Infrared and Motion Detection?

barberofcivil

268

Re: Infrared and Motion Detection?

« Reply #20 on: 26 / February / 2009, 22:22:08 »

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Quote from: waldo on 26 / February / 2009, 20:38:29

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- How does removing the IR filter affect the focus? Is it purely a spacing issue and the filter acts is also a physical spacer (i.e. with it removed, the lens is closer to the sensor)? Or is it a refractive issue through the extra thickness of glass?

The image sensor must be removed to access the IR filter. This means removing the screws that adjust the position of the sensor, both toward and away from the lens and that make the image sensor square with the image path. Putting it back so that it is perfectly adjusted is tricky. In addition, the IR filter affects the focus distance so this must be compensated for.

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- How does the automatic focus work? Does the camera try to detect the distance to the objects to be in focus and adjust according to preset internal settings? Or is it purely based on making the focus area the least blurry (as one would do with manual focus)?

On the small sample of cameras I have tried, the auto focus will try different focus settings over a range of possible settings and picks the one that gives the sharpest image. If the sensor is not properly positioned, the auto focus will never move the lens far enough to ever get to the proper setting. If the sensor is not square with the image path, you will get randomly focused pictures since one side of the image is focused differently than the other.

OK. So it is a physical spacing issue. I wasn't sure because I dissassembled one of my old unworking cameras before (Minolta) and the IR filter was pretty easy to remove and wasn't mounted such that it acted as a spacer.

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tgran

12
A570IS (101A)

Re: Infrared and Motion Detection?

« Reply #21 on: 12 / March / 2009, 00:57:51 »

Did disassembling the A590is require desoldering/soldering? Any pictures perchance?

I've seen some people stack microscope slip covers to get the right thickness.

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Chelidon

19

Re: Infrared and Motion Detection?

« Reply #22 on: 17 / April / 2009, 16:10:41 »

Quote from: barberofcivil on 26 / February / 2009, 22:22:08

OK. So it is a physical spacing issue. I wasn't sure because I dissassembled one of my old unworking cameras before (Minolta) and the IR filter was pretty easy to remove and wasn't mounted such that it acted as a spacer.

No, It isn't only a matter of distance and spacing. Because of the refractive index (n) and the thickness of the glass, it's an optic issue, there is a formula to find the difference in focus:

OutOfFocusDistance = (n - 1) / n * ThicknessDifference
(you can find explanaition in many page about cameras mod like this: http://astro.ai-software.com/articles/mod_350D/mod_350D.html).

Furthermore you have to consider the back-shift of the focus as the wave lenght increase, so even with the same thickness in the replacement glass you will notice different focus distance (infinite focus come earlier); in fact in some cases a replacement glass just a bit thicker may balance the opposite trends and give a near perfect focus...

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mgxxx

14

Re: Infrared and Motion Detection?

« Reply #23 on: 18 / April / 2009, 05:52:01 »

does anyone know at what wavelength the IR filter cuts in at? I guess it may vary between models but I haven't found an answer yet by searching the net.

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Chelidon

19

Re: Infrared and Motion Detection?

« Reply #24 on: 18 / April / 2009, 12:10:17 »

If you mean the ir-cut filter on the sensor then it's easy to find something about, even if the situation could vary a bit case by case: surely it starts cutting a little from the red spectrum. Thus in astrophotography it is usually removed: the main red source in the deep sky object is the Hydrogen alpha-line (here you can find an example of the band-pass http://sites.google.com/site/vincenzomiceli/pentax--ist-d-ir-mod).

Instead if you are referring to the ir pass filters, there are tables on the net in wich you can find more or less their lambda-cut to have an idea of the equivalent between different vendoor..

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