[kypfer]

I "play" with i/r filters occasionally, mostly with my *istDL2, but occasionally with my CMOS cameras if a particularly attractive cloud formation turns up, and have noticed an "unusual" effect if using my KP.
I take a white-balance reading through the filter from sunlit grass or similar foliage, to emphasise the i/r reflection, and where, on my other cameras, the LiveView results are "as expected", the KP image is a bright green, as though the KP sensor is more green-sensitive than the other cameras!
Basic tests have been conducted which involve using the same lens/filter combination and exposure settings on all bodies, so most variables have been eliminated.
The white-balance readings are taken with a wide-aperture lens to eliminate the [NG] or [The operation could not be completed correctly] messages.
The final monochrome results are usually fine, so no big deal, but I was wondering if there's "something" going on here that may be exploited for other purposes.
Any thoughts or experiences would be appreciated

[UncleVanya]

I "play" with i/r filters occasionally, mostly with my *istDL2, but occasionally with my CMOS cameras if a particularly attractive cloud formation turns up, and have noticed an "unusual" effect if using my KP.
I take a white-balance reading through the filter from sunlit grass or similar foliage, to emphasise the i/r reflection, and where, on my other cameras, the LiveView results are "as expected", the KP image is a bright green, as though the KP sensor is more green-sensitive than the other cameras!
Basic tests have been conducted which involve using the same lens/filter combination and exposure settings on all bodies, so most variables have been eliminated.
The white-balance readings are taken with a wide-aperture lens to eliminate the [NG] or [The operation could not be completed correctly] messages.
The final monochrome results are usually fine, so no big deal, but I was wondering if there's "something" going on here that may be exploited for other purposes.
Any thoughts or experiences would be appreciated

What does the histogram of the raw files look like compared to the same scene using another camera? The jpg preview may just be mislead by the filter in a slightly different way than previous bodies; the raw will tell you more.

[photoptimist]

The red, green, and blue filters are not perfect. Although each color filter type is predominately transparent in its respective color band, they may also be somewhat transparent (not perfectly opaque) in other wavelengths including IR and UV.

It's possible that the "green" filters on the KP's sensor pass more IR than do the filters on the *istDL2. Both camera makers and sensor chip makers adjust the color formulas for better visible light images and for manufacturability. I doubt that either Pentax or Sony pay much attention to IR response of the filter material so that might vary significantly across camera models.

Also, if you are using cameras that still have their IR-cutoff filters in them, then differences in the IR-cutoff filters (e.g., the cut-off wavelength and IR leakage) might cause differences in IR images taken with different cameras.

[Bob 256]

Try stopping the lens down a bit when taking the white balance and see if that makes a difference. I agree with the two previous posts but it might be a saturation issue if the KP has a higher than normal IR sensitivity. Reducing the incoming light (during white balance) might make some difference if that's the case.

[kypfer]

What does the histogram of the raw files look like compared to the same scene using another camera?

Good thought … I'm usually JPG only (having established a combination that works) so I'll check next time I'm out

The red, green, and blue filters are not perfect. Although each color filter type is predominately transparent in its respective color band, they may also be somewhat transparent (not perfectly opaque) in other wavelengths including IR and UV.
It's possible that the "green" filters on the KP's sensor pass more IR than do the filters on the *istDL2. Both camera makers and sensor chip makers adjust the color formulas for better visible light images and for manufacturability. I doubt that either Pentax or Sony pay much attention to IR response of the filter material so that might vary significantly across camera models.
Also, if you are using cameras that still have their IR-cutoff filters in them, then differences in the IR-cutoff filters (e.g., the cut-off wavelength and IR leakage) might cause differences in IR images taken with different cameras.

I was coming to these conclusions … mostly wondering if anyone else had similar experiences and what, if anything, they'd made of them

Try stopping the lens down a bit when taking the white balance and see if that makes a difference. I agree with the two previous posts but it might be a saturation issue if the KP has a higher than normal IR sensitivity. Reducing the incoming light (during white balance) might make some difference if that's the case.

The problem with "stopping down a bit" is that very often, with the filter fitted, the camera won't successfully complete the procedure … I've not gone into the whys and wherefores, just know it happens, so I'll take my reading with my 50mm f/1.7, 35mm f/2.4 or similar, then, if necessary, fit a smaller aperture lens for the required field of view.
At my latitude, 49N, the sun is rarely strong enough to get a reliable i/r white balance reading using f/4 or smaller!


Thanks for all your thoughts … certainly things to consider next time I'm "playing". If I make any positive conclusions I'll report back