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Thread started 22 Oct 2010 (Friday) 14:03
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AF in infrared light

 
Vermin87
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Oct 22, 2010 14:03 |  #1

General question:

Can Canon's AF system on the 5D mk II or any EOS digital camera focus in pure infrared lighting? Or does it need visible light in order to detect contrast and focus? I know the flashes and even the ST-E2 use that projected red pattern, but that is visible, or is some of it infrared also?


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Madweasel
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Oct 22, 2010 15:09 |  #2

The red focus-assist light is mostly infrared, but as you notice some of it is also visible. A deeper IR filter may well permit you to focus in visible darkness (if you see what I mean), but it would also dim the IR portion, probably limiting the range of useful AF assistance. I've never tried it though.


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apersson850
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Oct 22, 2010 15:34 as a reply to  @ Madweasel's post |  #3

How do you know it's mostly infrared?


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Csae
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Oct 22, 2010 15:34 |  #4

Interesting question, i know theres an IR filter on the sensor, but maybe not for the focusing.

It would be mighty helpful to be able to shine a IR light at something and use it for focusing.

I'm thinking like a concert, i could place a small IR Flashlight right near the performers and shoot from across the stage, the AF assist would never reach that far anyways.


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apersson850
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Oct 22, 2010 15:44 as a reply to  @ Csae's post |  #5

Which sensor are you talking about? The image sensor?

Or are you talking about the "Infrared blocking glass" that's a part of the AF sensor optics? That seems to rule out the possibility to focus with light that's IR only.


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Madweasel
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Oct 22, 2010 15:59 |  #6

apersson850 wrote in post #11146493 (external link)
How do you know it's mostly infrared?

Canon describes it as an infrared AF assist.


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apersson850
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Oct 22, 2010 16:10 as a reply to  @ Madweasel's post |  #7

OK. Since it's clearly visible it's obvious that they are lying to some extent. But since the whole point with IR is that we can't see it, it's impossible to tell how far into the IR spectra the red light on the flashes go, without a different instrument than just the eye.

Anyway, since the AF optics contains an IR blocking filter, it seems reasonable to believe that you can't focus with light that's IR only.
My thoughts are that the reason they use the red LED focus assist on a flash like the 580 EX II are not exactly related to IR or not, but instead for a couple of other reasons:

  • The bursts of "real" flash light other units use are rather annoying, and also consume a bit of the charge level from the capacitor.
  • Red LEDs were the first type that could be designed, and are still about the most efficient.
  • The optics in front of the LEDs make them project a pattern which allows focusing even on surfaces where there is no contrast to detect to begin with.
The last advantage you can try against a white wall in the evening. With the lamps lit, you can't focus on a wall that has no lines or edges. But turn off the light, to make the AF assist in a 580 EX II turn on, and you get a pattern the camera will focus on, with any AF point you prefer.

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AF in infrared light
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