, think about where your camera is going to take you, pick your ISO with confidence, and go shoot something amazing.