Spectral Testing of various filters

[Alan Smallbone]

Hi all,

Been meaning to post this, was originally going to write an article but never got around to it. I did some spectral testing of some filters that I own. I tested some screw on ND filters from Breakthrough Photography, some Lee 100mm filters, a grad ND at 0.3 and 0.9 gradient. A variable super dark ND from Syrp, supposed to be 5-10stops. Some 10 stop ND filters from Nisi and Lee, polarizer filters for the 100mm system from Nisi and Lee, and a Light Pollution filter from Nisi.

Testing was done using an HP Spectrophotometer, the filters where inserted into the device and it produces the transmittance level at various light frequencies. My reason for testing was to see if the ND filters had color casts and also just check the Light Pollution filters, so I tested most of what I had. The data was then plotted in a spreadsheet.

The LEE filters at least the gradient filters had very small color casts in the visible spectrum. Also the interesting thing is the just the clear part of the filter still blocked about 0.3 stops of light. Here is the Lee gradient result:
Lee gradient by Alan Smallbone, on Flickr

The 10 stop ND filters show quite a difference. The Nisi had no color casts in the visible spectrum while my older Lee Big Stopper showed the color casts that it has in several areas. Result here:
10-stop-ND by Alan Smallbone, on Flickr

The Breakthrough Photography ND filters were also fairly neutral across the spectrum.
ND filter by Alan Smallbone, on Flickr

The Syrp is shown here and three different positions since it is a variable filter. The first was supposed to be 5 stops and measured out at about 3 stops but the 7 and 10 settings were ok.
Syrp-variable-nd by Alan Smallbone, on Flickr

As for Polarizers I tested three of them. One was the Nisi which comes with 100mm setup and sits behind the filters and has a small wheel you can turn to rotate the filter. The Lee polarizer that fits on the end of the filter holder with an attachement and is rotated manually, and then the Lee 100mm square polarizer. The Lee and the Nisi for the filter holders are very neutral while the Lee 100mm square polarizer is not so neutral in color.
Polarizer by Alan Smallbone, on Flickr

The Light Pollution filter from Nisi actually works pretty well, it dims some of the green sky glow and notches out the sodium and yellow light spectrum and I have found makes a difference for night time photography. Here is the spectrum:
LiPo-filter by Alan Smallbone, on Flickr

I had tested another Light Pollution filter but it is not readily available so I left it out to not confuse the graph. Hope someone finds this useful. It was educational for me.

Alan

[NancyP]

How useful are the light pollution filters to actual astro-landscape photographers, as opposed to deep-sky imagers? I imagine that you could composite a non-foreground-lit LP filter shot and a foreground-lit no-LP filter shot.

[Alan Smallbone]

Nancy if you are shooting with a lot of sky glow for astro landscape they really do help a lot, it only knocks out the sodium vapor line, so the images look pretty natural and they still let the other bands through. I found it also helps the contrast quite a bit. I will try and post some examples, a lot of corrections can be done in post but it helps to have a good starting point. I found it useful, the Nisi is an expensive filters, Haieda makes some screw on filters that are supposed to be similar. 

For deep sky work it is not really necessary and you can remove the light pollution gradients with other tools, but with the dslr or mirrorless they are useful. If I do not have any of the ground landscape and it is a sky only shot, I will generally use Pixinsight to process the stack of images, this has a very powerful gradient removal tool.

Alan

[NancyP]

Thanks!
Interesting that they now have actual screw-on filters, and not just the "clip-in" type that sit in the lens mount (ick).