First time playing with IR
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Old 02-06-2017   #1
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Default First time playing with IR

I recently bought a cheap little sports camera clone and have been playing with it. First I bought a replacement lens and took the UV/IR cut filter off the back and then I bought a zoom cctv lens which had no UV/IR cut filter. I took the shots below using the latter lens with a 950nm IR filter held on the front of the camera.
I was pleased how they came out for such a cheap rig but have a question.
I set up the focus on the lens by viewing images in normal light or by connecting the camera to a monitor and then setting the focus by screwing the lens in or out etc.
I seem to end up with pictures where the focus looks ok in normal light but seems to more blurred when using an IR filter.
Is the apparent focus shift to be expected considering the shift in wavelength of most of the light when using the IR filter ?

Brian V.



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Old 02-06-2017   #2
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Default Re: First time playing with IR

I suspect that you are right about why the focus is a bit out, although the second one doesn't look that bad. Normal light runs from about 380nm to 700nm, so your filter is picking off light that is 250nm above visible light and thus it should focus at a slightly different point than that of visible light. You do not mention any details about the technical aspects of the exposure and whether it was tripod mounted using a cable release, etc. This would be nice to know for a complete consideration of your question.

I am also a bit curious because I would think that a 950nm filter would produce very stark black and white images and yours are not. I am wondering if this is an indication of how it was processed?

WesternGuy
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Old 02-06-2017   #3
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Default Re: First time playing with IR

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Originally Posted by WesternGuy View Post
I suspect that you are right about why the focus is a bit out, although the second one doesn't look that bad. Normal light runs from about 380nm to 700nm, so your filter is picking off light that is 250nm above visible light and thus it should focus at a slightly different point than that of visible light. You do not mention any details about the technical aspects of the exposure and whether it was tripod mounted using a cable release, etc. This would be nice to know for a complete consideration of your question.

I am also a bit curious because I would think that a 950nm filter would produce very stark black and white images and yours are not. I am wondering if this is an indication of how it was processed?

WesternGuy
Thanks for the comments WG
No the camera was handheld but seems to have a sensor very sensitive to IR - the apparent shutter speed was logged as around 1/900th sec assuming that means anything (it's an electronic shutter). The only PP I did was to use auto colour in photoshop which turned it from rather purple to more black and white.
Although the 950nm filter looks almost totally black the shutter speeds I saw only halved when I used it at the same ISO setting.
I've only seen very slow shutter speeds when shooting it in the dark with a fairly weak IR LED torch.

I will have to try setting the focus with the IR filter on and see how that goes.
The sports cam itself only cost £35 so I'm surprised at how well the shots turned out The tiny sensor is 14mp for stills.

Brian V.
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Old 02-06-2017   #4
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Default Re: First time playing with IR

Interesting colour in the shots. Surprising shutter speed. Be careful. IR can be addictive.

Chuck
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Old 02-06-2017   #5
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Default Re: First time playing with IR

Some of the older vintage digital cameras didn't have very effective infrared blocking filters, such as my Coolpix 950. Olympus and Sony digital cameras of late 1999 and early 2000s vintage had a similar problem, so are capable of handheld infrared shots when fitted with filters with an alpha cutoff wavelength of about 720 nm. These filters aren't completely black and allow some of the longer red visible light waves through, though they basically turn these cameras into infrared cameras. Later cameras have better IR blocking and don't work as well without some surgery to remove the infrared blocking filter. Typically, these cameras must also have their focus readjusted a bit so they can focus in the near infrared rather than the visible part of the spectrum.

