Stacking NDs loses sharpness?

[Cheng Han]

Hi there, if I’m shooting 16mm 500T and stacking one 85 filters and another ND to get a correct exposure. Am I running the risk of degrading the sharpness? (Assuming the lens, perform similar wide open and stop down) Would love to know your opinion on this!

[David Mullen ASC]

Two filters isn't too bad, it depends on the focal length -- it starts to affect longer lenses but I'm referring more to ones with a telephoto effect. I suggest a test.

[Cheng Han]

2 hours ago, David Mullen ASC said:

Two filters isn't too bad, it depends on the focal length -- it starts to affect longer lenses but I'm referring more to ones with a telephoto effect. I suggest a test.

Would you think I should to do the test on a digital camera instead of 16mm? Digital is sharper so might show the result bit clearer?

[David Mullen ASC]

Maybe — but you’d probably want to put your 16mm lenses on the digital camera and crop the sensor to the 16mm area. You’re testing the degree the stacked filters affect front-focus as well as sharpness on certain focal lengths. I’d be concerned that the results might be different if using a larger sensor and lenses made for that larger sensor in terms of the degree of shift in front-focus. I could be wrong though. Ultimately what matters is how it affects your 16mm photography so the best thing would be to test it in 16mm, shoot some line resolution charts, and zoom way in on the scan. But checking it in digital might also reveal something.