How to avoid light leakage when using neutral density filters

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When you are shooting long exposure images using neutral density filters, a very common issue is the appearance of light leakage in your images. The longer the exposure, the more likely you are to see light leakage if light is not evenly and uniformly being distributed across the camera sensor. This is due to the amount of time brighter, unfiltered light is allowed to hit the camera sensor. By holding your shutter open for longer, you are allowing the captured light to hit the sensor for a longer period of time. If some light is passing through a neutral density filter, and light is also entering in via a path that is not being filtered correctly, that brighter light will cause parts of your image to be overexposed, have reduced contrast, show optical aberrations like reduced sharpness or the introduction of color banding.

Light leakage will cause a very noticeable degradation of image quality. But in most cases it can be prevented by ensuring square neutral density filters are being inserted correctly into their holder, and that any other possible light entry points such as the optical viewfinder on DSLR cameras are being covered. Let’s take a look at what causes light leakage in photography and how to prevent light leakage from occurring when using neutral density filters.

What causes light leakage in photography?

Light leakage is caused when light enters the camera via any opening other than through front element of the lens. Even when the only light that enters is via this path, if the light is not uniformly and evenly distributed across the entire sensor light leakage can occur. When you are using square neutral density filters, it is extremely important to ensure your filters are placed in the correct order in the holder, with filters such as the 6 stop ND or 10 stop ND placed as close to the lens as possible in the available slots before any others such as graduated ND filters.



Our NiSi neutral density filters have a foam gasket on one side of them. This provides a light seal between the filter and the holder, ensuring no light can enter through this gap. The longer the exposure, the more important it is to ensure the filters themselves are providing this light seal. Any uneven light entering the camera through any gaps will be much brighter than the light filtering through the front of the neutral density filter, therefore creating an exposure that is uneven across the entire frame. You will notice brighter, possibly overexposed parts of your image as well as uneven color banding and a lack of contrast.

One possible cause of light leakage is due to a damaged or worn foam gasket. This is most often caused by filters being inserted incorrectly into the filter holder, but also just from age or overexposure to direct sunlight. Replacement foam gaskets are available to be purchased. Here is more information and a video showing how to replace foam gaskets on ND filters.

Neutral density filters that have been damaged, scratched or have had any of the outside layers removed may cause light leakage due to uneven light distribution across the sensor. In this case, it is recommended to replace the filter.

Light coming in through the viewfinder on SLR cameras is a common cause of light leakage which results in image quality issues. Covering the eye piece with either your finger or a dedicated eye piece cover will prevent this.

Unwanted light can also pass through the optical viewfinders of DSLR cameras. The viewfinder provides a direct path for light to enter straight towards the camera sensor. Even the smallest bit of light entering the camera via the optical viewfinder can severely degrade your image quality. If there is a bright source of light behind you such the sun itself during daytime shooting, bright street lights, passing car headlights or reflected light from buildings at night, you will need to cover the viewfinder to protect the camera from light leakage.
 
When you are using graduated neutral density filters or filters such as the Natural Night Filter, if they are not placed as close to the lens as possible (after and ND filters that have the foam gasket) in your available filter holder slots, light reflecting from the back of these filters way also cause reflections from the back of the filter and uneven light to enter the camera.
 
A less likely but still possible scenario is light entering the camera via any faulty lens connecter seals, or through any gaps in the lens body or even camera body itself. While highly unlikely, older or damaged lenses and damage to the camera body could cause light leaks if any of the manufacturer seals are not in place correctly.

By ensuring the ND filters you are using are placed in the correct order, the rubber seals should prevent any light from entering behind the filters between the holder.

How do I prevent light leakage from occurring in my images?

To prevent light leakage from occurring in your images, you need to ensure light can only enter through the very front element of the lens with uniform distribution across the entire exposure. Any other sources of light entering the camera will cause an uneven exposure to occur, impacting either on the entire image, or localised areas of the image depending on how bad the light leakage is.
 
Below are some of the most common ways to prevent light leakage from occurring in your images –
 
– Ensure your neutral density filters are in the first available slot in your filters holder and the rubber or foam gasket is sealed against the holder.
– If the foam gasket on the back of the filter is damaged you can purchase replacements here. Check out this article and video for more information on how to replace them.
– Ensure graduated neutral density filters and the Natural Night Filter are placed as close to the lens as possible (and next in order if using other neutral density filters) within your available filter holder slots.
– Cover the viewfinder eyepiece. Some cameras such as recent full frame Nikon bodies come with an inbuilt eyepiece cover. Others such as Canon may include an eyepiece cover on the strap. Otherwise use a piece of black electrical tape to cover the eyepiece.
– If there is a bright source of light either directly above or behind the camera, using a cloth of some sort to cover the filter holder and lens to ensure light cannot enter from these directions.
– While not likely, it is possible that light can enter from around the seals of the lens itself. In this case you could use a solution like cutting a hole in the bottom of an old lens bag, and sliding it onto the lens when it is connected to the camera. This could help prevent light entering in the case of faulty or old lens seals.
– Again while far less likely than the above scenarios, light can potentially enter through any broken or damaged seals within the camera body itself. In this case use a cloth to cover the entire camera body while ensuring the front of the lens is uncovered.

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