ND Filter Calibration
Blaven and Clach Glas

A while ago I managed to get a Lee Big Stopper and Little Stopper and I’ve been mostly messing around with the 10 stop big stopper. However I noticed it wasn’t quite blocking 10 stops of light, rather it was going a little bit further and the images were slightly underexposed. This is normal as each filter is handmade and varies in stopping power. How to find out how much light it stops? Roll out the Chezzas!

Calibrating the filter is pretty easy. I just put the camera on a tripod, metered f11 @ 18sec and used the remote release to take a family portrait of the Chezzas. I then used the excellent iPhone app from Lee, chose the 10 Stops option and it gave me an exposure time of 2mins.

As expected, this resulted in a little underexposure of Queenie and the boys. So I reduced exposure by a 13rd of a stop to get 2mins 40secs and that resulted in a slightly ’lighter’ image. It wasn’t overexposed, no highlights were blown and it was a good representation of what was in front of the camera. The histogram was pushed a little to the right but not overly so.

The histogram is key here, especially the combined one. I don’t find the channel histograms that useful for working with the filter as it has a blue cast so I prefer the overall exposure to guide me when calibrating it. It all boils down to capturing the first, non filtered image and tweaking the long exposure time until the two histograms are as near each other as you can get. You need fairly constant lighting to do this so a sunny day with clouds coming and going isn’t the one to try calibration. Anyway, with weather like that I’d be up a hill!

The final result of keeping Queenie and the boys posing is shown below. I incresed the non filtered exposure in Lightroom by 0.45 and WB to 5500K. This more or less matched the filtered image with the WB increased to 8200K to get rid of the slight blue cast from the filter.

So now I’m happy to increase the exposure for the Big Stopper by a 13rd of a stop. Combined with RAW it should give me the best possible data to work with.

I repeated the process with the Little Stopper (6 stops reduction) and it was spot on 6 stops.

Below are a couple of images I created with the Big Stopper on the lower slopes of Blaven on the Isle of Skye a while ago, before I calibrated it. Challenging conditions of gusting winds, lowering banks of black clouds scudding down the coire and unspeakably monstrous hoardes of midges! Ah the joys of long exposure photography on the Atlantic seaboard!