Camera modification for IR sensitivity
A warning: messing around with your camera and opening it by yourself is always at your own risk and will void any warranty on the camera. If something does not go completely as planned, it is not unlikely that you will destroy a fine camera and will be left with nothing more than a neat paper weight.
The goal of each infrared camera conversion is to remove the internal IR-block filter and to replace it with an IR-pass filter, clear glass or any custom filter you would want. The basic workflow to do this is:
- Remove the back panel and sometimes some other body-parts
- Take the main circuit board out to gain access to the sensor
- Remove the sensor assembly from your camera
- Remove the anti-aliasing\dustshaker and IR-block filters from the sensor
- Install your own filter in the filter frame
- Reattach the filter to the sensor
- Reassemble the camera
Detailed workflow for:
- » Sony NEX-6 (16.1 Mpix APS-C, live view, HD video, 2012)
- » Canon EOS 550D\T2i (18.0 Mpix APS-C, live view, HD video, 2010)
- » Canon EOS 50D (15.1 Mpix APS-C, live view, 2008)
- » Canon EOS 450D\XSi (12.2 Mpix APS-C, live view, 2008)
- » Canon EOS 40D (10.1 Mpix APS-C, live view 2007)
- » Canon EOS 30D (8.2 Mpix APS-C, 2006)
- » Minolta Dynax 7D (6.0 Mpix APS-C, sensor stabilisation, 2004)
Infrared camera focus
The focusing of IR converted cameras knows some problems which have two main causes: 1) Differences in optical thickness between the original IR-block filter and the IR-pass replacement. 2) Difference between the focus of visible light and infrared for the used lens.
Both problems occur because the camera has actually three surfaces that are important for the focus. The First is the Image sensor, the part that is actually important. Besides that there is the focus screen that is used for manual focusing trough the viewfinder and the autofocus sensor that is a separate sensor in all DSLR cameras and is used for the autofocus.
1) Replacing the filter Changing or removing the IR-block filter causes a shift of the focal point caused by refraction of light at the filter. The filter sits only in the optical path of the sensor and not in front of the autofocus sensor or focus screen and therefore results in pictures that are out of focus. But this problem can be solve quite easily by adjusting the position of the sensor in the camera. The older canons use shims at the sensor mounts and the newer ones (500D and up) have spring loaded screws that can easily adjusted.
The shift of the focal plan introduced by a filter is roughly 1/3 of its thickness (this is a rule of thumb, but a very effective one!). If you replace a 1mm filter by a 2mm filter, the sensor position has to be corrected 0.33mm backwards, because the difference between the filters 1mm. If you have all characteristics of the filters, you can calculate the focal shift exactly.
Focal shift = ((filter refractive index – 1) / filter refractive index) * filter thickness
The refractive index of commonly used glass for these filters is between 1.5 and 1.55. For the camera internal filters the exact refractive index is usually unknown. Replacement optical filters are often Schott glass and have detailed descriptions of their optical properties available (example: Schott RG715 @850nm = 1.53).
Let’s give an example: the two internal filters of a 550D (total 1.2mm, assumed refractive index 1.5) are replaced by a 2mm thick Schott RG715 (refractive index 1.53).
Original: ((1.5 – 1)/1.5)*1.2 = 0.40mm Focal shift backwards
New filter: ((1.53 – 1)/1.53)*2 = 0.69mm Focal shift backwards
Difference to be corrected = 0.69 - 0.40 = 0.29 mm.
Second example: Only the IR-block filter of the 550D is removed, the AA\dust filter is reinstalled. We do not use a replacement filter
Original: ((1.5 – 1)/1.5)*1.2 = 0.4mm Focal shift backwards
New filter: ((1.5 – 1)/1.5)*0.6 = 0.2mm Focal shift backwards
Difference to be corrected = 0.2 – 0.4 = – 0.2 mm (the sensor has to be shifter forward!)
2) IR focus shift of a lens.
