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Colour temp of a domestic bulb

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    12 Dec 2012 - 3:46 PM

    Hi all,
    Can anyone tell me what the difference is between the Colour temp in image processing software and the new domestic light bulbs.

    In light room, if I shoot an image with daylight WB, the colour temp is 5500k. Raising this number warms up the image and lowering the number cools the image.

    I think I am correct in that.

    Recently my wife bought some of the new type bulbs that turned out to produce a very cold white light. She dident keep the packaging so we dident know what temp they were.

    When I went into the shop the assistant offered me bulbs that were rated 2700k and said that these were very warm coloured.

    Me, being the expertTongue, told her that the 2700kwas a very cool temp and that I needed a higher number something like 6500 or higher.

    I now have even cooler bulbs in my house, with my wife laughing at my expertise.Sad

    Can anyone please explain where I obviously have my thinking wrong?

    To warm up the lighting in a photo, you raise the colour temp.............right?

    So if you want warmer light from a bulb, why is it not the same?

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    12 Dec 2012 - 3:46 PM

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    12 Dec 2012 - 4:01 PM

    When you go into software and adjust the temperature, what you are setting is the value that you believed the colour of light was at the time you took the photo. You are not adjusting the temperature of the photo, you are telling the software what the temperature was when you took the photo.

    If when you took the photo the colour temperature was actually 3000K, and you set it the that the picture will look as it did when you took it.

    If however you set it to 4000K, it would look a lot warmer, so I understand your thinking, but it needs to be reversed.

    When you set the colour to 4000K the software now assumes that the light temperature was this when you took the photo, this is actually a colder temperature, the effect of this is that it renders the image warmer as it assumes it was taken in much colder light.

    Not sure if that makes sense, but I don't know how better to explain it.

    Helpful Post! This post was flagged as helpful
    lawbert  71713 forum posts England15 Constructive Critique Points
    12 Dec 2012 - 4:03 PM

    Had a similar problem at home and manually changed the white balance on the camera to suit the low energy bulbs.
    you can do this by shooting a grey card if Im correct and there must be a setting wb tutorial here somewhere.
    I just set the white balance until I got the nearest to a normal exposure that I would expect under any lighting by firing off a fair few shots and reviewing on the cam screen.
    If you try and adjust the white balance by a huge increment in pp then its pretty much unachievable in my experience....minor tweeks only....Its the age old get it right in camera first and then tweek and play afterwards!!

    Helpful Post! This post was flagged as helpful
    12 Dec 2012 - 4:27 PM

    There is a full explanation on Wikipedia. Gets very technical (as in degree level physics) at the end but the first part is very readable. Suggests that as photographers we need to think in terms of white balance rather than using colour temperature as a an interchangeable term. It took me a while to get my head round this as well.

    Helpful Post! This post was flagged as helpful
    GlennH  91918 forum posts France1 Constructive Critique Points
    12 Dec 2012 - 4:31 PM

    If you were going to use CFL or LED lighting regularly as a light source it'd possibly be worth creating a custom profile as opposed to merely adjusting white balance. This would be to counter the inherent colour problems in those lights, although at this juncture I don't know how successful that would be (I noticed the recommendation in a Jeff Schewe book). Ironically the bulbs that invariably have warm colour temperatures (i.e. incandescent) have much better underlying colour accuracy and can therefore be reasonably corrected with a simple white balance adjustment.

    Last Modified By GlennH at 12 Dec 2012 - 4:41 PM Helpful Post! This post was flagged as helpful
    GarethRobinson e2 Member 8992 forum postsGarethRobinson vcard United Kingdom2 Constructive Critique Points
    12 Dec 2012 - 4:36 PM

    Shoot raw, use a grey card, on import of raws sample the grey card and apply to all images. Job done.

    12 Dec 2012 - 5:06 PM

    Thanks all, very confusing subject, but I think I have a grasp of it now.

    Last Modified By canonfan46 at 12 Dec 2012 - 5:06 PM
    lawbert  71713 forum posts England15 Constructive Critique Points
    12 Dec 2012 - 5:28 PM

    Quote: Thanks all, very confusing subject, but I think I have a grasp of it now.

