Having used Cokin filters for many years and having been satisfied with their performance, it was only when I took my photography more seriously that I realised that Cokin's ND filters had disappointing flaws. Whilst taking some early morning shots at Buttermere, I took a number of photo's of Hassnesshow Beck just as it winds it way out of Crag Wood on its way down to Buttermere.

The photo at left was solely responsible for my move away from Cokin and towards LEE Filters. But given the huge cost differences between the two systems I wanted to understand whether LEE would truly resolve the problem and why.
The following is my understanding of why the colour cast appears, particularly when using Cokin ND's. The photo was taken with a Cokin 3 stop ND filter to smooth the water and a 3 stop ND grad to balance the strong light of the sun rising to the south east. The resulting exposure time was a full 8 seconds. The red colour cast is the result of excessive infrared entering the sensor of the camera, which is most noticeable in the lighter areas of the scene; typically the sky.

The photo to the right was taken a few minutes earlier with the same filters but had a slightly shorter exposure time of 6 seconds (evident in the slightly darker foreground). The difference in the result is quite obvious. It isn't without a colour cast though, when expanded to 100% a cast is still to be seen.

Both have a WB of 5000K and no digital editing has been performed; they are straight out of the camera.

So what is causing the colouration? As mentioned, it is due to infrared reaching the camera's digital sensor. Digital camera sensors are incredibly sensitive to IR and if left unprotected would detrimentally affect all photographs. Manufacturer's attempt to resolve this problem by placing a filter, called a high-pass filter, in front of the sensor. This filter aims to remove the IR whilst still allowing all of the visible spectrum of light through.

Visible light has wavelengths ranging from 400nm (the blue end) to 700nm (the red end). Beyond the visible red end is the near IR spectrum which ranges from 700nm up to 1400nm (1.4µm). The blue end has a higher frequency than the red end and it is for this reason that the filter over the camera sensor is called a high-pass; that is, it passes the higher frequencies.

The graph to the left shows a section of the electromagnetic spectrum plotted against atmospheric opacity; the amount by which the atmosphere filters radiation. Fortunately (for us all) the ultraviolet, X-rays and gamma radiation to the left of the visible region are blocked. Most of the visible region passes through and it is clear that much of the near IR radiation also passes through.

Unfortunately, the high pass filters are not perfect and do allow varying amounts of IR through. The amount is very variable depending on manufacturer, camera model and invariably price! However, for most photographs the exposure time is small and the high pass filters adequately reduce the IR to a negligible amount.
When an ND filter is used though, the intent is to reduce the amount of visible light entering the lens. It is in this respect that differences in ND filters become apparent. Irrespective of price, all ND filters reduce the amount of visible light by the specified amount (i.e. 1, 2, 3 stops etc) however the cheaper ones are not so good at filtering IR. The longer exposure times, coupled with an ND filters failure to reduce the IR radiation by the same amount as the visible light hitting the filter, results in a disproportionately high amount of IR entering the camera. In those cameras with less effective high pass filters, the IR passes through to the sensor.
This relatively high level of near IR is interpreted by the sensor as a reddish colour.

When using non-apochromatic lenses, even small amounts of IR can be visible as chromatic aberration, particularly noticeable along the outermost edges of tree branches against bright skies. This is because IR is not brought into focus in the same way as visible light. The APO lenses are however designed to deal with this.
The solution, especially when using non-APO lenses, is therefore to use ND filters which are better at reducing the amount of IR allowed to pass through. This is where the Cokin A & P ranges are not very effective and where LEE filters are much better. Depending upon the camera (and high pass filter) used, some colouration can still occur even when using LEE ND filters. Some Nikon cameras are understood to be not so good in this respect, however again this presumably depends upon model.

It is suspected that the times of the day when IR is at its strongest is during the classic landscape photographers periods around sunrise and sunset. This is because Rayleigh and Mie scattering causes the shorter wavelengths of light (the blue end of the spectrum) to be scattered, allowing the longer wavelengths to penetrate the atmosphere and appear as the warm colours we associated with these times. It's likely that the presence of IR in our many photo's at these times is masked by the red skies which we seek.

For those who regularly use two or more NDs at the same time, LEE do produce the Proglass range which is suggested as reducing IR transmission even further than their standard resin range. The cost for these filters is even higher however.

My opinion therefore is that if you are serious about correct colour rendition and regularly use ND filters, you should be considering the higher end of ND filter rather than going for the cheaper option. I can testify that the cheaper filters are not cost effective in the long run.

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