Thursday 16 January 2020

Filtering a full-spectrum camera

My full-spectrum camera, a FujiFilm IS-Pro, has no built-in filter and shoots from near UV to near IR unimpeded. This also means that when I put a filter over the lens it also affects what I see through the viewfinder. With, say, a 720nm filter I am unable to see through the viewfinder so would have to use a tripod. Sadly the Fuji's live-view is pretty-well useless.

Many people replace the high-pass IR-blocking filter over the sensor with a low-pass filter such as a 720 or 820 nm to get around this problem ... at the small expense of loss of versatility.

In recent years I have used a deep blue filter, which I can see through to frame. Blue filters usually pass a lot of near IR. You can use a red filter of course, as you might have done with film, but I find the blue filter sometimes produces an interesting colour balance with minimal post-processing. Interestingly, the auto-focus works most of the time, which helps.

I recently bought a specially-designed filter for a type of colour infrared photography from the American company, Kolari Vision. It's called their IR Chrome Lens Filter, which I'll come back to in a moment. This nudged me into looking at results of a number of filters with the full-spectrum camera.

First, here is the camera output without any filter. This is basically a 'normal' colour image but with infrared contamination. You can click on the images to make them larger.

Next is a minus-blue (ie yellow) filter. This can be used to emulate the old infrared Ektachrome film ... see this blog page for more details.

Next comes the red (#25) filter.

Now the blue filter. Different black and white results can be achieved by either removing saturation or by selecting individual channels. (This also applies with other filters of course.) The green channel is useful because, with a Beyer filter camera the green channel has twice as many pixels as the red or blue. With this filter I find I need to under expose (according to the camera) by 3 or 4 stops.

This is Kolari's IR Chrome Lens Filter, which gives a good approximation of the old Ektachrome images. However, it is not exactly the same so is not as useful for foliage health analysis: but it's not a bad approximation.

I had achieved good results using neutral density filters in the past, with Sony's Night Shot, since the ND doesn't apply at IR wavelengths. So I bought a variable ND filter, which is basically two polarising filters together. You rotate one with respect to the other in order to reduce the amount of visible light going through. In this case once I had frames with minimal density I simply rotated the outer filter until I could only just see anything then fired the shutter. Autofocus worked and by trial and error found the exposure change: in this case under by 4 stops. There is a little (false) colour information left but this method works best for a monochrome result.

Finally, for comparison, here is a 720 nm filter result.

One thing this experiment also showed me was how bad the chromatic aberrations are around the edges in the lens I am using, which are quite noticeable with colour shots but usually vanish when reducing to monochrome.

For more on this subject, here is Kolari's page outlining the characteristics of their various filters.