It has been over a year since I modified my Pentax K110D camera by removing the internal IR blocking filter. Some new information has surfaced so it is time for an update on the project. I am not an expert, just someone who has enjoyed using a filterless camera which may be of interest to other people. This message is rather long if you are not interested in the subject.
Warning: I am not advising that anyone remove the internal filter in their camera. It is not a project for the faint at heart, and will void any warrantee. The filterless camera will not be good for normal daytime photography.
I’ve been very happy with my K100D purchased in November 2006. In the Spring of 2007 I purchased a K110D with the intent of making a dedicated Astrophotography camera by removing the internal filter. Why a K110D? At $349 it was the lowest priced DSLR at the time. My wife and I have several manual lenses. The sensor and filter are mounted solidly on the circuit board, I did not want to deal with a floating Share Reduction system.
Why remove the internal filter?
Two reasons: Astrophotography and Infrared Photography
Most all digital cameras have an Infrared blocking filter mounted internally. The filter severely blocks IR from the sensor to prevent it from distorting the visible light image the camera is intended for. Many people don’t realize the low cost filter manufactures use also attenuates deep red visible light. The manufactures boost the red response (white balance) but the camera still has weak response to deep red. Astrophotographers know the there are many red nebulas that emit light at the visible wavelength of 656 nm, called Hydrogen Alpha band. My K100D was near dead at that wavelength, from my light polluted location most nebulas were not imageable.
The poor response of DSLRs to IR is well known, requiring long exposures in bright daylight.
How did I removed the filter?
After trying out the new camera a few days I opened it up. It was not easy, but eventually I reached the filter which comes out with four screws. Unfortunately the camera was near dead when re-assembled. The symptoms were exactly like the memory door was open. It took many hours for me to realize the door open switch had been mounted on the main board. I had broken it off so clean I could not see how the door open was detected..
Knowing I was licked I sent the camera to Pentax Colorado with a note as to what I had done and why. It took 5 weeks and $212 but I received the camera back without the filter. I had feared Pentax would, “Do me a favor” and fix the camera to work like new.
I was a happy camper, the total of the camera plus repair was still less than the cost of a professionally modified “Big Two” camera.
My results for Astro?
I’d estimate sensitivity to deep red visible light went up by a factor of four. After years of unsuccessful attempts with a $3000 astro camera my DSLR produced a usable color image of the Rosette Nebula when combined with an external Ha filter. Several other nebulas were now possible in color for the first time.
My results with IR?
Though not my original intention, I realized filter removal will improve IR response. I bought a Hoya R72 filter and found the modified camera to be a delight. Awhile back I posted my results of comparing daylight IR images between my filterless K110D, my normal K100D, and my wife’s istD. The reduction in shutter time is about 100:1. IR photography went from tripod images of 1/10 second to handheld at 1/1000 second or less.
Are there any problems with filterless camera?
Yes. When you have a professional modify a “Big Two” camera for astrophotography, they replace the poor cutoff internal filter with a fast cutoff filter that blocks IR but passes all red. This makes it possible to use the camera normally with some color balance. This was not a concern to me since I had my K100D for daytime shots. At least one company makes an external filter that simulates the response of the original filter, this may be an option to get normal daytime use from a filterless camera. I think it is better just to have two cameras.
If a professional modifies a camera for IR they replace the internal filter with a clear filter. This is because having NO filter affects the focus slightly, usually at the infinity end. I had no problems with focusing on the Moon and stars with either a telescope or my manual Sigma 70-210 mm zoom from K1000 days. I thought perhaps the Pentax was immune to this problem the “Big Two” have. Recently I read the “Digital lenses” are less forgiving than the older film lenses. I slipped on my Tamron 75-300 mm AF lens and found it will not quite focus on stars. It will focus on objects a half mile away, but hits the stop before reaching infinity. This does not bother me, there is no need for autofocus in Astrophotography, but something one should be aware of.
The bottom line?
If I had to do it over I’d do everything again, except perhaps break the camera.