What is variable aperture? A feature as old as film finally comes to smartphones

galaxy s9 Plus hands-on review back full

Julian Chokkattu/Digital Trends

Samsung calls the camera on the new Galaxy S9 and Galaxy S9 Plus “the revolutionary camera that adapts like a human eye” because of a new feature called dual aperture, a form of variable aperture. But what exactly is a variable aperture, and why is it revolutionary?

The variable aperture is “revolutionary” in that the feature has never been included on a smartphone camera before — but it’s actually an option that has been around since early film photography when variable aperture was added to a camera lens around the mid-1850s.

Variable aperture simply means that the Samsung Galaxy S9 and S9 Plus can actually change the aperture of the lens. Aperture is a term that refers to how large the opening in the lens is — a wide-open aperture will let in more light, while a narrower aperture will let in less light. In dark scenes, a wide aperture will create a brighter image. Because of how small the lens is, smartphones use a fixed aperture — you can’t adjust the aperture like you could in a DSLR or mirrorless camera.

What the Samsung Galaxy S9 and S9 Plus do that previous smartphones did not is allow users to change the aperture. The feature isn’t quite the same as the aperture adjustments on a film lens or a DSLR. While a dedicated camera offers a range of different aperture settings, the S9 offers just two settings: an f/1.5 and an f/2.4, the f/1.5 being the brightest or widest of the two. In comparison, the earlier Samsung S8 uses a fixed f/1.7 aperture.

While the smartphone can’t go from f/1.8 to f/22 and a few dozen settings in between like a DSLR lens might, the dual aperture lens allows the smartphone to switch between the two, with the widest used for a low light mode and the narrower option for the regular camera mode. (Along with automatically switching between the two based on how bright the scene is, the two apertures can also be switched inside a manual mode).

The dual aperture is available from the same lens thanks to a moving piece called an iris that adjusts the size of the lens opening, which means you don’t have to get the pricier S9 Plus and the dual lenses to get the variable aperture. On the S9 Plus, the feature is only available from the wider rear-facing lens, not the second lens with zoom. The feature may not be revolutionary for photography, but fitting moving aperture blades inside a tiny smartphone is no small technical feat.

So why not just use a brighter f/1.5 that’s wider than the camera on the older version? Aperture changes the amount of light that’s let into the photograph, but it also changes how soft or out-of-focus the background is. The effect is less drastic on smartphones because of their smaller sensor sizes, which is why dual camera smartphones use a fake computed bokeh, but the f/2.4 aperture in general will shoot sharper images than the low light mode f/1.5.

Along with the variable aperture, the S9 Plus also has a bokeh mode created from the dual-lens view that will adjust the shape of the out-of-focus lights. Like the variable aperture, this is a trick done before by changing the shape of the aperture using a special lens cap on a DSLR, though in Samsung’s case, the effect is from computational photography, not actually changing the shape of the aperture opening. The bokeh filters on the S9 Plus rear camera will change the out of focus lights in the background from circles to starbursts, snowflakes, butterflies, or a handful of other shapes.

Variable aperture has been around since photography’s early days — but for a tiny smartphone lens, the feature is a first.

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