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I have a D3s with no infrared filter (it has been removed). When I take dark images in high ISO with body cap on and covered in a wrap (so no light gets in), and no lens mounted, the images are not dark but surprisingly bright.

Is it because of internal heat and/or infrared radiation? Can I do something about it? enter image description here

Here's an image with IR filter on and same conditions (ISO:51200, Expo. time 1/400 s) enter image description here

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  • That you are referring to the body cap and have no lens mounted is potentially very useful information to anyone trying to answer your question. I have proposed an edit to make this clearer, based on your comment in response to Michael Clark's answer. – a CVn Feb 8 '16 at 17:42
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    Is the viewfinder also fully shielded from stray light? – HamishKL Feb 8 '16 at 22:27
  • Is there the possibility of taking the same exact shot under the same conditions using a D3s with intact IR filter in order to compare the two results? – Michael C Feb 9 '16 at 0:12
  • There's no way light can get through as I covered the camera pretty well and the viewfinder is also closed and shielded. – faf Feb 9 '16 at 9:45
  • I have already took same pictures with same conditions with the IR filter on and they were completely dark (except for some dark noise, obviously). – faf Feb 9 '16 at 9:47
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It seems that with the Nikon D3 series (D3, D3x, D3s), the D4 series, the new D5, the Df, the D700, and the D7000 a small infrared emitting diode is used inside the light box while the shutter is open as part of an infrared self-diagnostic shutter monitor circuitry used to confirm shutter timing and function.

When the standard IR filter is on the front of the sensor stack as is the case for all models when they leave the Nikon factory the IR light emitted by the diode is prevented from reaching and being recorded by the imaging sensor.

But when the IR cut filter is replaced with a piece of clear glass to enable infrared photography the IR light emitted by the shutter control system is detected by the sensor and at higher than base ISO and longer exposures it is potentially enough to make the image unusable.

With the camera models listed above that have undergone an IR conversion, staying at base ISO and limiting the length of exposures is necessary to avoid the light emitted by the diode in the infrared self-diagnostic shutter monitor system from unduly affecting the image.

Edit: the following was basically grasping at straws in the dark prior to the discovery of the above information.

Most of what the camera is picking up might be the heat of the body cap itself. Try placing the lens cap in a freezer for several hours, then placing it on the lens and taking a shot. How much darker is the image?

Also, are you blocking stray light from entering via the viewfinder? Even with the mirror in the up position sometimes light can leak around the edges of the mirror. The light entering via the lens during a typical exposure will overpower any such stray light under normal circumstances. But with this experiment you're also capturing near-infrared and there's no visible light coming through the lens to compete.

Also keep in mind that with infrared, visible light that makes it into the mirror box via the viewfinder or an uncapped mounting flange prior to when the shot is taken will create a miniscule amount of heat in the surfaces it falls upon that may affect an exposure made at high ISO and no visible light striking the sensor. Even if the viewfinder and mounting flange are blocked during the exposure, it will take a little while for the small differences in heat to dissipate and for thermal equilibrium to occur within the mirror box.

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  • When I remove the cap hand just hold my palm on the aperture, I get the same shape as above, I don´t think cooling the cap would help, because the internal body of the camera is still a bit above room temperature. P.S. No lens is mounted. – faf Feb 8 '16 at 16:34
  • Your palm is likely warmer than the cap... Heat is infrared radiation. – Michael C Feb 8 '16 at 22:51
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    What the camera sees is near infrared radiation- just beyond the visible spectrum. It can't see heat radiation (Long-wavelength infrared) - you need special sensors for that. Perhaps you're seeing leakage through your viewfinder or the body cap. Just because something is visually opaque doesn't mean it's infrared opaque. How long is the above exposure? Could be electronic/sensor/readout noise and have nothing to do with imaging per se. – BobT Feb 8 '16 at 23:08
  • @BobT Yes the strongest heat radiation is long infrared, but just as visible light spills over into near infrared at much lower energy levels, so does heat spill over into visible light in the same way. That's why you can see a electric stove eye or an infrared radiant heater glowing when it is hot. – Michael C Feb 8 '16 at 23:35
  • @BobT The unevenness of the distribution of energy in the sample image strongly suggests this is more than read noise. There is way too much difference between the brightest and darkest parts. – Michael C Feb 8 '16 at 23:37
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"According to the information from Nikon, this is a well-known effect, because of the IR-sensor of the shutter, which is emitting minimal (IR)light."

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