Some lenses, especially the older prime lenses, have a red mark for infrared use. The lens is focused with visible light, the IR filter installed, and the focus is shifted so that the number that was formerly under the main index mark is now under the red IR mark instead. Lenses for which some kind of IR adjustment has not been made will often show some focus misadjustment when used for infrared photography.
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Old 02-17-2017   #6
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Default Re: First time playing with IR

the main reason why you are having focus issues particularly at infinity focus is because you removed the hot mirror/low pass filter in front of the sensor of your action cam. naturally the focus will shift. this is more evident if you put a wide angle lens in front of the sensor. you can test it on those zoom lenses (cctv zoom lenses) and the camera will be able to focus when zoomed in but cannot focus to infinity when the lens is at it's widest.

an alternative is to adjust your sensor to make it as close as possible to the lens mount (depend on the thickness of the removed hot mirror or by replacing the hot mirror with a clear glass preferably the same thickness as the hot mirror you are replacing. it doesn't need to be the same thickenss as long as the sensor distance to the lens mount can be adjusted.

i have the same problem with a DIY converted panasonic GF2 wherein i removed the hot mirror and the dust reduction filter without replacing any clear glass in fornt of the naked sensor. i did however adjusted the sensor closer to the lens mount. im using manual adapted lenses and so far i have no issues with zoom lenses even with IR 720nm filters infornt of the lens, but when it comes to an adapted kit 18-55 kit lens, it is blurry at 18mm but tend to be sharp mid zoom (around 28mm).

i have already cut a clear glass out of an old filter and hopefully i can open up the body again this weekend and put the clear glass in front of the sensor and see how it would help in focusing to infinity for wide angle adapted lenses
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Old 02-17-2017   #7
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Default Re: First time playing with IR

Quote:
Originally Posted by scoundrel1728 View Post
Some of the older vintage digital cameras didn't have very effective infrared blocking filters, such as my Coolpix 950. Olympus and Sony digital cameras of late 1999 and early 2000s vintage had a similar problem, so are capable of handheld infrared shots when fitted with filters with an alpha cutoff wavelength of about 720 nm. These filters aren't completely black and allow some of the longer red visible light waves through, though they basically turn these cameras into infrared cameras. Later cameras have better IR blocking and don't work as well without some surgery to remove the infrared blocking filter. Typically, these cameras must also have their focus readjusted a bit so they can focus in the near infrared rather than the visible part of the spectrum.

Some lenses, especially the older prime lenses, have a red mark for infrared use. The lens is focused with visible light, the IR filter installed, and the focus is shifted so that the number that was formerly under the main index mark is now under the red IR mark instead. Lenses for which some kind of IR adjustment has not been made will often show some focus misadjustment when used for infrared photography.
Quote:
Originally Posted by fatoldbaldguy View Post
Interesting colour in the shots. Surprising shutter speed. Be careful. IR can be addictive.

Chuck
Quote:
Originally Posted by moodless View Post
the main reason why you are having focus issues particularly at infinity focus is because you removed the hot mirror/low pass filter in front of the sensor of your action cam. naturally the focus will shift. this is more evident if you put a wide angle lens in front of the sensor. you can test it on those zoom lenses (cctv zoom lenses) and the camera will be able to focus when zoomed in but cannot focus to infinity when the lens is at it's widest.

an alternative is to adjust your sensor to make it as close as possible to the lens mount (depend on the thickness of the removed hot mirror or by replacing the hot mirror with a clear glass preferably the same thickness as the hot mirror you are replacing. it doesn't need to be the same thickenss as long as the sensor distance to the lens mount can be adjusted.

i have the same problem with a DIY converted panasonic GF2 wherein i removed the hot mirror and the dust reduction filter without replacing any clear glass in fornt of the naked sensor. i did however adjusted the sensor closer to the lens mount. im using manual adapted lenses and so far i have no issues with zoom lenses even with IR 720nm filters infornt of the lens, but when it comes to an adapted kit 18-55 kit lens, it is blurry at 18mm but tend to be sharp mid zoom (around 28mm).

i have already cut a clear glass out of an old filter and hopefully i can open up the body again this weekend and put the clear glass in front of the sensor and see how it would help in focusing to infinity for wide angle adapted lenses
Thanks again for the comments and help - appreciated

Moodless, the focus shift I mentioned only occurred when I put the IR filter in front of the lens. I had already focused the lens in normal light as these lenses have no preset focus distance- you screw them in until you get a sharp image.

Brian V.


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