Fun with Filters

[Keith Walters]

I’m not sure what the best folder for this is. To me it would seem most relevant to the RED, since that is the only single-chip HD camera currently being used in any quantity, and quite a few problems have been discussed involved standard ND filters and infrared.

 

This is an illustration of the very different properties photographic filters can have under visible light and under infrared.

 

Here we have some test tubes filled with ordinary water, to which I’ve added a few drops of ink from a colour inkjet refill kit. The first three test tubes contain yellow, cyan and magenta inks respectively. (The pictures were shot with a domestic Sony Digital Handycam, so the colour rendition is a little off, but not to worry).

 

The fourth test tube contains a mixture of equal amounts of the contents of the first three, and it looks totally black. This is understandable, since yellow dyes absorb blue light, cyan absorbs red light and magenta absorbs green. Mix the three together and everything gets absorbed, hence, it looks black.

 

Now you could actually make perfectly workable filters by boiling up some gelatin, adding equal amounts of the three inks, dipping a piece of plain glass into the mixture, allowing the excess to run off, and then allowing it to dry. (This exactly how it used to be done in the old days, and was also how they used to make glass photographic plates).

 

You would need to get the dye mixture right, because you want your ND filters to be, you know, “neutral”, but that could be done by trial and error. (Most filter manufacturers use some variation of this approach, rather than just using a single black dye, since it allows them to “fine tune” the colour balance of the final product.)

 

Unfortunately, your new filters will behave somewhat deficiently when Infrared light is involved!

 

This is a photo of the same four test tubes shot with the handycam in the “Nightshot Plus” mode. In Nightshot Plus, the camera has its infrared filter physically pulled away from its image sensor, and a small bank of infrared LEDs is switched on. This allows you to shoot monochrome images in total darkness, so you can take pictures of sleeping babies, wildlife etc.

 

Notice now that the fourth test tube suddenly looks totally transparent! Here’s a couple of close-ups to further reinforce the point. You can clearly see my fingers through the liquid under IR!

 

 

Now imagine what would happen if you made up some gel filters using the same mixture. Suppose you were shooting outdoors and you wanted a shallower depth of field. You might decide to put in 4 stops worth of ND, and open the iris up 4 stops to compensate. If you are using a RED the picture may suddenly start to look strange.

 

 

A 4-stop filter will reduce the light transmission to one sixteenth, allowing about 6% of the light through. Well, 6% of the VISIBLE light. If it was made using the sort of dyes shown above, it would also let through near 100% of the infrared!

 

Suppose the light falling on your subject consists of 6% IR and 94% visible light. After it passes through one of those filters you will be left with about 5% of the visible light, and about 100% of the IR. So the light will now consist of about equal amounts of visible light and infrared.

 

All single-chip colour cameras have an IR filter, but it can only do so much. A more aggressive IR-cut filter will also tend to cut into the red response, upsetting the colour fidelity. A less aggressive filter will be less effective with a strong IR component. A compromize is usually reached which works for most normal light sources.

 

Not too many cameras would be happy with about equal amounts of IR and visible light, which is far from being a "normal" light source.

 

The only solution is to use filters with a linear response down to the infrared region, or use a dichroic “hot mirror” in front of the lens to deflect some of the IR out of the light path.

 

I have done experiments with smoked glass filters. They do definitely work just as well on IR as visible light, and they appear suiaby colour "neutral" but I don't have accerss to a camera that is both troubled by IR and is good enough to accurately assess the resolution hit (if any).

[Keith Walters]

It's important to remember that before cameras like the RED came along, video cameras generally didn't respond to infrared at all and neither did movie film, so there was never any need to engineer an IR blocking characteristic into filters. Generally, whatever IR reduction was there was was completely undefined or possibly non-existent. I don't know why RED haven't explored this market.

[Robert Niessner]

It's important to remember that before cameras like the RED came along, video cameras generally didn't respond to infrared at all and neither did movie film, so there was never any need to engineer an IR blocking characteristic into filters.

 

There exist a lot of video cameras which have trouble with infra red. Just point your remote control into the optics and watch your display.

A lot of Canon and Sony cameras show the IR response to some extend. Especially the Sony EX1/3 are very sensitive to IR, resulting in brown colors on dark gray or black cloths when light with tungsten lights. DSLR seem to have better protection, e.g. my Canon EOS400D does not show any problems with IR. CCDs and CMOS sensors always needed IR cutoff filter.

[John Sprung]

I don't know why RED haven't explored this market.

 

Indeed, as I've said before, we should regard this as a feature, not a bug. Stack up a s--tload of conventional ND, say 3.6 - 4.8, and your Red gives you access to the surreal world of IR. Well worth checking out for nightmares, alien abductions, etc.

 

But remember the three most important things in shooting IR: 1. Test. 2. Test. 3. Test.

 

 

 

 

-- J.S.