The amount refraction of light, and therefore the effect of a lens is dependent on the wavelength. The result of this is that infrared light has a different focal point then visible light with many lenses. The same would be true for the difference between blue and red light, but most lenses are pretty well corrected for these differences within the range of visible light. You could say that this lens problem is no problem at all, because the lens is in the focal path of all our sensors (image, autofocus and focus screen), but sadly the problem is real. The IR-pass filter is installed only on the Image sensor and manual or autofocus still use the visible light, resulting in the IR photos being out of focus. To fix this we could put the IR-filter in front of the lens (or at the back, in the lens mount), but then we see nothing (humans can’t see IR) and the focus sensor will also not function well.
Some lenses (mainly the older and pro series) have an IR focus mark. This is used by focusing in visible light and then turning the focus ring by the difference between the normal and IR focus point before taking a picture.
Live view: The solution!
The newer generation DLSR cameras and also all compact cameras have a live view function and can also use the live view for metering and focus. For IR converted cameras this is a great solution because it uses the image on the main sensor to determine the focus and metering. This means that you do not need to synchronize different sensors anymore and also the metering is based on the incoming image after your IR-pass filter. Live view focus is a little slower than the normal focus system of your DSLR, but if you want critically sharp pictures in the wavelength you are working with, this is the way to go.
It is still very advisable to correct the sensor position for differences in filter optical thickness. A well corrected camera will also function well with the phase detect autofocus, and composing and focusing trough the viewfinder is much easier. Some modern cameras also have a per lens micro adjustment of the focus, this may allow for perfect correction of the IR focus for most lenses (some zoomlenses may even have varying focus point in IR depending on the zoom, so live view focus may still get better results). Another pitfall may be that if you do not correct for the removing of a filter or installing of a thinner filter, you camera may not reach infinity focus anymore. Even live view cannot solve this.
In the 30D fine tuning the focus can be done using the adjustment screw in the mirror housing. The screw is in the right side (mount facing towards you) just in front of the shutter and fits a 1.3mm or 0.05” hex key. I used a standard straight hook type, bent a bit to provide better fitting. You need two of those with the bends in different direction, as the space inside the mirror housing is too tight to make a full 1/6th turn. Adjustment is done as an itterative proces (turn, check, turn, check etc.) using a printed focus test chart. Please notice that the IR focus point may differ between lenses (and zoom positions). I would advise to use a simple standard prime for the calibration, but you may also want to check different lenses or use the lens you will use most in IR.
In the 50D and up, you can use the software focus calibration, so you don’t need to fiddle in the mirror housing. The xxxD series (rebel) do not have a focus adjustment. These methods only calibrate the autofocus, the focus screen form manual focus trough the viewfinder is not adjusted and may result in pictures out of focus.
Cutting your own filter to size:
It is possible to order a specific filter for your camera type, but these filters are very expensive. Cutting some standard filter glass from an optics store, or even from a mounted filter, may be much cheaper. If you cut yourself, the choice in filter glass is also larger than then when ordering camera specific filters.
Cutting your own custom glass can be done either with a standard glass cutter, by scribing and breaking, or using a diamond cutting disk on a light power tool (dremel). I found that using a glasscutter was very effective and also quite precise on 3mm thick glass, but when trying 1mm thick clear filter it resulted in splinters only…
For scribing and breaking you will need a glass cutter (I used a tungsten wheel type), a ruler (preferably metal), and some fine grain wet&dry sandpaper to fine-tune the size or take of the sharp edges.
- Mark out the cutting line, and using a steel ruler and a microfiber cloth make one straight pass with the glass cutter.
- Break the glass over the cut line. (I used a book, as a straight edge support)
- Check the size and sand of any sharp edges.
- Be careful not to scratch the glass and clean using plenty of running water, then dishwasher detergent, and finally some cleaning solution (eclipse) and a optics cleaning tissue.
For diamond cutting:
- Cover your filter with fine quality masking tape
- Mark the cutting lines
- Cut using the diamond disk
- If needed, sand the edges to take of any sharp parts and finetune the size
- Clean under plenty of running water, remove the masking tape, clean using water and detergent and finally use some optics cleaning solution and lint-free non-abrasive tissue.
For gluing the new filter in its frame, fine black silicone glue is advised, as it is also used with the original build. I used standard superglue (thin 2seconds glue) which works fine, but only when the fit is tight as is has little filling capacity (Watch out with the modern superglues as the vapor may cause a haze on the glass filter).