Digital Infrared Capture & Workflow
Filter Choices
(Based upon information courtesy of Advanced Camera Services, UK)
Clive R. Haynes FRPS
720nm
This is the most commonly fitted IR filter. It allows only IR light to pass from about the point where visible light ends and allows the sensor to 'see' IR frequencies up to around 1100/1200nm, or as far as the sensor will allow. You can shoot false colour IR images, or convert to B+W as you wish. The 720nm filter can produce stunning B&W images, but has slightly less contrast than the 830nm images. Contrast can be added in post production to achieve very similar B&W results. This makes the 720nm a good all round conversion. Once converted with this filter, your camera will shoot roughly the same shutter speeds as an ordinary visible light camera, except on very overcast days where the moisture held in the clouds will mean there may be less IR light coming through.
830nm
This is essentially a black and white only filter. It allows the camera to see only the part of the IR spectrum from around 830nm to the end of the sensor’s capability. The filter material itself is slightly ‘darker’ than the 720nm, and you would normally lose around 1-1.5 stops of light as compared to a visible light (or 720nm) camera in the same conditions. The 830nm has slightly more contrast in B&W images but the 720nm can match it with a little tweak of the contrast in post production. Being that the 830nm filter allows the narrowest band of wavelengths, it could be said to be a tiny touch sharper than filters that pass a wider bandwidth. Some 'false colour' can be recovered but rather less than with the 720nm filter.
780nm
This filter is not commonly requested. It is effectively a mid-point between the 720nm, and the 830nm. It is a filter which will give you colour images like the 720nm, but the colour is very muted, or less saturated, but not totally absent as with the 830nm. The 780nm filter will give you the same shutter speeds/exposure as an ordinary visible light camera just as the 720nm does. This is a useful filter for black and white IR photography but without the 1 stop loss arising from the darker 830nm filter material.
665nm
This filter allows IR light to pass, just as the 720nm filter does but also lets through red visible light. This gives you a little more colour to play with when processing with 'channel swapping' and produces images with stronger colour. The plants and foliage in colour images taken with the 665nm are not pure white as they are when taken with a 720nm filter, but they have more of a light blue tinge, which turns to a slight yellow tinge when you perform the red/blue channel swap. (Swapping the red and blue channel output can give you a blue sky on your IR image). For black and white images the 665nm will have less contrast, and may not be as good as a 720nm or 830nm conversion.
590nm
Allows IR light, and red visible light as per the 665nm but also allows the camera to see orange, and even yellow visible light. This produces a slightly different colour scheme to the other filters. Plants and foliage taken with the 590nm filter are a strong blue colour, and once you perform a channel swap you will end up with very strong yellow foliage. This colour scheme is very striking and interesting. It can be further manipulated to produce other hues, but it is not to everyone’s taste. One other consideration is that this filter allows a wide bandwidth compared to the 720nm or 830nm and due to the fact that each individual wavelength will focus at a very slightly different point, the wider the bandwidth you are looking at the softer the image will be. The 590nm images are not soft as such but they are perhaps a touch less sharp than the 720nm or 830nm.
The results from these filters can be compared by searching for ‘720nm’ or ‘590nm’ etc. on Google Images. This will give some idea of the different colour schemes that each of the filters will offer. If you see colour IR images with blue sky, then these will have been manipulated in post production by swapping the red and blue channel output. Please be aware that what you may find that some images may not always be correctly labeled.
Quartz Glass / Full Spectrum
Quartz glass conversions allow the camera to see the full spectrum that the sensor is capable of seeing (from IR, through visible wavelengths, to UV). You can then use various filters in front of the lens to filter or pass different portions of the spectrum and narrow the bandwidth in any way you require. This conversion is commonly used for scientific, and industrial applications and often needs manual focusing via live view to give the sharpest results.
With SLR cameras converted to full spectrum, the AF system of the camera can be set to be correct for visible light but it will not be correct for IR light when using an IR filter for example as the IR wavelengths will focus at a different point. Also, the AF system of many SLR cameras will not work with dark filters. For these reasons you would need to use manual focus with the live view function to accurately focus your images. Some SLR cameras have Auto-focus in live view which works directly off the sensor but these systems tend to be very slow. Please note some older digital SLR cameras do not have the live view function.
With many compact or micro 4/3rd cameras the AF system functions directly from the sensor itself and so in theory they will always give correct focus. This makes them very suitable for full spectrum conversions if required.
Full spectrum cameras can be used with a wide range of front-of-the-lens filters to cut or narrow the spectrum to various bands of wavelengths depending upon the filter specification.
Comparing Filter Types - Images
I carried out a series of tests using a Full Spectrum camera with different front-of-lens filters. The results give a general indication of the differences between them. To view the results, click the link below.