I took these a couple of weeks ago but only just got round to posting them, sorry about that
It's a comparison to show the effect of the Astronomik CLS filter. The images are just 15s at f2.8 ISO 800. I picked the most light polluted part of my sky, just above the roof line These images were taken 10 minutes apart.
You can see that the filter gives a much darker image with much less red. This is just as well since red is blown out in the non-filtered image. The filtered image has more stars visible and allows for longer exposures before the histogram reaches the middle of the graph. The histogram peak is narrower too, indicating less gradient. The gradient is still there though, so you still have to do some processing to flatten the sky intensity.
The clip filter itself is in a round metal frame which is sprung and fits into a recess in front of the camera's mirror when you have the lens off. It is quite a tight fit which means you have to apply some force get it in properly, and you might need to (carefully) use a tool to lever it out afterwards. Hopefully this will get easier with time.
The clip filter fits most Canon crop sensor cameras. Their website doesn't mention the 7D... I would be interested to know if it fits. It doesn't work with full frame cameras and it doesn't work with EF-S lenses. For me, it means I can't use it for very wide shots... perhaps an excuse to look for another lens.
Of course I have used the filter with a darker part of the sky and the longest exposure I have attempted is 3 minutes. The histogram reached almost half way, so that is probably the limit for my back garden. One day I'll process those images and post the result