Quote Originally Posted by Old-N-Feeble View Post
If you decide to try filters on the rear of the lens then consider gels. Those won't cause focus shift nor image degradation.
As much as thicker glass or acrylics.. Anything behind the rear element, regardless of its thickness, will cause a focus shift of 1/3 rd the thickness of whatever it is. But a scratch, fingerprint, dust, etc on a gel behind the lens is just as detrimental as if they were on glass behind the lens. So is lack of filter flatness with a gel.