Infrared conversion of Nikon 1 S1

Digital cameras have gotten very good at representing the reality the way the human eye sees it. Several key indicators have gotten better, for example, colour accuracy, resolution, dynamic range, noise performance, lens rectilinearity, etc. All this to make your pictures look more natural.

But there are times when you want to take unnatural pictures. I have written about such examples previously. One way to take unnatural images, is to photograph outside of the visible light spectrum, usually infrared.

Back in the film days, you could buy specialist film which was sensitive to infrared light. Normally, you would combine this with a visible light blocking filters, somewhat confusingly referred to as "infrared filters", to put in front of the lens. These filters block the visible light you see with your own eyes, only letting through infrared light. That way, you can photograph outside of the colours that you can see yourself, for interesting effects. These filters usually have a cut-off wavelength of around 650nm or higher, see the illustration below.

With digital cameras, this is somewhat simpler. The sensor itself is able to see infrared light, so you can use the normal live view process to compose your image, and even use autofocus. However, virtually all digital cameras have a strong infrared blocking filter built in. This is to avoid stray infrared light hitting the sensor, which would reduce the contrast.

Some specialist digital cameras come without the infrared blocking filter, for example the newly announced infrared version of the Fujifilm XT1. This camera is "full spectrum", meaning that you must yourself add a visible light blocking filter to the front of the lens to achieve the infrared effect. However, at a price of US$1700, this camera is not for everyone.

A cheaper option is to take a digital camera you already have, and convert it. This can be done by third party companies, or, if you are adventurous and dexterous, you can do it yourself. In this article, I show you how to do this with the Nikon 1 S1, an old, basic Nikon 1 camera. Contrary to what you may expect, this camera is serviceable, and you can disassemble it to access the filter stack above the sensor.

Nikon 1 S1 with Nikon CX 10-30mm VR in orange(!). There is a 760nm cut off infrared filter attached using a 40.5mm to 46mm step up ring and a 46mm to 52mm step up ring.

To do the conversion, I bought a cheap set of instrument tools. This was sufficient to do the disassembly and re-assembly process.

Of course, there is a big risk that you damage the camera. You can even risk injuring yourself, if you touch the circuits related to the flash unit, which can carry a high voltage. So do this at your own risk!

See how I did it by watching this video:



Some comments, though. After removing the LCD screen, and the metal screen below, you get to the motherboard. The cables you need to disconnect before proceeding to remove the motherboard, are marked with a red ring.

I've set another orange ring around one cable: This is the one I did not disconnect when removing the motherboard. This resulted in the cable breaking! Luckily for me, this cable goes to the flash. While it could have been fun to test if the flash is compatible with infrared photography, it is not a huge loss for me to lose the flash. But when you do this process, please disconnect the cable before lifting up the motherboard.

When you come to the sensor unit, you'll see that there is one clear looking filter mounted at some distance from the sensor. It has a hot pink reflection when viewed at 45 degrees, consistent with common infrared blocking filters.

Beneath this clear/hot pink filter, there is another cyan coloured filter. I have tested extensively, and concluded that both of these block infrared light, by approximately the same amount. Both filters, individually, block out about 6 stops of infrared light.

So to make the camera full spectrum, you need to remove both. This will leave the sensor completely exposed, so be careful with dust contamination.

Top filter, hot pink when viewed at 45 degrees angle, 24mm by 22mm. This filter is quite thin	Bottom filter, cyan colour, next to the sensor. 20mm by 18mm. This filter is fairly thick, around 1mm. It probably doubles as an anti-aliasing filter (AA)

What professionals do, is to replace one of these filters with a visible light blocking filter. That way, the camera is infrared only, meaning that you don't need to mess with adding IR cut-off filters on the front of the lens.

On the other hand, not mounting such a filter in front of the sensor means that I have more flexibility later: I can experiment with different cut-off frequencies, and I can even use the camera for full spectrum photography, even if this is not usually very interesting.

Other cameras

The Nikon 1 S1 is contemporary to the Nikon 1 J3 and they probably share the same internal layout.

Other cameras tend to have mostly the same construction. To access the sensor unit, you must generally open the rear cover, move the LCD screen and mother boards, and access the sensor from the rear.

Problems you may experience, include that the filter stack may be glued to the sensor unit. If so, you need to cut and pry it off, which adds more risk of breaking the camera to the process.

