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IR question

Discussion in 'Black and White' started by rklepper, May 26, 2013.

  1. rklepper

    rklepper Mu-43 Top Veteran

    733
    Dec 19, 2012
    Iowa, USA
    Robert
    So I really admire the ir shots I have seen. I really want to be able to do some. I am hoping someone can answer a couple of questions I have about them.

    1). I have read that not all cameras can be used for ir photography. Is is true and if so is there any way to know? Are there some cameras that are best to use?

    2). I see that some convert cameras to do ir and some just use filters. Does one block more other types of light? What is the real benefit of one over the other?
     
  2. Petrochemist

    Petrochemist Mu-43 Top Veteran

    651
    Mar 21, 2013
    N Essex, UK
    Mike
    Unconverted cameras have a 'hot mirror' in front of the sensor, which is designed to block infra red and ultra violet. Different cameras have different types of hot mirror, so that some are more aggressive in their blocking, but all will transmit some IR.
    If you're trying to use an unmodified camera with a strong hot mirror you will need very long exposures, and the range of infra red filters that will give proper effects is more limited. I can see through my 720nm filter on a bright day but on my Pentax k100d (which I've kept mainly because it has a fairly weak hot mirror) gives good monochrome infra red images with it (and can be hand held with a fast lens). When I tried it on my K7 (stronger hot mirror) a tripod was essential and the infra red effect was less convincing, the image being considerable more from the traces of visible light that leak past the filter. I've only had a brief try with the G1 (the weathers not been really hot since I got it) I've been able to hand hold it with fast lenses but I was using weaker IR filters and the results were not convincing.
    I believe it is possible to record IR with any camera, but those with weaker hot mirrors are much better. Very careful optimisation of filter may be required if the camera has a strong hot mirror, unfortunately I don't know of any camera maker that actually discloses the type of hot mirror installed, but older cameras are likely to have weaker hot mirrors.

    Conversion involves removing the hot mirror and replacing it either with a piece of silica, or a infra red filter of some sort.
    The Silica option transmits IR, visible & UV, giving a full spectrum conversion. Filters on the front of the lens can then determine which combination the camera sees - to record visible only you need a hot mirror in front of the lens, for UV you need to block visible & IR (typically needing 2 filters), while IR can be recorded without visible if an IR filter is fitted. It is also possible to record combinations of the different parts of the spectrum, a normal UV filter will leave visible & IR, or no filter gives all three together. LOADS of options for experimenting, but if you have many filter sizes the filters will get awkward.
    The infra red filter conversions leave the camera unable to see visible light, but easily give IR images with any lens fitted (some lenses will produce 'hot spots' though).
    Converted cameras no longer having a hot mirror allow much more infra red to reach the sensor, so that shutter speeds are similar to everyday visible photography.

    Fortunately very cheap IR filters are available, so it doesn't cost a fortune to have a go. It's not worth trying filters below 720nm with unconverted cameras though - to much visible will be transmitted.
     
  3. rklepper

    rklepper Mu-43 Top Veteran

    733
    Dec 19, 2012
    Iowa, USA
    Robert
    Thanks that gives me a lot to think about. I think I will start with a filter and go from there.