I'm pretty sure that it is not recommended to point a digital camera at the sun for a picture because of possible damage to the sensor. Does anyone know if it is possible to take a picture of someone welding metal without causing damage (I need to get some shots like this). But this creates an extremely bright light that will damage your eyes after a few seconds. Have any of you ever tried it?
I think you are going to need more than "a ND filter or two". Blast shields for welders are the equivalent of I believe about 15 stops of ND. You can look at the sun safely through a blast shield.
With that said, you can buy ND filters which are that strong. They are designed for photographing the sun, but should work in this situation as well. Being so specialized, I bet they are spendy. I also bet you could get some cool effects as the picture would likely turn out with a very black background and only the reflective glow of the arc lighting the subject.
If you're willing to experiment (at your own risk of course), close the aperture all the way, set shutter to fastest possible (1/8000 for d70?), and step away at least 10 meters. And yes, put some ND filters too. Then, if you haven't burned your sensor yet, keep coming closer and closer until you're happy.
Check if your camera is covered before embarking on this endeavor.
I've had a few ideas about using shots of someone welding for the challenges here (Faceless being an example). This leads me to suggest composing your shot such that the actual arc is out of sight- either obscured by the workpiece for instance or simply outside the frame. That way, you could use the bright light to pick out the smoke and fumes and bring out the feeling of the work being done (and illuminate the welder himself if you so desire) without the ridiculously high-contrast situation that would exist if you included the arc.