Dust on the sensor

One big risk when doing this procedure, is exposure to dust. However, even if I was not very careful, this has not been a problem to me. The sensor is now completely exposed under the lens, but I don't see any dust problems.

To avoid dust, I have kept the same lens mounted all the time since removing the filter stack.

Focus

The filters, especially the cyan one, are fairly thick. Removing them means reducing the optical distance to the sensor. This affects the register distance, it becomes longer. On top of this, infrared light also focuses slightly differently, but I think this effect is fairly minor compared.

This means that you may experience problems focusing on infinity, especially for wide lenses with a short focal length. With the Nikon CX 10-30mm VR, I generally cannot focus on infinity wide open below around 15mm. To overcome this, I use a smaller aperture (larger f-number), which helps keep infinity in focus.

Beyond this, autofocus works just like before.

IR hotspots

A common problem when converting a camera to infrared, is that some lenses are not suitable for IR use. They show a "hotspot", meaning a bright centre area, usually in the shape of the aperture opening, making the image difficult to use.

A common cause for the hotspot is the lens coating, which could be unsuitable for infrared use. This is not seen with normal photography, as the infrared light is filtered off anyway before reaching the sensor.

I have only used one lens so far, the original kit zoom lens 10-30mm f/3.5-5.6 VR. It does not show any hotspot.

Results

To illustrate the effect of various cut-off wavelengths, I have taken a series of photos of the same subject. Click for larger images:

	Out of camera picture	Converted to black/white
Normal camera, no conversion ISO 200, f/7.1, 1/640s
Full spectrum, no cut-off filter ISO 100, f/7.1, 1/800s
680nm cut-off ISO 100, f/7.1, 1/400s
720nm cut-off ISO 100, f/7.1, 1/320s
760nm cut-off ISO 100, f/7.1, 1/250s
850nm cut-off ISO 100, f/7.1, 1/125s
950nm cut-off ISO 250, f/7.1, 1/60s

With a sub-700nm filter, you can usually see some red through it. Here is the 680nm filter held against the sun:

But with a larger cut-off wavelength, the filter is completely black to the human eye, completely blocking all visible light.

Which cut-off wavelength to choose? 680nm is too little: You will get some visible red in your pictures. And 950nm is too high, this is beyond the peak sensitivity of the camera sensor, so you need to push the ISO more. Around 760nm cut-off seems optimal to me.

When having a high cut-off wavelength, you don't get any colour information at all. All the channels show the same profile.

With the smaller cut-off wavelength, e.g., 680nm, you can get some false colours. E.g., I have found that the material wool can get a cyan colour tint. This effect can be amusing to play with, so you may want to get a filter with a smaller cut-off as well.

Note that you cannot make a heat sensitive camera by choosing a high cut-off frequency here. The sensor is simply not sensitive to so long wavelengths. To take heat pictures, you need a specialist camera.

And don't expect to be able to photograph in darkness. You need an infrared light source. And the sun is the main source for infrared light, just like for visible light.

In the pictures, you can note the characteristic infrared effect: Leaves become very bright. This is due to the Wood effect, not named after the material wood, but the person: Robert W. Wood. In infrared, leaves reflect light much like snow, which gives these bright leaves in the pictures.

Generally, you'll want to photograph in RAW format, and use a program like Lightroom to desaturate them to black/white.

Example images

When using a moderate cut-off wavelength, around 700nm, the sensitivity is much like a normal camera. So you can use the converted camera handheld, just like you would commonly do.

Here we see the effect on clothes and people. Clothes from artificial fibres tend to become white, and peoples faces become pale, while eyes become dark:

30mm, ISO 100, f/6.3, 1/160s

Vålerenga Church: Even if this photo is taken midday, the sky is dark. Also, the weed on the wall becomes very bright due to the wood effect:

13.6mm, ISO 140, f/7.1, 1/125s

The lawn looks like it is covered with snow. The cloudy sky becomes more dramatic, a bit like when using a red filter on a traditional black and white film camera:

17.5mm, ISO 140, f/6.3, 1/125s

17.5mm, ISO 100, f/7.1, 1/125s

Example video

You can record videos just like before, too. Just keep in mind that you normally want to convert the video to black/white.

Conclusion

If you are reasonably handy and patient, removing the IR filter of a camera is not very difficult. You can use it for interesting effects. The camera can be used like any other cameras, except that the images need a bit of post processing to work well.