Blocking light An SLR has an infrared blocker on its sensor, so why do you have to add a filter for infrared shots?
QDigital SLRs have a blocking filter in front of their sensor. This wasn’t very efficient in earlier models, but it’s been improved. However, it’s still not 100% efficient in the majority of cases, so some IR wavelengths can get through if you expose for long enough.
An IR filter, like the Hoya R72 shown above, is actually used to block visible light. This then allows some IR light to sneak through during the longer exposure and get recorded on the sensor. So although effectively you have a two-filter system designed to block light, they are working at different wavelengths, and one of them is holding back normal light so enough IR light can get through. When you put the Hoya R72 filter on, it is blocking light up to 720nm, but it is in the wavelength above this where ‘interesting’ IR effects start happening.
With most digital SLRs, you will be able to recreate the classic monochrome infrared appearance when using a Hoya R72 filter. Of course, to get an image to record on the sensor, you will need to slow down the shutter speed.
Using an R72 with my very modern Canon EOS 5D Mk III, I was able to get excellent results. It’s not as easy as using the IR-converted camera I own, because I can use this and expose with almost no obvious differences to a normal camera – except the results of course.
In my example photos, the colour image is shot without any filtration. I manually focused on the tree and set an aperture of f/8 at ISO 100. This required a shutter speed of 1/200 sec in the bright conditions to give the right exposure.
With the camera in exactly the same position on a tripod and focus unchanged, I put the Hoya R72 on the front of the lens. I switched on Live View and changed the shutter speed (I worked in manual) until I could see an image appearing in the screen. In this case, it was at six seconds. Using a cable release, I fired the shutter and the result is the ‘red’ scene.
I brought the raw file into Lightroom and converted it to mono for the final image.