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This is a specialization of this question.

When there is the option to use glass or gel filter on certain wide-diameter lens: How would the optical performance of these two types of ND filters compare?

Consider:

  • The position of the filter is the opposite. Does front-vs-back position impact image quality?
  • Are glass filter inherently sharper than gel filters?
  • Which type of filter would deliver a more uniform effect?
  • In all likelihood there's far more difference between the best and worst glass filters, or the best and worst gel filters, than there is between the best glass filters and best gel filters. – Michael C Nov 9 '17 at 6:14
  • It may be unfounded but I am worried that gel filters are flexible. On the other hand a 30x30mm gel is much cheaper than a 95mm glass which is why I'm asking. – Itai Nov 9 '17 at 14:13
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Quality gel filters are optically flat, they tend to preform better than their glass counterparts. The thickness of the filter plays a part. Impose a filter and the focal length of the lens slightly shifts, it gets shorter. Nobody notices because we compose and focus with the filter mounted.

I don't think you will see any difference between glass or gel unless you are making billboard size images.

  • If the filter is completely flat and in front of the lens, then for an object at infinity or very far away, nothing happens except the introduction of more stray light. – Brandon Dube Nov 8 '17 at 21:24
  • @ Brandon Dube -- A filter stops some light energy and if colored, some frequencies. A filter thus passes some light energy. The filter has an index of refraction greater than air. Because of this, it slows light while it transits the filter. This act shortens the focal length ever so slightly. Same effect, under water camera or how a diver sees under water. The thinness of the filter makes this practically unnoticeable. Nevertheless a filter shortens focal length. – Alan Marcus Nov 8 '17 at 21:46
  • If you insert a window into a collimated beam, nothing happens. Snell's law. Angle of incidence = 0, QED angle of refraction will be 0. There is some phase shift due to the index break, but uniform phase is not relevant to imaging. – Brandon Dube Nov 8 '17 at 21:50
  • @ Brandon Dube -- The focus shift when a filter is added is approximately 1/3 of the thickness of the filter. Think about why in an underwater view, objects appear to be closer. – Alan Marcus Nov 8 '17 at 23:39
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    @ scottbb -- You are correct – a filter mounted after the lens shortens by approximately 1/3 the thickness of the filter. This happens because the image forming rays are converging. A filter mounted before the lens has little or no impact if the light rays are coming from infinity. However if the filter is before the lens and intercepts diverging rays, the object appears to be close. This induces a focus shift, we compensate by racking the lens forward. What happens is the back focus is elongated. – Alan Marcus Nov 9 '17 at 7:38
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A gel filter placed behind the lens will always reduce image quality, though the amount may not be noticeable. A perfectly flat filter placed at the front of the lens will only reduce image quality by introducing stray light, and even this can be highly mitigated.

Neither filter is preferable in terms of any sort of uniformity, and neither is inherently sharper. A high quality glass filter will likely tend to be better because they are easier to coat to reduce flare, and more rigid to prevent the surface from becoming warped, which will damage image quality.

  • The question specifically states a use case of large diameter lenses where either the gel filter or the glass filter is placed at or near the rear of the lens. It is not a comparison between a rear gel filter or a front mounted glass filter. (E.g. a Canon Super Telephoto EF 600mm f/4 IS II with provision for a 52mm drop-in filter and no filter threads.) – Michael C Nov 9 '17 at 6:15

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