Clear skies ahead: Reduce the effect of light pollution with Lonely Speck’s PureNight filter

by Jeremy Gray

posted Friday, December 16, 2016 at 3:00 PM EDT

Light pollution is a serious problem for night sky photography. Lonely Speck, purveyors of both excellent night sky photography tutorials and products, have a new premium glass filter to help you capture better night sky images in camera, even in light-polluted areas. This special filter, which is available for preorder now, is called the PureNight and it specifically targets the transmission of light from sodium vapor lamps, a very common source of light pollution.

By reducing the transmission of light from sodium vapor lamps, resulting images shot through the PureNight filter will have colors truer to life and looking much better, as you can see in the sample shots below.

Without PureNight filter. Sony A7S, 18mm at f/2.8, 25s, ISO 3200, Daylight WB.

With PureNight filter. Sony a7S, 18mm f/2.8, 25s, ISO 6400, Daylight WB.

You may be wondering how much the PureNight filter affects brightness. Lonely Speck had to use increase exposure by +0.7 EV with their prototype filter, but they expect the final version to display a roughly -0.3 to -1EV reduction in brightness. You can download full-size RAW files of their sample images here. (NOTE: Links on this page start automatic downloads).

The PureNight works due to its didymium glass construction. Didymium “greatly reduces transmission of yellow and orange light in the 575nm to 600nm wavelengths, the same wavelength of light that is emitted by high pressure sodium vapor lamps,” which are commonly used for street and industrial lights.

Wavelength chart. The PureNight filters light in the 575nm and 600nm wavelengths, commonly emitted by street lights and industrial lights.

PureNight is polished and features anti-reflective coating to reduce ghosting and internal reflections and comes in a universal square shape in two sizes, 85mm and 100mm. The filter requires a special filter holder, although it is compatible to fit Cokin P (85mm), Lee 100mm, and Formatt Hi-Tech filter holders (85mm and 100mm are available). The PureNight is crowdfunded and will have a limited run, so if you want one, head here and order soon. The filters, if funded, are expected to ship in March. The 85mm filter costs around US$220 and the 100mm filter around $240. Every order includes a protective case in addition to the filter. To reiterate, filter holders are sold separately.