Wednesday, March 7, 2007

IR for Beginners - Fighting Filters

This time we will talk about Filters. Specifically, filters that are used for photography. Basically, by definition, a filter is a device that will prevent something from passing thru it. In this case various wavelengths of light that would reach the sensor or film in our cameras.

For Infrared Photography, there are mainly 2 different filters to identify and understand.

Infrared Pass Filter – This filter allows (or passes) the wavelength of light in the infrared spectrum. (Remember our first discussion of IR Light). In the process, this filter allows IR, but “filters” or does not allow the visible wavelengths below IR to pass thru it. It is actually a Selective Filter, selecting what is desired to pass, and blocking the rest.

Infrared Block or Hot Mirror Filter – The two terms are used interchangeably, they are referring to the same filter. As you might expect, this type filter allows Visible Light to pass thru it and Blocks the IR spectrum. Again a selective process.

So, the obvious question you might ask goes something like this. “I have a digital camera, and although I really am interested in learning how to shoot IR, I don’t remember seeing either of these filters included in the box with my camera, so what gives?”

Your observation is correct. You didn’t get either of these filters as an assessory in the box, but you do have an IR Block filter. It is installed inside your digital camera right in front of your sensor. They all have them.

The purpose of the IR Block filter is to prevent IR light from reaching the sensor. If the IR light were not blocked, colors would not be rendered accurately by your digital camera. Here is an example of a photo taken with a camera that has been modified to remove the IR Block filter and replace it with clear glass. In this photo you can see the effect that the IR light would have on a color picture. So it’s obvious, your camera, unless you modify it, won’t allow IR light to reach the sensor. (Example Photo by Jules Alexander, used with permission.)

So, you have probably already been thinking, “In order to shoot IR, I must need an IR Pass filter to deliver only IR light to the sensor in my camera”. Right you are, you do need an IR Pass filter to selectively filter out the Visible Color light wavelengths and deliver that beautiful IR image.

But, if you were to add the IR Pass to the camera that has an internal IR Block filter inside, by the definitions that we have described, absolutely no light would be delivered to the sensor, so how are we going to get an IR image out of this combination that gets down to no light passing thru?

This is why I have coined the term “Fighting Filters”. Seems like what we have, right? Each one is blocking out what the other one is allowing to pass thru. OOPS! Houston we have a problem. No light, no picture!

Well, the reality is that neither one of these filters is 100% capable of blocking and passing the light that they are designed to block and pass.

The IR Pass allows the IR wavelengths and blocks MOST of the Visible Color Light, but not all of it.

The IR Block allows the Color Visible and blocks MOST of the IR wavelengths, but not all of it.

So it is possible to have both filters in your light path to the sensor, and if you take a photo with a long exposure time, you can get enough IR light to the sensor to produce an IR Image. The filters fight each other, but each has sufficient weakness to deliver enough IR light to make an image.

We will add one more idea to this lesson, and wrap it up. This is important to your understanding for IR Photography, so get this idea tucked away in the ol’ grey matter.

Not all IR Blocker or Hot Mirror filters are created equal. Some are more aggressive at blocking IR light than others. The “aggressiveness” of the blockers is a matter of what the camera manufacturer chose to include in the design of their camera. Don’t forget, the camera manufacturer’s main goal is to produce the best possible Color image. Period! They are not factoring in the off chance that you might be interested in also using their product for IR.

When people remark that this camera or that camera is more or less “sensitive to IR” what they are really referring to is how aggressive the IR Block filter in their particular camera is in blocking the IR light from reaching the sensor.

In the next post we will talk about how we can add an IR Pass Filter to your stock camera and get an IR Image.

-=- Jerry -=-

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