Digital SLR Camera Notes
IR photography using a modified digital camera is more complex than most people realize. If you understand how the IR camera, sensor, and lens interactions better, you will be able to take better photographs. To start, the camera, sensor and lens were designed to function well as a system in the visible light range. The system may perform much differently when modified to take infrared pictures.
1: Camera: All cameras are designed to focus sharply in the visible range. The optimal focus point in infrared is slightly different in infrared. Older lenses often had a red dot on the focusing ring that was the point the user was to turn the lens after visible focus was achieved when using infrared film. Most lenses today have omitted the infrared focus dot. Another way to achieve correct focus is to move the sensor in the camera backwards to compensate for the IR focal shift. On our infrared enabled cameras, we modify the camera so that the camera will focus correctly in IR when using a standard lens focusing in visible light.
2: Sensor: All digital color cameras and camcorders have an IR Cut Filter (ICF) in the optical path because the image sensor color dies that separate color open up in the infrared range to varying degrees. When we modify a camera, we open the camera up, remove the ICF/Anti-Aliasing (AA) filter, replace the stack with a 715nm IR filter and modify the Auto Focus function so that the camera focuses correctly in the infrared band. You can shoot at any F-Stop with any lens. The camera is modified in a Class 100 clean room with ionized air and static control surfaces.
Most current digital SLR cameras incorporate the AA with the IR cut filter in a 3 layer glass sandwich. When we remove the ICF, the AA is also removed. This means that the camera will be susceptible to Moiré patterns under certain conditions such as photographing a screen door. Another term for the AA is a Blur Filter or Optical Low Pass Filter (OLPF) because the filter is blurring the high frequency visual information from reaching the sensor. Basically, the idea is to introduce enough blur to the image that any repeating patterns are wider than the pixel spacing on the sensor. Aliasing occurs when the visual pattern from the image matches a multiple of the spacing of the pixels on the sensor. When a repeating image pattern matches the sensor pixel spacing, you get doubling and subtracting effects that give you a Moiré patterns. The upside of removing the AA is that the camera will take slightly sharper pictures at the expense of possible moiré patterns. For landscape photographers, this is a tradeoff well worth making since they would rather get the sharpest image possible from the camera. Some of our customers have used our IR+Visible converted cameras for normal photography because of the increased resolution possible.
3: Lens: Camera lenses are designed to be sharp with correct color registration, no barrel distortion, hot spots and such in the visible range. The lens may perform much differently in the infrared range. Keep in mind that lenses are not designed for the infrared, so you should experiment with your lenses to learn how to get the best performance from your lens. We have seen excellent visible light lenses perform poorly in infrared which is not a fault of the lens, camera or conversion. It is simply that the lens was not designed for the infrared. Common distortions in infrared:
- Barrel distortion: Wide angle lenses (e.g. 10-22mm zoom) can have out of focus pictures around the corners. The wider the angle, the worse the problem usually is.
- Infrared Registration: Lenses are designed to that the Blue, Green and Red (400nm to 650nm) colors all come into focus in the same spot. Infrared light from 700nm to 1100nm may not focus as sharply. For instance 700nm may register in a slightly different spot than 1100nm light which will cause a loss of sharp focus.
- Infrared Hotspots: Lenses are usually coated with anti-reflection coatings to minimize the effects of light bouncing inside the camera. Light may bounce off the silver colored image sensor and/or between lens elements. Infrared hotspots will usually be worse at small apertures (High F-Stops) and at full zooms on zoom lenses. Lens designers rarely care about infrared reflection problems since the ICF is blocking IR from the sensor. To minimize IR hotspots, shoot with a wider aperture and wider lens.
Getting The Sharpest Picture
Before doing serious work with your camera in infrared, you should take some test pictures using various lenses, zoom setting and F-Stops. Learn what works for your camera and lens combinations first. In general, our observations are:
Aperture: Contrary to common knowledge, your sharpest picture will usually occur at the widest or 1 to 2 F-Stops from the widest aperture setting. While small apertures give you the greatest depth of field, the small apertures also increase
Lens Diffraction. Furthermore, any dust spots, IR glass imperfections will be much more visible. Our modified cameras are designed to work at all F-Stops. Only use small apertures if necessary.
Wide Angle Lenses: Please test any wide angle lenses. Look especially around the corners to see if you are experiencing barrel distortion. If you have a lens with barrel distortion in the infrared, there is not much of a solution other than using a different lens.
Exposure Compensation: Infrared data comes in primarily on the Red Channel Because of this, cameras will sometimes over expose the red channel. Try shooting with -1 or -2 stops of exposure compensation.
White Balance: Try setting a custom white balance with a piece of white paper in outdoor sun. Some cameras such as the Fuji won't allow such a white balance setting, but the Canon cameras are capable. While this won't increase sensor resolution, it will give you close to a black and white picture.
Image Size and Format: Shooting RAW will maximize picture information. Processing is best done afterwards on a computer using software such as Adobe Photoshop. In Photoshop, users will typically optimize the color channels, desaturate and sharpen the image.
Pixel Size vs. Dynamic Range: As pixels get smaller, the photon wells get smaller resulting in the well unable to gather as many photons which means less dynamic range.
Pixel Size vs. Airy Disk: As pixels get smaller, they run into something called the Airy Disk which is a description of the best focused spot of light that a perfect lens with a circular aperture can make, limited by the diffraction of light. The Airy disk varies with the frequency of light. The shorter the wavelength, the smaller the airy disk. The airy disk looks like a doughnut of light on the sensor. Below around 4 microns pixel pitch, even the best lenses cannot resolve light due to the limitations of optics. So if you have a tiny point and shoot sensor with a sensor the size of a fingernail with 20 megapixels of pixels, it is highly doubtful you are getting 20 megapixels of information because the lens can't resolve that fine of detail even if it were perfect. And that small point and shoot camera with plastic parts that are telescoping in and out is capable of far less. At some point, all you are doing is making a bigger file with no more real optical resolution.
A excellent source for IR photography and post processing is from Lloyd Chambers Guide to Digital Infrared