Very interesting line of inquiry. I've always been a bit confused about the role of the filter material itself in focus -- my assumption has been that the focus algorithm may be "confused" by monochromatic images; i.e. it may use a combination of channels to calculate focus, and have trouble focusing if one or more of those channels has very little light.

But I hear people talking about filters having physical optical effects, and you mention:

the glass filter should preserve the optics of the original IR block filter

If the filters are not curved at all, how can this be? They also don't change the geometry of the existing lens/sensor arrangement; and since it's rubber-gasketed in place, I had guessed that the precision of its placement was not very important.

Chromatic aberration seems like a reasonable possibility though -- especially with infrablue, where the span of wavelengths (blue vs. IR) is quite wide. With Ned getting good red-filter infragrams, the two channels would be much closer in wavelength, and maybe the lower chromatic difference would make for better overall focus. But even in infrablue images, we should be able to see some difference in focus between the blue and IR channels, unless the focus algorithm just focuses quite evenly halfway between the two channels, in which case they'd be equally blurry.

I was assuming that the best way to preserve image quality when removing a glass filter would be to replace it with another glass filter. Plastic, gelatin, or film filters are not the same thickness, do not have optically smooth surfaces, may not be flat, and may not be internally homogeneous (film and sometimes plastic filters have layers). All those things could influence image clarity. I was also very impressed with the quality of the Superblue photos from Ned's Canon G11 with a glass BG3 filter inside, and was naively hoping to match that with the small sensor and lens of the A2200.

There is some chromatic aberration evident in some of these photos -- a faint blue ghost on the outside edge of objects near the edge of Wratten 25A photos. Further investigation is warranted.

The filter should have nothing to do with the difference in quality between the center and edge of the image since it is uniform in density. Lenses, by their nature do not focus in a plane and thus this is one of the common quality measures for lenses. F stop with film cameras effects depth of field. Just like a pin hole camera, if the aperture is small enough the depth of field is infinite. Thus if we had an infrared sensor with a pin hole for the lens it would be in focus automatically. I do not know if these digital cameras that we are using physically change the aperture or algorithmically accomplish the equivalent. What is needed is to determine 1) the focus algorithm, and 2) the aperture mechanism.

Have you considered watching other sites for cameras that are refurbished? One I would recommend is canonpricewatch.com. It also shows prices for other point and shoots if you might want to look at another one. Just an idea to try and help out.

The aperture mechanism definitely varies among Powershots. By peering into the front of the lens I learned that the high end ones have an actual iris with leaves and can produce an aperture of multiple sizes. The A495 apparently has only two different apertures which seem to be round but I don't know how one replaces the other (elastic?). The simpler cameras have no variable aperture and instead move one or more neutral density filters into the optical path. So f-stop does not influence depth of field in the SD1100 and A2200.

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Okay, the markdown above displays properly in preview mode, but not when published. So...

Camera --- Variable Iris--------ND filter(s)

SD1100------no----------------yes
A2200--------no----------------yes
A495----------yes (2 round holes)---?
SX120--------yes (~6 leaves)-------- ?
S95------------yes (6 leaves)-------- ?
.

Assuming the BG3 glass filter I installed is seated properly, the soft edges in the A2200 photos must be due to different optical properties of the glass compared to the original IR block filter (different thickness or glass density) or a failure to properly focus NIR light compared to visible, or both. First I should open up the camera again and wiggle everything to see if that helps.

There are lots of A2200 cameras for sale for $70, and some for $55, but those don't include shipping. The Canon refurbished store sells them for $70 including shipping, and they have periodic sales. For this week, you get $20 off if you spend $100.

The Canon A2200 (refurbished) is on sale today (November 21) for $48.99 (free shipping) with a one year warranty: http://shop.usa.canon.com/shop/en/catalog/cameras/refurbished-powershot-digital-cameras/powershot-a2200-blue-refurbished (red, blue, or black).

Couple quick thoughts on the filter/clarity issue. For starters, I just ordered an a2200 for the infra blue purposes from eBay, so I haven't had a chance to look at the ir block filter myself yet, but:

1. if the block filter is merely coated with some dye or material to block ir, could it be possible to chemically remove the coating without damaging the filter's glass? If yes, then the ir filter could be placed back in the camera and this should resolve the focusing issue...

2. Alternatively, I read on another thread that the camera developers build in a negative refraction factor into either the firmware and/or sensor hardware to adjust for the known optical refraction of the ir block filter, and therefore removing the ir block still leaves in place this other built-in compensation. If this is the case, and the correction is built into the firmware, perhaps there could be some form of chdk correction to undue the negative compensation?

These ideas occurred to me as I fell asleep last night forcing me to wake up and write them down so I wouldn't forget, haha.

1. Interesting idea. Some IR blocking filters are "hot mirrors" which are coated to reflect IR. Maybe it's possible to remove the coating (but maybe not so easy). Other IR blocking filters are absorbtive glass, so they cannot be modified.

2. Tom Benedict started a good thread at the KAP forum on single camera IR systems and mentioned that:

   "In the case of IR blocking filters, these are fixed characteristics of the optical system at the time the lenses are designed. So most lens designers will design in a little negative spherical to compensate for the IR blocking filter. When you remove the IR blocking filter, you essentially introduce negative spherical into the optical prescription. It helps if you can put that back in. That's why a number of places that do full-spectrum conversions will stick an AR coated BK7 window in front of the detector."

He was referring to optical compensation. Correcting for image distortion in software is possible, but correcting for spherical aberration in software might be tough (how would the software know which of the photons hitting a pixel did not belong there?).

@Cfastie,

That post from Tom was the one I was referring to in my idea #2 but yeah, I must have misread it.

Still, regarding #1, I do understand that some materials are coated and others are actually naturally absorbative, my question is if anyone knows which the A2200 filter is? As for the difficulty of removing a coating, I'd be willing to try it when my camera arrives provided it's a coated filter.

Chris, do you remember/know the dimensions of the A2200 hot filter?

I think most small Powershots use an IR block filter that is 8.9 x 7.9 mm. That's the advertised size of the BG3 glass filter I put into an A2200 and it fit fine. I measured the filter I removed from an old A590 and it was essentially the same: 8 x 9 mm. The BG3 filter was advertised as 0.55 mm thick, and I measured the A590 filter with cheap calipers as 0.36 mm.

I did a request for quotation at Alibaba for AR coated BK7 glass in that size and got some quotes, but they were mostly unintelligible and unhelpful. There is a piece on ebay for $49 that he will cut to size.