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I have the following issue with my ND filters 100x100 square (K&F concept), I am using them with my Sony A7 and with several manual lenses: when I am using the ND1000 alone I do not have any issue, it doesn't matter which lens I am using

When I am using two filter the ND1000+ND64 I start to get issues with 2 lenses, the Canon FD 20mm f2.4 and the Canon FD 100-300 f5.6; all the pictures are partly washed out. I tried different angles, different time during the day but more or less I am getting always the same result, washed out pictures.

When I am using the Flektogon 35mm or Helios 58mm or Sony 50mm I don't have any issue with the ND1000+ND64. I tried to realized a hood to cover the filter (made by paper), it helped a bit but not really a noticeable difference. I don't know how to solve this issue.

I do know that ND filter are not really great with wide angle lenses and that can explain the issue with the 20mm but what about my 100-300 mm (btw the issue is more visible at 300mm rather than 100mm)/

Any idea? potential solutions?

This is one example of a washed out pic with the 20mm

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    Looks like there is light leaking between the filters. The combination you are using blocks very much light and any minimal leak could bleach the image. I could also think of reflections or flaring. Is the filter surface exposed to direct sunlight? Try to use something as a shade to prevent direct light hitting the filter.
    – Arjihad
    Dec 29 '20 at 0:22
  • @Arjihad Your comment is most of an answer. Please put your answers in the answers section, even if they're short
    – scottbb
    Dec 29 '20 at 0:46
  • Is this image a crop? I ask because the shadow on the left and upper-left corner look like a crop from an image that is hard-vignetted. If it is a crop, can you post the uncropped image?
    – scottbb
    Dec 29 '20 at 0:53
  • Also, which filter is mounted first (i.e., closest to the camera)? Do either of the filters have a foam gasket mounted on their backside?
    – scottbb
    Dec 29 '20 at 0:56
  • This is one of the downsides of using gel filters. Some holders probably are better for this than others. Also, stacking filters is something to be avoided when possible. Each filter adds 2 more optical surfaces Jan 1 at 13:54
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I believe this is due to a light leak entering between the two ND filters, and the one in front (farthest from the camera body) does not have a light-blocking foam gasket.

Light coming in through the top or bottom of your square filter stack will be able to bounce around and reflect off the parallel surfaces of the two filters. If the filters do not have substantial anti-reflective coatings on the surfaces facing each other, the light leak from the ends will easily cause the kind of glare you are seeing.

The solution is to shield your filters from light leaks. Use a towel, thick shirt, or other material to block light from entering between the filters. Obviously, the more darker and opaque the material you use, the better. Woven fabrics like towels and shirts will help, but if you hold them up to the sun and can see through them, that means light can get through. Felt is better, more substantial coverings like pleather, leather, or multiple layers, are best.

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  • Thanks a lot, i will try today your suggestion and will post the result, i was thinking also to buy some rubber gasket to try to see if it helps. Dec 29 '20 at 12:56
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    Gaffers tape is a better choice than a towel or shirt for this use. Dec 29 '20 at 22:26
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This looks like a light leak/alignment issue. Is one of the filters warped? Also, wide angle lenses have such a large dof that they see the filter on the lens. ny abnormality will be magnified w/ a wide angle lens.

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