    Perhaps a like or 2 and maybe the odd green post or 2 may show your appreciation a little moreTongueWink

    Its fantastic to recieve info and very little effort to show a little gratificationTongueWink

    dcash29  81908 forum posts England
    12 Dec 2012 - 7:20 PM

    I use these

    12 Dec 2012 - 7:29 PM

    I'm sorry Lawbert but I dident realise what those things were for. I am an old foggy who, given the lack of the ability to give a gratuity, is used to saying a very sincere thank you for favours received.
    But now that you have pointed out the errors of my way, I will indeed press these buttons for you.
    Thanks again everyone, much appreciated.

    Sooty_1 Critique Team 41207 forum posts United Kingdom198 Constructive Critique Points
    12 Dec 2012 - 10:29 PM

    Most of the confusion centres around terminology.

    Normal daylight is around 5500K (though blue sky/sunny 16 conditions are actually often a bit higher). If you increase the colour temperarure, above this, you are going to the bluer end of the spectrum, and the colour temp looks "cooler" to our eyes, though the Kelvin rating is higher.
    And going the other way, reducing the colour temperature down to say, 3500K is moving towards the red end of the spectrum, and it appears "warmer" because we associate heat and warmth with redder colours.
    It is common to talk of using a 'warm up' filter to make the image more red, though the colour temp is actually being lowered, and so on.

    Older domestic light bulbs are usually in the range 3000-4000K, and we used to use tungsten film indoors (which had a blue bias to counteract the redness of the light). It should be possible to find the Kelvin rating of bulbs to get a more accurate balance, though it can vary with the age of the bulb and how long it is switched on for.

    With digital processing, the same applies...increasing the colour temperature (in LR, PS or whatever) moves the picture towards the blue end of the spectrum, and vice versa. This is also modified by the White Balance selection on camera, or if you choose autoWB in processing, though if you use RAW, you have the option of changing it later during processing.


    Helpful Post! This post was flagged as helpful
    12 Dec 2012 - 10:52 PM

    Thank you Nick, believe it or not, I do understand what you are saying, but it is hard to make it stick in the brain as everything goes against our conditioning with regard to hot/cold and red/blue.
    Much appreciated.

    GlennH  91918 forum posts France1 Constructive Critique Points
    13 Dec 2012 - 7:49 AM

    It's easy to see how you were confused by the Lightroom slider, John, since its blue to yellow scale is a reversal of a regular colour temperature chart, whilst the numerical scale remains the same. It's the worst possible way of picking a light bulb! The colour in LR represents the correction rather than actual colour temperature, which is Adobe's way of trying to make it more intuitive I suppose.

    The reason why white balance adjustments don't work across the board for various types of lighting is simple: white balance assumes everything is equal; it's an adjustment of neutral colour that works best with a continuous spectrum of light. Artificial CFL & LED lighting is anything but predictable in that sense, and so the very subjects people usually take under household lighting (i.e. each other) are likely to look pallid and even a little extra-terrestrial. Taken to extremes, our inability to discern colour under street lighting is a demonstration of the same problem. It's not something a grey card can sort out.

    Helpful Post! This post was flagged as helpful
    13 Dec 2012 - 9:39 AM

    Quote: LR's blue to yellow scale is a reversal of a regular colour temperature chart.............Adobe's way of trying to make it more intuitive I suppose.

    That's the bit I was missing.
    With that last piece of the jigsaw in place, everything is very clear now.

    Thanks GlenH

    GlennH  91918 forum posts France1 Constructive Critique Points
    13 Dec 2012 - 10:29 AM

    I was just reading Martin Evening's explanation of the LR colour temperature slider, and I suppose he'd rightfully disagree with me that the design is an Adobe thing. He gives tungsten film as an example, which is inherently corrected with blue to counteract the warmth of the lighting. So yes, the number (colour temperature) you see in Lightroom is meant to represent the lighting conditions that prevailed when you took the picture, whilst the colour of the slider is indicative of the correction needed for restoring neutral colour.

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