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If I choose to remove the hot mirror off the sensor of my D70, should I absolutely drop in an IR filter in it's place? I understand that dropping an IR filter shall put an end to all visible photography.

What is the best choice I can make to keep the camera versatile?

http://www.lifepixel.com/ has a clear filter (UV+Visible+IR)that can be dropped in and external screw in filters may be mounted on the lens for the desired effects. Additionally, they have a choice of 4 IR filters that mount on the sensor. What are the advantages of use the sensor mount filter? It seems choosing one of the 4 limits the kind of pictures that I can take.

What happens if I just remove the hot mirror and do not replace it with anything and just choose to use screw on IR filters?

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3 Answers

up vote 7 down vote accepted

You basically have three options:

  • Hot mirror in front of sensor (e.g. stock camera)
    • Only good for visible light, IR exposures are possible with a lens-mounted IR filter, but exposure times are on the order of minutes.
  • Cold Mirror (e.g. IR Only filter on sensor) - Camera is only good for IR Photography.
    • If you have this professionally done, (or are into DIY), this will involve recalibrating the focus sensor for the new filter and your lens's IR focusing offset.
    • This is definitely the easiest to use - the viewfinder remains useable at all times, and the AF will always set the focus correctly.
  • All-Pass filter on sensor - Camera is good for IR and Visible lights, but there are caveats to using in either mode.
    • You will have to always have a hot mirror (IR Filter), on the front of your lens, or you will get unusual colors/overexposure from IR light in visible light photos. This will likely entail buying a hot mirror for every lens you own.
    • Shooting IR requires a cold mirror, and it has to go on front of the lens. Therefore, you will not be able to use the viewfinder when shooting IR.
    • The camera's AF sensor has a separate IR Filter. Therefore, AF will not work when a cold mirror is on the lens. Every IR shot will have to be composed and focused in visible light, and then you will have to mount the IR filter to the lens, and take the shot.
    • You can calibrate the AF sensor for either visible light or IR light. In one shooting mode, you will have to dial a certain amount of focus compensation in. This is simple (basically, you just shift the focus ring by a known number of degrees), but you have to do it ever time.

I strongly recommend having separate camera bodies for visible and IR. A clear (allpass) filter involves many compromises, and is generally a pain in the ass. the only reason I think it could be a good idea is if some bizarre situation means you can have only one camera body with you.

A note on focus correction:

The AF can be corrected for one or focus offset. This is why places like lifepixel ask for you to send the lens you plan to use the camera with to do focus correction. Basically, the way it works, is the lens either front or back-focuses IR by a certain percentage (this is what the IR focus mark on some lenses shows).

Correcting the focus involves basically inserting a corresponding amount of front or back focus into the AF system by physically moving the AF sensor using it's adjustment screws (e.g. how the focus is tuned at the factory, and what they change when you send in your camera to have front/rear focus issues corrected.

The end result is a camera that always front or back focuses by a certain amount. However, this focus offset is the opposite of the IR focus offset, and the two cancel out.

The camera is still focusing using visible light. However, because of the offset, it ends up with the focus set correctly for the lens it is compensated for.
Therefore, if you use a different lens, with a significantly different focus offset, or a telephoto where the focus offset changes over the zoom range, it will be blurry. However, you can get tack-sharp results with the original lens.

The only way to have a IR camera focus correctly with any lens in IR is to remove the hot mirror from the AF sensor, and no one on the market does this service. I removed the hot mirror from the AF sensor on my D80, but it's a really involved process, and I nearly broke the thing in doing so (it's glued together).


In the comments on imre's answer, Matt Grum seems to be confusing the sensor cover glass with the sensor filter. These are separate elements. There are some cameras that combine the two (The Sony Alpha A200, at least), but these cameras are basically impossible to convert, unless you have access to a cleanroom. All the other cameras have separate pieces of glass.

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I don't think I am confusing anything, the company I was referring to is called MaxMax and they do use a cleanroom and will even remove the Bayer colour filter array from a sensor! –  Matt Grum Jun 25 '11 at 12:32
    
@Matt Grum - Removing the bayer filter is a very different process from just removing the IR filter. –  Fake Name Jun 26 '11 at 2:33
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If you remove the IR-blocking filter and do not replace it with anything, you will be removing a section of glass from the optical system. As a result, your viewfinder and auto-focus will be out of calibration; removal of refracting glass makes the light path between lens and sensor slightly longer and as a result some lenses will not focus to infinity on large apertures. Your sensor will be more vulnerable to various particles in the air and cleaning - any possible damage will happen to sensor, not the interchangeable filter designed to be cleanable. To avoid these pitfalls and use your camera for full spectrum, you should use clear glass with similar refractive index as replacement for the infrared filter.

An advantage of sensor-mounted filter is that you can use optical viewfinder for composing your photo. If you use an infrared filter in front of lens, the optical viewfinder will also show only the (invisible) infra-red light. Since your D70 does not have Live View, you will essentially either be screwing the filter on and off between shooting and composing, or be composing without using the viewfinder.

Another advantage is that even if you have lens with different filter sizes, you will not need several filters or a filter system.

Note that infrared light focuses on a bit different plane than visible light - is's always an issue with different wavelengths and lenses are usually corrected to focus visible light wavelengths in within the range of depth of focus. So you should either have your AF re-calibrated for IR (if you have installed IR filter on sensor) or compensate focusing when shooting IR (read more here).

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@Imre - Thanks. That was helpful. I'd have voted up the answer if I had enough points to do so. –  Lord Loh. Jun 24 '11 at 22:06
    
Can you explain a little more how the IR filter affects focus? Isn't it just flat glass? –  mattdm Jun 24 '11 at 22:24
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Furthermore, removing the filter does not leave the raw CCD/CMOS sensor exposed to the air. All sensors are manufactured with a glass cover permanently bonded to the CCD case. Some sensor include the IR filter in the CCD cover glass, but this renders the IR filter effectively unremovable outside of a semiconductor lab. All other cameras have the filter as a separate element. –  Fake Name Jun 25 '11 at 7:16
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Lastly, focusing with an uncompensated AF sensor is pretty easy, but it is more work. Basically, you let the camera focus normally, and then turn the focusing ring a small amount in one direction. On my Nikon D80 conversion, it was about 10°. Depending on your lens (and how zoomed it is, if it is a telephoto), the focus offset may vary, but at the same zoom length, it is the same for any focal distance. –  Fake Name Jun 25 '11 at 7:17
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All sensors (except really exotic stuff) have a permanently bonded cover glass. Some camera's use a hot mirror deposited onto the cover-glass (a hot mirror is a special coating, not actually a type of glass, it's just usually deposited on a piece of glass for convenience, using vacuum deposition), which effectively renders them unconvertable to IR, but there is still a piece of cover glass. –  Fake Name Jun 25 '11 at 7:49
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In my opinion by far the best way to do IR photography is to have the IR blocking filter removed from the sensor and a visible light blocking filter installed.

If you leave the sensor filterless, so it sees all wavelengths then you will always need a filter on your lens which will cause problems with AF and obstruct your view from the viewfinder.

If you do go the dedicated IR route, then yes that is the end of visible light photography for that camera. But you do get an IR cam that's easy and fun to use, no need for a tripod (like a stock camera with visible light blocking filter on the lens), you can still use autofocus (though accuracy suffers a little) and compose shots unhindered through the viewfinder. I took mine out last weekend on a whim and shot this:

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Great shot, Matt. –  Damian Powell Jun 13 '12 at 17:34
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