A couple months back, I posted some photos from Mount Robson, in British Columbia. Recently, I was wondering how my full spectrum channels correspond to the infrared or ultraviolet photos. I decided to do a comparison, and after looking at the results, I thought I’d post them.
For a recap, here is Mount Robson (in the visible spectrum):
And here it is again, in my camera’s full spectrum. Like usual, I have set the white balance myself; otherwise, it would be much more red(I might actually compare the white balance settings sometime, but not in this post).
The full spectrum’s main difference from visible spectrum is that the red channel captures both red and infrared light in full spectrum, but only red in the visible range. The sky is a slightly different shade, but the main difference is in the foliage.
So, lets compare the visible red channel, the full spectrum photo’s red channel, and a near infrared photo(I’m going with my Zomei 950 nm filter for this one):
There’s quite a difference between all photos. The trees look lighter in the full spectrum Red/IR channel than in the visible red channel. The mountain and sky are a little darker and clearer, but it’s not immediately obvious. The 950 nm photo, on the other hand, is clearly different from the other two. The sky is black, and the trees are white. This photo is monochromatic- all three channels look the same, so there wasn’t any purpose in separating the red channel.
It seems, from comparing the three photos, that the full spectrum red channel is still strongly influenced by red light- it records the infrared light, but red seems to be detected in greater quantities. It makes sense- infrared light is not what the camera is designed to pick up. As well, some ultraviolet light might be recorded in the full spectrum red/IR channel- violet light is detected in both the red and blue channels(obviously, as purple/violet is registered as a combination of red and blue), and without compensating the white balance, UV photos on my camera appear reddish. Given my past experiments with UV light, I’d assume that it lightens the sky and darkens the trees of the red channel, but only minimally.
Of course, that brings up another point. A full spectrum camera looks past the other end of the visible spectrum as well, into the ultraviolet range. So, maybe I should compare the blue channels as well. My hypothesis… well, I’m expecting the photos at this end of the scale to look fairly similar. Unlike the stark difference comparing visible to infrared photos, true UV photos look similar to the blue channel in my camera. My camera also isn’t good at picking up UV light, so the full spectrum photo will contain mostly blue light, with only a small amount of violet/UV light. I’ll compare the three channels the same way as before; visible blue channel, then full spectrum blue channel, and finally ultraviolet (using my combined Schott BG40 and UG11 lenses- there might be some infrared leakage, but that’s the only UV photo I took at the time).
Yeah… fairly similar, but definitely not identical. The full spectrum blue channel seems to be the ‘best’ of the lot. The contrast between the trees and the sky is the lowest- the trees can be clearly seen and distinguished from one another. The visible blue channel has darker trees, and the UV channel has trees much too dark to distinguish individual plants from one another. The FS blue channel also has the least haze- it might be infrared light affecting the blue sensors in the camera, as photos in the 680-760 nm range seem to register some colour in the blue channel. The UV photo has the hazy, indistinct skies that I’ve come to expect from my UV photos, and the visible blue is at an intermediate point between the full spectrum and ultraviolet blue channels.
Of course, it’s hard to compare the photos by looking the channels separately- it would be much easier to recombine them and see how it changes the photo.
So, here is the visible spectrum photo… kind of. The red and green channels are from the visible spectrum camera, but the blue channel has been replaced by my ultraviolet channel.
Because I was using two different cameras with different lenses, and it was a windy day, not everything matched up. Ignore the greenish look of the nearest trees and the sign. That was just because I couldn’t line the objects in the channels up perfectly. As well, there is some minor distortion near the edge of one of the cameras(or maybe both of them); it’s almost unnoticeable when comparing them side by side, but when trying to line the photos up perfectly, the sides were slightly distorted and didn’t match up.
As for the photo itself, it doesn’t look too far off from the visible spectrum. The mountain looks a little bluish and hazy, and the foliage is slightly darker. if it wasn’t for the near trees looking off-colour, I wouldn’t be able to tell the difference between this and a normal photo.
Finally, on to a combined spectrum photo. This time, I replaced the blue channel of the visible spectrum photo with the UV photo and the red channel with the 950 nm IR photo. I first left the green channel unaltered from the original photo…
but after thinking about it, I thought that as I was expanding the spectral range in the photo, I’d also try expanding the green range as well. I replaced the green channel with a combined red/green/blue channel(ie a monochrome photo of the visible spectrum):
So, the results.
This is my ‘ultra-full’ spectrum photo, where the blue channel has been replaced by ultraviolet, green channel is unaltered, and the red channel is replaced by 950 nm infrared. Again, the photo didn’t line up perfectly.
And here is the other experiment(blue channel replaced by UV, green replaced by monochromatic visible spectrum, red replaced by 950 nm IR).
They’re very similar, but the red trees in the ‘green=green’ photo look slightly less oversaturated than the trees in the ‘green=visible spectrum’ photo. I like the former slightly better, but both are very unique.
By the way, I tried swapping the visible red channel for the 950 nm IR photo, and leaving the blue and green channels the same. I didn’t bother including those two photos, as there was almost no difference from the above two photos and I couldn’t think of anything unique to mention about them.
Comparing these to my camera’s normal full spectrum photo… well, there’s really no comparison. If my camera gave equal weight to the IR and UV ranges as it does to the visible range, its photos would probably look similar to these, but as it is designed to only focus on the visible spectrum, the photos usually look like normal, but with trees looking yellowish. It’s not a fault of my camera, just a reminder that ‘Full Spectrum’ means ‘Primarily visible spectrum, with some IR light and a touch of UV light included.’
The visible spectrum photo was taken with a Canon Powershot SX600 camera. The full spectrum, infrared and ultraviolet photos were taken with a modified Canon Rebel T3i camera, with the default Canon 18-55 mm lens. I know from experience that using a different lens or camera would give different results outside the visible range.
Parallel Views 