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Tim Parkin
Amateur Photographer who plays with big cameras and film when in between digital photographs.
The issue of sharpness and filters comes up regularly in forums and the general consensus seems to be that any filter affects sharpness a bit and poorer glass filters can be worse. If you’re using resin filters you might as well just smear your lenses with vaseline though!
As usual, the armchair experts can’t be completely right as there are enough people out there with 50mp+ cameras that are still using Lee grads and I don't hear them complaining. The only way to know for sure is to test things though. And being as I love testing things so much** I immediately lined up my vast array of ND graduated filters and a resolution target***
Sharpness
First things first though, how do I go about testing for sharpness? I figured I wanted to do some real world testing with various lenses and I ended up using my Canon 100-400mm plus my Sigma Art 24mm and 50mm lenses on an A7R3 using various apertures.
But first I wanted to try something that would really show up any differences first. I used my Canon 100-400mm lens at 400mm, placed each filter at a 30 degree angle to the front of the lens and took a photograph through the clear part of each ND. I also used pixelshift, Sony’s ability to microshift the sensor to create a sharper image with no moire. (the 30 degree test is contrived in order to make light rays pass through the glass or resin at an angle other than 90 degrees).
This demonstrated two things very clearly. Every glass filter passed the test with flying colours (which is why I don't list them all). The resin filters were all soft to some degree, but the Lee was by far the best of these, closely followed by ProGrey and 84.5 and then the SRB. The Cokin and Zomei were dreadful, so bad I had to repeat checking things a few times. See the results of these above. Now I’m not sure this matches any real-world shooting scenarios but I wanted an indicative test to see if differences could be seen. So now we know what sort of thing we’re looking for!