To channel-swap or not?

I’ve been experimenting with the Fuji X-T1 which was recently converted for full spectrum photography using three IR filters.  The Fotga variable IR filter is adjustable from 530nm through to 750nm.  The first image  here was shot with the filter set to 750nm.  The resulting RAW file is blue as can be seen on the left hand side of the first image below. In the past, when I was using an 720nm infrared converted camera,  I have channel-swapped the reddy/brown RAW files which created a blueish file and I wondered whether it was necessary to do so with these. The right hand side of the first image below shows the effect of channel-swapping.

© Dave Whenham

I then converted the image to mono using Silver Efex Pro. In the image below the left hand side of the frame was converted to mono using the RAW (blue) file whereas the right hand side was converted to mono using the channel swapped (red) version of the RAW file. I can see no real differences between the two.

© Dave Whenham

But what of an image shot at 530nm?  In the image below the RAW file is again on the left and note how different the  tones are as more visible light is allowed to hit the sensor.

© Dave Whenham

© Dave Whenham

Now, whilst there is a definite difference in the tonal values of the image shot at 530nm once again there is no appreciable difference between the mono conversions directly from the RAW file compared to the channel-swapped version.

Now it has to be acknowledged that I didn’t chose the “best” conditions for this type of test, most people reckon on a bright sunny day for the best from infrared photography, but this non-scientific test nevertheless suggests that channel-swapping may not always be necessary for mono conversions.

The next comparison is from an image shot with a 950nm filter. I have carried out exactly the same process but this time the split runs diagonally from top right to bottom left.

Again, there is virtually no difference, the conversion from RAW being arguably a tiny bit darker but we are talking about fractions here. Incidentally, this was shot in bright but cloudy conditions.

The final image below has been treated exactly the same (the split running top to bottom middle) as the other two above. This was taken on a bright sunny day with the Fotga variable infrared filter at 530nm.

© Dave Whenham

Again, there is virtually no difference. There is a slight tonal variation, particularly in the water, circled, but even that is not a major variation.

The upshot of this experiment is that I’m not certain I need to channel-swap for monochrome conversions going forward. I will probably continue to do so for a while until I’m confident of the process though. False-colour infrared is a totally different kettle of fish though and one which doesn’t interest me quite so much although I’m sure I will be having a play at some point.

 

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