Search for "long exposure in daylight" and you'll quickly run into ND filters: dark pieces of glass that photographers screw onto their lenses. Clip-on versions exist for iPhone too, and they're not cheap once you buy decent ones.

So do you actually need an ND filter for your iPhone? Short answer: no, not for long exposure. Computational photography solves the same problem in software. Here's why the filter exists in the first place, and why frame stacking makes it unnecessary on a phone.

What Is an ND Filter?

ND stands for neutral density. An ND filter is a darkened piece of glass that sits in front of the lens and reduces the amount of light reaching the sensor, without changing colors. Think of it as sunglasses for your camera.

Photographers buy them in different strengths. The darker the filter, the longer you can keep the shutter open before the image overexposes.

Why Traditional Cameras Need ND Filters

On a DSLR or mirrorless camera, a long exposure means physically holding the shutter open. That works fine at night, when there's little light to collect. But try a 10-second exposure at noon and the sensor drowns in light. The photo comes out pure white.

The ND filter fixes this by cutting the incoming light, so the shutter can stay open for seconds or minutes in broad daylight without overexposing. For traditional cameras, it's the only way to get silky waterfalls and smooth seas under the sun. The cost: more gear to carry, fiddling with strengths, and color casts on cheaper glass.

Why iPhones Have the Same Problem (Worse)

The iPhone has a small sensor and no physical way to hold a single exposure open for long. In daylight, the stock camera defaults to a fast auto shutter, and it has no shutter speed setting at all (we cover that in detail in our iPhone shutter speed guide). Even with a clip-on ND filter, you'd still need an app that can slow the shutter down, and the sensor's own limits cap how far that goes.

So the traditional path on iPhone means buying a filter, mounting it straight over a tiny lens, and still fighting the hardware. There's a better way.

The Computational Alternative: Frame Stacking

Instead of one long exposure, frame stacking takes many short ones and blends them. Lento captures frames continuously at 30fps and stacks them in real time using Metal compute shaders on the GPU. Each frame is exposed normally, so daylight can't blow out the image. The motion blur comes from the math, not from starving the sensor of light.

The result is the same look an ND filter delivers, with some practical advantages:

  • No gear. Nothing to buy, mount, or lose in a bag.
  • Works in any light. Noon sun, golden hour, night. The exposure of each frame adapts; the stack handles the rest.
  • No color cast. Cheap ND glass tints your image. Stacking doesn't touch color, and Lento exports in P3 wide color.
  • Live preview. You watch the long exposure build on screen while shooting, instead of guessing at filter strength and exposure time.
  • Handheld is possible. Because every frame is a normal exposure, hand shake doesn't smear the whole image. A tripod still helps for very long stacks, but it's optional. More on that in our no-tripod guide.

You also keep editing flexibility: Lento can export 16-bit RAW DNG at 12MP with full EXIF, which gives you far more room in Lightroom than a JPEG shot through dark glass.

When Would an ND Filter Still Make Sense?

If you shoot video on iPhone and want cinematic motion blur at a fixed frame rate, an ND filter still has a real job, because video frames can't be stacked the way photos can. And on a dedicated camera, filters remain the standard tool.

But for long exposure photos on iPhone, the filter solves a problem the phone doesn't have to have. A daytime waterfall shot, a smooth sea, ghosted crowds in a plaza: all of it is doable in software. Our daytime long exposure guide walks through the exact settings.

FAQ

Do ND filters work on iPhone at all?

Clip-on ND filters physically work, but the stock Camera app can't take advantage of them because it has no shutter speed control. You'd need a manual camera app on top of the filter, and the sensor still limits how long a single exposure can run. Frame stacking avoids the whole chain.

Can I really do a daytime long exposure without any filter?

Yes. Because stacking blends many normally-exposed frames, bright daylight doesn't overexpose the result. Pick a moving subject like water or traffic, set a duration, and the blur builds up in the live preview.

What does a frame stacking app cost compared to ND filters?

Lento is free to download with Smooth and Light Trail modes and a 5-second limit. Pro is $9.99/year or $29.99 lifetime, which is in the range of a single decent clip-on filter. It requires iPhone XR or newer on iOS 18.