Do you need a filter for astrophotography?
For wide-field night sky photography with a normal lens, you usually do not need one. A dark sky, a fast lens and the right Milky Way camera settings matter more. Filters become more useful for specific goals: reducing the effect of light pollution, isolating nebula emissions, softening stars, or adjusting a camera that has been modified.
This article describes common filter types. Results depend on your sky, camera and target, so treat the descriptions as general guidance.
Light pollution filters
Light pollution filters block parts of the spectrum emitted by common artificial lights, such as some sodium and mercury lamps. The aim is to improve contrast against a bright sky.
- They are usually described as broadband filters.
- Their effect depends on the local lighting. Modern LED street lighting covers a wider spectrum, and some filters help less against it.
- They can shift color balance, often adding a color cast that needs correcting in editing.
- They also reduce the light reaching the sensor slightly, so exposures may need to be a little longer.
Narrowband filters
Narrowband filters pass only a few narrow ranges of wavelengths, such as hydrogen-alpha. They target emission nebulae, which glow at those wavelengths. They are very effective at cutting skyglow, but they also block most starlight, so they suit nebula imaging rather than Milky Way landscapes. Because they pass little light, they generally need long total exposure times and a tracking mount. With an unmodified DSLR, whose sensor already blocks much of the deep red light, the benefit is often limited. Dual-band versions are popular for cameras with color sensors.
UV/IR cut filters
Sensors respond to ultraviolet and infrared light that human eyes do not see. A UV/IR cut filter blocks both, which can improve star color and sharpness. Standard DSLRs already include a similar filter in front of the sensor, but cameras that have been modified for astrophotography often have the internal filter removed or replaced, and an external UV/IR cut may then be used for general shooting. For deliberate infrared work, see infrared photography at night.
Clip-in and drop-in filters
On many DSLRs, a filter can be placed inside the camera body, behind the lens mount. This is useful for lenses with a bulging front element that cannot take a screw-on filter, and for telescopes. These are available for several popular camera mounts, but you must choose the correct size for your model. Also check that the filter does not interfere with the mirror, and handle it carefully since it sits near the sensor.
Diffusion and soft-focus filters
Diffusion filters, sometimes called star softeners or mist filters, create a subtle glow around bright stars so constellations stand out. Opinions differ on the aesthetic, and the effect is sometimes done in editing instead. It is a creative choice rather than a technical need.
Neutral density and other filters
Neutral density filters are not used for typical night sky photography, because the sky is already dark. They are more useful for long exposure night techniques at dusk. Standard UV or clear protective filters are also not necessary for the night sky and can cause reflections or flare around bright stars. Some photographers remove them for night shooting.
How to choose
Ask these questions:
- What will you photograph? Wide-field Milky Way, deep sky objects or the moon?
- How polluted is your sky? A very dark site gains little from a filter.
- Does it fit your gear? Check thread size, clip-in compatibility or mount type.
- Will you track the sky? Narrowband imaging in particular benefits from tracking.
- Are you willing to adjust color afterward? Many filters add a color cast.
Filter use and star shape
Filters can add reflections, halos or flare around bright stars, especially cheap or poorly coated ones. If stars look odd in the corners, remember that lens aberrations such as coma can also be to blame. Test with and without the filter and compare.
Alternatives to buying a filter
- Move to a darker site.
- Shoot when the moon is down.
- Stack many frames in software to reduce noise.
- Use editing tools that reduce light pollution gradients.
For more on getting a good night exposure without extra gear, see the best settings for night photography.