Stacking ND filters

runnah · Mar 6, 2014

Gavjenks said:
Not terribly surprisingly, the welding glass is crap. * It is 16 stops exactly as I suspected, which is nice, but... * You cannot recover colors. It blocks everything but green so completely by comparison to green that the other two channels look like the most heinously jpeg artifacted garbage imaginable. You are pretty much obligated to dump both channels and make the green black and white. * Even then, it does some strange stuff with images across the frame. Dead center is usually normal looking. As you go further out to the left or right (it seems to be a linear pattern not circular), the image starts splitting up and down. Like, it looks as if you jostled the camera up or down halfway through the exposure and it ghosted half and half up and down. But only on the sides of the image. Esentially impossible to get a sharp image end to end. * Even though I glued it to an empty filter ring it still flares weirdly, so it's coming through the glass with the weird flares, not just from the edges. Gavin is sad. Back to resigning myself to shelling out way to much money for gray glass.

I admire your effort. Why did you go with green?

Gavjenks · Mar 6, 2014

The only ones I could find were green. When I search for black, I get a lot of hits from places like alibaba for 50,000 pieces minimum order, etc. Didn't see any black tinted ones sold single. If you know of one, lemme know, I'll try that.

480sparky · Mar 6, 2014

Shoot a gray card, and adjust in post.

Or, can you change your WB manually enough to correct for it?

Gavjenks · Mar 6, 2014

I am adjusting in post, with RAW. It's simply too green. There is literally almost no data in the red or blue channels by the time that the green channel is properly exposed. Doesn't matter how far you slide the sliders, the information simply is not there.

I am looking at other filters, and it looks like they sell "gold" ones that primarily reflect light rather than absorb, and then let more color information through amongst the light that passes. But I'm getting wildly different reports from the scant amount of information on welding forums about whether the resulting image is normal looking, blue tinted, amber tinted, green tinted, etc. Probably changes by brand, seems like a crap shoot.

I'd be happy to buy one more if I had more information about specific brand and what color tint and how much tint to expect, but I don't want to run around buying 7 different welding filters and ending up spending just as much as a proper 3.0 ND filter.

480sparky · Mar 6, 2014

I've got a Singh-Ray VND that, like many NDs, does have a color cast. But it's very minor and easy to correct in post. Same for my B+W 10-stop.

480sparky · Mar 6, 2014

Isn't there a way to boost just the red & blue channels in the camera?

Seems like I toyed with that at one time.

Gavjenks · Mar 6, 2014

You can't really change anything in camera at a sensor level. Custom white balance merely automatically applies conversions from RAW to jpeg if/when you save in jpeg. It's not any more powerful than post processing the RAW. And it's not like it's actually changing the sensitivity of some of the pixels versus others, etc.

Yes I can expose a lot longer so that red and blue get enough data, but then green is almost completely blown out. Still can't get realistic color.

I could do an "HDR" for green versus Red/Blue separately, but since the goal is usually to blur moving things, so the frames wouldn't have the same content and wouldn't line up. If everything is static, then there's no point in using a filter at all.

Gavjenks · Mar 6, 2014

Maybe I'm just doing it wrong... Well the color I'm not doing wrong. But the flares and the weird non-cirspness I might be.

I will try again soon in conditions with absolutely no wind and rock solid anchoring, and with some sort of lens hood for the filter. I may still have to do B&W only, but if I can at least get nice crisp clean B&W, then this is useful.

480sparky · Mar 6, 2014

Shooting Menu > White Balance > choose white balance setting > change tint along A/B and/or M/G axis.

Gavjenks · Mar 6, 2014

Sparky, that doesn't do anything. All the custom white balance does is take the exact same RAW file and essentially automatically slide the sliders for you based on those settings and save the resulting jpeg. It doesn't do anything more powerful or useful than sliding the sliders yourself in post, which is what I was doing.

The physical sensors themselves are the problem here, no amount of settings will fix it completely--the green sensors are getting saturated beyond their maximum range before the red and blue sensors start to register anything but the very bottom of the histogram. So you can't have both colors not-clipped at the same time.

Also, random Fun Fact I just tested: It goes from 16 stops in visible light to being a 23 stop filter on my infrared camera. Guess that kinda makes sense for a welding shield.