dtoh posted a link about this a week or so ago, and I decided to have a crack at it after ordering the bits and pieces. Taken today at Lane Cove National Park.
Shade 8 welding glass lens.
Generic cokin p-series holder and 77mm adaptor ring.
30D, 17-55mm lens, and cable release.
At first I tried to fit the welding glass into the p-series holder. While the glass was the right dimensions length/width for the holder unfortunately it was just a bit too thick (by a mm or so) and I didn't have the patience to sand it down. The edges were pretty sharp too - as my poor fingers discovered - I'm still sporting bandages.
I'd read about light leak issues with the holder as well, so abandoned that idea and set about fitting the glass directly to the adaptor ring. Used blu-tak (or yellow-tak, in my case) - rolled into a long thin piece, and stuck on the filter edge.
Tip - don't go too close to the inner edge as when you press it down onto the glass it squashes down and if you have it too close it may intrude into the field of view. The bonus of directly attaching like this is you avoid light leaks around the glass.
Here's the setup (sorry about the poor quality - phone pic):
and from the other side:
(you can see I applied some duct tape to save my digits any more impromptu surgery).
I also ordered shade 10 and shade 12 lenses, but settled on the 8 as it was the first to hand. I've read that a shade 8 is equivalent to about a 9 stop ND, but I'm not really sure how to go about measuring it - any thoughts?
Focusing through the glass didn't work, so I auto-focused without the welding glass on it, and then switched to MF, then re-attaching the welding lens filter.
The lens has a strong green cast to it (this is autoWB, out of the camera):
I'm shooting in RAW, so I could correct it in PP, but it was easier to use this shot as a custom WB. That got rid of most of the green.
I set aperture at f/18, ISO 100, and turned on long exposure noise reduction. Figuring the exposure time took a bit of trial and error - I started at 30 seconds and worked my way up, checking the histogram on each subsequent exposure and looking for blowouts. I'd read that you also need to cover the viewfinder to stop light leaking through that, so I used a hanky. It fell off on a couple of exposures but that didn't seem to affect things much.
The day was fairly still so there wasn't the cloud movement that I hoped for, but I think it's worked, and definitely worth pursuing.
The shot that turned out best was a 105 second exposure. The out of camera shot still had a slight green tinge to it, so I corrected that, and a few other minor tweaks.
I'm pretty pleased with how it's turned out and had a whole lot of fun to boot, particularly given that the single filter cost, including (redundant) holder and adaptor ring was around $7.
What do you think?
I can see myself having some fun with this