Saturday, July 29, 2017

Travelling light: at all frequencies

Although I have an infrared converted camera (a 50D), it is rather large. Therefore, if I wish to travel light, the 50D and my 5D3 just don't fit...literally!

For visible band photography, my go to travel camera, because it runs Magic Lantern and my focus bar script, is my EOSM. I did have a Sony 6000 as my travel camera, but I missed Magic Lantern so much, that I sold that, and now all I buy are 2nd hand EOSMs. Yes I have two.

So last week I decided to get one of them converted to IR. I decided to keep with the 720nm cut, that I have in my 50D, as I believe it to be a good middle ground between a 'full spectrum' conversion and a deep IR one.

Here is a chart to illustrate how a digital sensor responds to light:

The black curve shows the typical response of a CMOS sensor, showing that the sensor can capture more data that we can see, which is reflected in the 'hot mirror' curve.

Of course in IR photography we are not looking at thermally-emitted IR, ie from hot objects, but reflected solar IR:

The IR conversion removes the hot mirror and, in my case, replaces it with a 720nm filter, giving the following sensor characteristics: 

As I wait for my EOSM conversion to arrive, I thought I would revisit an IR capture that I made with my trusty 50D. The image is of the Seven Bridge and as a RAW capture, ETTRed with the help of Magic Lantern, it doesn't look that impressive:

The LR histogram clearly shows that this was an ETTRed image:

Here we see one of the first 'problems' with IR photography. Bluntly, just looking at the image, you would initially be tempted to mark this as a 'reject'. In other words, never discard an IR image by just evaluating the RAW: always post process first!
As usual with digital photography, there are many ways to post process an IR image. The only thing you can say with confidence is, that there is no right or wrong way.

The general workflow I follow goes like this:
  • First I use a custom profile in LR that I have made. This processes the image with a linear gamma curve, and pulls out more highlights, which goes with ETTRing. This profile also reduces the red cast, because LR on its own can't handle IR images;
  • I then get the basics of the image looking right in LR;
  • I then do a round trip to Photoshop to tiding things up, ie clone stamp etc and use luminosity masks to give the image the look I'm after;
  • Finally, in LR I will carry out any last minute tweaks.
Following the above, the Seven Bridge image is transformed into the following:

Look out for more IR postings once I get my EOSM back from its conversion.

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