How does the sensitivity of camera sensor behaves with the infrared filters?

Question

I am puzzled about the behavior of camera sensor to the infrared spectrum.

I have heard that the you must remove the internal infrared filter to capture infrared pictures. So, I removed it and used a 760 nm infrared pass filter took some pictures. It looks like this (from Pentax point and shot):

Now, if I am going to put the same 760 nm infrared pass filter to another camera (without removing its internal infrared filter), it also takes the infrared pictures albeit with higher exposure time. Images in this case look like this (from Canon point and shot):

In both of cases, a 760 nm infrared filter was places before the lens of two cameras. The white balance is different but it will give an idea about what is happening. First image is blurry because no glass replacement is done yet to compensate for the internal infrared filter that I removed.

So, first camera is with infrared pass filter and second camera is with infrared pass + infrared cut filter.

My question is: If camera can take infrared pictures without removing internal infrared filter what is changing in these two scenarios? Change in the sensitivity of sensor to the particular wavelength or the overall sensitivity in infrared spectrum? Is the remove of internal infrared filter only reduces the exposure time? What is the overall logic in these two cases?!

Answer 1

The factory installed IR cut filter on most digital cameras is very thin, and whilst it cuts out most of the unwanted IR spectrum, some IR light does get in. Screw in IR pass filters designed to go on the front of the lens are thicker and usually better at stopping light from the visible spectrum.

So what is happening in the second case is that you are using a good IR-pass filter combined with a weak IR-cut filter, so no (or very little) visible light gets through the first filter but some IR light gets through the second, hence you're able to get IR photos.

The strength of the factory fitting sensor IR cut filter varies between manufacturers and models, some have very weak filters and are thus prized by people who want to dabble in IR without going through the expensive or risky filter replacement option.

You will get slightly different frequencies transmitted with both filters in place, but fundamentally the advantage to removing the sensor filter is that exposure times improve (quite drastically if the IR filter was strong to begin with). In some cases it can go from 10 seconds (requiring a tripod) to 1/200s (allowing hand held shooting). With a DSLR not having a visible-light cut filter on the front of the lens allows the viewfinder and autofocus to work as normal, which aids composition and general shooting!