Will the real infrared photography please stand up?

b-w_ir_filters

The strange thing about infrared photography is that it represents a very small piece of photography in general, but there is apparently no space in photography so small that it can’t support some form of snobbery. And in infrared photography, it is the idea that there is “near” infrared versus “true” infrared. Not only does this convey a false sense of exclusiveness to people who shoot 850nm and up, it’s also not accurate.

When you shoot a normal camera in daylight, there is a small amount of infrared contamination – it’s about 10 stops less than daylight, or coming in at 1/10 of a percent or 1/1024. Tiny, even on something with big infrared contamination like a Leica M8. So any particular shot is overwhelmingly lit by visible light.

A dark red filter (RG630, 091, 8x, #29, etc.) flips the equation: the average blockage of visible light is 3 stops, or 75%.The reality is that most skylight scenes are predominantly blue, and this filter cuts a lot more than three stops. Even if you are shooting objects that are middle grey, these filters reject 75% of all light – meaning that when you shoot them on a camera with no other IR rejection, deep red and infrared light make up 75%+ of the light. The false color you are obtaining is infrared light that is still being blocked in part by green and blue squares on the Bayer filter on the sensor.

The case for the “near” classification is even weaker with the 695-720nm filter (RG695, 092, R720). First, consider that wavelength ratings on filters are at the 50% mark. So a 720nm filter really starts passing 100% of its light around 750nm. On a short exposure, which you will see is commensurate with a normal visible-light exposure, infrared light is providing almost all of the illumination.

Going the other way, “true” infrared is not that advantageous – and may not be something to commit to in an IR conversion. First, even though the Bayer filter does not affect 830nm+ light, the decoding algorithm in your camera still compensates for it. So if you dump the RAW file into DCRAW, what comes out still has something of a checkerboard pattern. Second, the false color effects generated by mid-band IR actually allow for more contrast control because there are multiple channels of useful information (and with 850nm+, you really need this, since everything likes to come out bright white in sunlight, especially around dusk). Eliminating this effect means that you have less ability to rebalance the tones in a scene.

None of this is to say that it’s good to meet one form of snobbery with another technical one. But let’s just keep the infrared world big, okay?

# # # # #

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: