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What is the most common absorbing material used for professional quality ND filters, and can that material be etched away with acids or bases, while leaving the substrate (BK-7?) intact?

Basically, I am after an apodizing filter, similar to this one offered by Thorlabs:

https://www.thorlabs.com/thorproduct.cfm?partnumber=NDY10B

The issue is that the center OD I am after is 0.3, not 1. Based on experience with other items, if I asked TL for a custom one, they would ask for a minimum order of 5, at 2x the price of the stock item, so I would be looking at thousands.

So, how crazy is it to get the stock filter and to etch away the absorbing material? I can monitor the progress with a power meter, and I can get whatever acid or base needed. Perhaps, I could get a uniform NDF made with the same material (if they can disclose it), experiment with that, and then have a go at the apodizing one.

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    Hi and welcome to Photo.SE. I think the question is on the edge of what's inside the scope of this site. However, perhaps your final goal is fully in scope and there might be other ways to solve the problem. Would you mind sharing what your final intention or desired result is? Why do you need a center OD of 0.3? – Saaru Lindestøkke Sep 17 at 15:11
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    I think this will probably vary significantly based on the actual construction of the filter in question. Is it a film/coating applied to one or both sides of the glass, or is it sandwiched between two pieces of glass, or maybe it's even an actual tint that's integral to the glass used... Or some combination of the above... Also, uniform etching to preserve an optically flat, minimally reflecting surface may be more challenging than you might think... – twalberg Sep 17 at 15:27
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    Photographers DIY things for artistic purpose. Like smear Vaseline on a UV filter. Any accomplishment like what you’re asking would be so unscientific it’d be useless for you - though a Jackson Pollock inspired acid wash of a filter may yield some cool artistic results – Hueco Sep 17 at 16:50
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    I'm voting to close this question as off-topic because nothing in consumer or professional photography supports scientific level exactness. Also - use case doesn’t pertain to the making of a photo and is unlikely to be relevant to the photo community at large. – Hueco Sep 17 at 16:53
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    On the other hand, it is entirely conceivable that someone could desire to make their own DIY graduated ND filter (with the lowest density in the center and the highest in the corners) to be an apodizing filter, or another DIY device to get the same effect, for artistic purposes. Apodization was a fad in the late 1970s-80s, and seems to be making a comeback in the late 2010s. – Michael C Sep 17 at 20:58
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I don't have a good answer for your specific question, but my desire for precise ND graduated filters has led me to think about solutions somewhat similar to your problem.

To begin with, it's instructive to understand the current state of the art for how graduated ND filters are created. As an example, here's a video from Lee Filters about how their ND grads are made. Personally, I'm appalled at how imprecise and error-prone their filters are created. The technician is literally hand-dipping the plastic filter into a hot dye bath, where the duration of submersion determines the ND factor, and the rate of slow removal from the bath determines the hard/soft transition region (while the technician talks to the interviewer, looks over her shoulder and causes the filter to dip up and down). This is something that really should be automated by a robot.

Furthermore, you can understand how the demonstrated dip technique cannot easily create filters such as reverse-ND grad strip filters, or apodization filters.


One technique that I think could create precise graduation regions of arbitrary shape would be to control a vapor deposition process (such as chemical vapor deposition (CVD) or physical vapor deposition (PVD)) by some means, such as electrostatic control, to deposit the vaporized ND material in precise locations and amounts (i.e., density). Essentially, it would be somewhat analogous to laser printing the gradient of ND material across the substrate.


Speaking directly to your idea of removing material from an ND filter to create an apodization filter, I'd be inclined to try mechanical removal rather than chemical. I'd experiment with different semispherical shapes/radii of soft buffing wheels mounted to a mill or drill press, different buffing rotational speeds and durations, and possibly different buffing/rouge compounds, to remove the deposited material from a ND filter. Almost certainly, you'd want to use glass ND filters rather than optical resin filters. The glass is much harder (i.e., resistant to scratching on the Moh scale) than resin, so if you use soft enough buffing wheels, you shouldn't get circular micro-scratches (on the substrate, at least) that act almost like a Fresnel lens. You'd need to be careful about scratch patterns in the ND material, of course. Again, I'd emphasize soft buffing wheels, and slow drill/mill speeds, to determine the ND material remove rate and pressures.

Additionally, to help minimize the neat pattern of strictly circular buffing, you could use something like a random-orbit sander motion to introduce more-or-less random motion in the buffing disc.

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