Sunday, 12 July 2015

Hot photography

I've been fascinated for a long time by infra-red photography. Partly it appeals to my inner geek  due to the science involved, partly because I love the surreal landscape and images that it produces, but most of all because I love hacking stuff to make them do cool things.

In the days of film, you could take infra-red photos by substituting normal film for one that was sensitive in infra-read. The results were very high-key black and white photos. Now that effect can be simulated in Photoshop today, but they are not as effective as doing it in camera.


Due to the sensitivity of the sensor, digital cameras are excellent at recording infra-red wavelengths

The above image (copyright ) shows the sensitivity of camera sensors compared to the human eye. It is is easy to see that cameras sensitivity extend well into the infra-red. In fact they are so sensitive in this areas, manufacturers go to great efforts to reduce that sensitivity.If they didn't photo's taken on digital cameras would look much redder than images seen by the normal eye.

To stop this camera manufacturers put something called a hot-filter between the lens and the sensor that cuts out a lot of the infra-red wavelengths(In fact despite that many DSLR's cameras are still sensitive to infra-red, but require very long exposures to get anything out of them.) and make the actually sensitive more akin to the human eye.

Therefore to take Infra-red  photos, we need to remove the hot-filter in your camera. However once you have done this the camera will only be suitable for infra-red photos so you don't want to do this on your main camera (it is is strange that no main stream manufacturer allows the in-camera switching of filters[I think Sigma did one once, but it is no longer available], it would be a pretty cool feature).

The camera

So basically you need a second camera. The conversion itself requires taking the camera apart and removal of the the hot filter. There are many firms who will do the conversion for you, however these are quite expensive and I'm cheap so I decided to do it myself.

First I  needed a camera.

As a happy accident I had a spare camera. My late father had bequeathed his collection of cameras , mainly were of film vintage,but he did have a Finepix S5700, which as far as I can tell, had never been used.(my dad was never big with computers, which made owning a digital camera relatively useless exercise)

Now this camera is not exactly a Nikon D800, being of a certain vintage and limited to only 7 MPixels (which is less than you would get in a high end mobile phone today), but it was a decent bridge camera in it's day and has some significant advantages.

Firstly it allows you to set a custom white balance. Infra-red skews the standard white-balance, and unless you have a camera that does raw you need to modify the white-balance before shooting.

Secondly It will take screw in filters. More on that later.

Thirdly, being mirror-less you get continual live view. You can do Infra-Red with DSLR's, but you can only see the actual result in live view, which means using the back-screen and not the viewfinder. This camera has a EVF viewfinder which makes it more convenient.

Also focusing is based on the on-sensor chip. A lot of DSLR's struggle to focus in infra-red because they rely on the phase detection chip in the viewfinder, which calculates the light path differently for visible and infra-red light.

Best of all someone had already done the conversion and posted  instructions on the internet.

One thing I did need however to complete the conversion,  was a replacement filter.

Now it is not clear why this is required. It seems that for some cameras, once the hot-filter is removed, they do not focus well unless a equivalent piece of non-filter glass is used to replace it. So I needed to make myself a filter made of plain glass of the same dimensions as the hot filter.

This sounds simple, but glass is not an easy material to work with and the size of the sensor means it was always going to be fiddly.

What I needed was some glass (microscope slides work perfectly here), some method of shaping the glass to the right dimensions and a way of measuring it, I duly purchased  a glass cutter and glass files, a digital micrometer and 50 microscope slides (that was the smallest quantity I could get, but hey, at least I had plenty of raw material. )

Ready for conversion

Duly armed I set about with my task. However I quickly hit an issue. In my haste to do the job I had not actually tested the camera. I had assumed since it was basically unused, it would work.

No such luck. When I put the batteries in, it was as dead as the proverbial dodo. So I was forced to buy a second camera of flea-bay. Not a good start.

Now I had a spare, but non-functioning camera. So I decided to do a trial run on that one.

The dis-assembly  

It's amazing how often you read instructions on the internet and it seems so simple. Normally it is fine until the point of no-return and then goes pear shaped.

I generally have no problems taking things apart. Putting things back together in something akin to the original state I find is the challenge and so it proved in this case.

Taking the camera apart proved quite simple, but I just could not get the thing together. The main issue was the ribbon cables that linked the boards together. They were small, fiddly and just would not re-connect. It took me 3 days until finally I managed to re-assemble the camera (losing only 1 screw in the process).

This was the problem one (copyright

However all in all it proved a useful exercise. One other thing I learned was the dimensions of the hot filter given in the original instructions was in fact in-correct meaning I needed to create a new one of the correct size.

The old filter (copyright

However it was here that something strange happened.

I decided for no good reason , to test the camera again and to my surprise it turned on. OK, it would not focus, but it showed that perhaps the original issue was to do with the connectors and maybe I could fix it.

So I decided to take the camera apart again, partly for practice and partly to see if I could fix the focusing issues. The 2nd time was a lot easier and to my surprise everything worked....

So if I learnt anything, it is assembling and dis-assembling cameras is not easy  and be prepared to get it wrong.

(see here for a better example of the pitfalls  ),

First Light

So how do you test your camera is picking up infra-red?

Well, just pointing out of the windows may not show much, especially if the camera is getting all the other visible wave lengths as well. The best way is to take a TV remote and point it at the camera while pressing a button. Normally the camera will not see the light, but now you should see the LED flash, like below

Phew, it works
These are the images taken outside, with no changes to the white balance. As you can see they are now a lot redder, due to the extra light from the red side of the spectrum getting through

What Now?

So far so good. But we are a little way to getting those infra-red only shots. The problem is that now the camera is actually getting too much light from the visible end of the spectrum. We are only interested in the infra-red stuff.

What we need is to put a filter on to take the visible stuff away. This was point about why this camera is a good fit for this type of conversion,  since it will allow you to mount a 46mm filter on the front.

I believe if you send your camera off to be converted, they will often replace the hot filter with a standard IR filter. However there are many variations of IR filters, each with advantages and disadvantages.  For example the more light you block off, the longer the exposures will be. By allowing the changing IR filters we have more flexibility on when and where we take our photos.

Generally IR starts at about 700nm. So a 700nm filter will pass all light at a larger wavelength. However filters are not perfect and will pass other colours as well. A 950nm filter will pass far less of the visible light and have a pure IR affect

Anyway I am going to get a 720nm and a 850nm filter to start and see how I get on. Certainly I expect exciting times ahead.


Someone has corrected me that it is still possible to purchase a Sigma SD1, and a very interesting (but probably technology dead-end it is). However it could never be called a mainstream camera

No comments:

Post a Comment