I have mentioned before that I have a 10 stop ND circular filter but I have not used it because of the difficulty of focussing. The problem is that you only see a black image when you look through the view finder - because of the filter. and with my camera, even for landscapes, where you might be able to focus at infinity, this isn't at the 'end of travel'.
So the danger is that in screwing the round filter onto the front of the lens you ruin sharpness of focus. I have tried several times to find a solution to this and failed
I have just downloaded and read:
http://www.digitalmonochromeforum.c...rence-guide-to-long-exposure-photography.354/
which Ian posted on the Members part of the site.
AND THIS GIVES A SOLUTION

You put your camera on HIGHEST ISO solely for the purposes of composition/focussing. And then (this is a direct quote from the book):
"Here’s the simple process for getting tack sharp long exposure images:
1. Switch your lens to Manual Focus
2. Turn on Live View and while mounted to a tripod
3. Select the Magnify button to toggle between 5x or 10x magnification.
4. With the focusing ring on your lens, dial in the sharpness so the edge of
your subject is as sharp as it can be."
And don't forget to switch back to desired ISO for taking the actual image
I have tested this and it works
So the danger is that in screwing the round filter onto the front of the lens you ruin sharpness of focus. I have tried several times to find a solution to this and failed
I have just downloaded and read:
http://www.digitalmonochromeforum.c...rence-guide-to-long-exposure-photography.354/
which Ian posted on the Members part of the site.
AND THIS GIVES A SOLUTION
You put your camera on HIGHEST ISO solely for the purposes of composition/focussing. And then (this is a direct quote from the book):
"Here’s the simple process for getting tack sharp long exposure images:
1. Switch your lens to Manual Focus
2. Turn on Live View and while mounted to a tripod
3. Select the Magnify button to toggle between 5x or 10x magnification.
4. With the focusing ring on your lens, dial in the sharpness so the edge of
your subject is as sharp as it can be."
And don't forget to switch back to desired ISO for taking the actual image
I have tested this and it works