How Clear Sky Filters Work
Top 5 Reasons to Own a Clear Sky Filter
- Great for Astro and Milky Way Photography
- Reduces the orange/yellow haze from night sky caused by city lights
- Capture more accurate color at time of exposure
- Reduce editing time with a "cleaner" RAW File
- Filter also enhances sunsets and fall colors without effecting cool colors
How It works
The filter targets specific wavelengths of light emitted by common Sodium and Mercury-vapor street lamps and blocks a portion of the orange/yellow glow as it passes through the filter. The effect can be dramatic or subtle depending on how strong the light source is and how much atmospheric haze is in the air.
As the light passes through the filter it selectively blocks specific areas of the light spectrum that most commonly cause that orange/yellow color spike. The light transmission chart below illustrates what parts of the color spectrum that are blocked by the filter and what colors pass through.
Field of View - How to Determine the Right Size Filter:
Cokin square filters are manufactured in three sizes. The size of filter and corresponding adapters you decide on will depend on a number of factors. The most important thing to determine is your field of view. You want to have a filter that will ultimately cover the range of your widest focal length:
If your 16mm lens is wider than what the filter can cover, then you will see vignetting and/or the filter holder in the final image.
Here are some other features to look for in deciding which ND filter is best for you:
- Color Shift: Should be non-existent or at minimum predictable results
- Vignetting: A quality filter does not amplify vignetting from a lens.
- Durability: Filter is resistant to inevitable dings and knocks
- Simplicity: The system is easy and quick to set up and use on location and/or during a shoot.
It is really important to note that this filter is not adding blue or changing the color of the scene. It is literally stopping specific areas of the color spectrum from passing through the filter. The camera never sees that light or color.
The end result is a "cleaner" RAW file without the orange/yellow glow polluting your image. And, it allows you to color correct and enhance colors without needing to compensate for the "pollution".
NOTE: The filter has a base filter factor (light loss) of 1/2 stop, but you may see a 1-2 stop difference in exposure depending on how much of your scene is lit by Sodium or Mercury Vapor lights.
I'll Just Fix it in Photoshop
You can to a point but it's not the same. Digital color correction normally works on 2 simple linear scales; Color Temperature and Tint. Color Temperature controls yellow to blue, and Tint controls magenta to green. If you adjust for the yellow/orange color in your image you will effectively add blue to the entire scene changing other colors in the scene as well. A skilled Lightroom or Photoshop user can use more advanced tools to essentially achieve what the filter will do at the time of capture, BUT that takes a lot more time and skill.
Not Just for Light Pollution
The filter can also be used during the daylight hours to enhance red, orange and yellow colors. It was originally created for film photographers to "intensify" warm fall colors on overcast and cloudy days. The filter is still used today by digital and film photographers to target those specific colors for enhanced saturation while leaving the blue and green colors alone. Add a circular polarizer to this and you will have the ultimate filter combo to capture the best possible fall colors especially on those rainy, overcast fall days.
These examples were created using the Nuances Graduated Neutral Density Filters. The Nuances GND lineup is manufactured with hardened, tempered optical glass. The G2 series is manufactured with optical resin.