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ND Filter Guide Numbers

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    AdrianTurner
    14 Jul 2005 - 5:06 AM

    Can anyone explain why different filter brands publish different Guide Numbers for filters which achieve the same reduction in luminosity? Is the numeric part just an identifier pertinent to that manufacturer or does it relate to a standard? If the later is true then are there more than one standard or perhaps more than one scale i.e. celsius / fahrenheit?

    COKIN:
    ND2 = 1 STOP
    ND4 = 2 STOP
    ND8 = 3 STOP

    LEE:
    ND0.3 = 1 STOP
    ND0.6 = 2 STOP
    ND0.9 = 3 STOP

    I know that its the result that counts, its just been bugging me that I dont know why they are described differently.

    Adrian.

    P.S. did a search, couldn't find a relevant answer.

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    14 Jul 2005 - 5:06 AM

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    baxter
    baxter  9164 forum posts England
    14 Jul 2005 - 6:59 AM

    ND2 needs 2x light if filter wasn't fitted=1 stop
    ND4 needs 4x light if filter wasn't fitted=2 stops

    The Lee method comes from Log values where for reference Log (base 10) 10 =1.0 and Log(base 10) of 100 is 2.0. The Log is the power the number is raised to ie 10 to power of 2 is 100 and 10 to power of 1 is 10 and 10 to the power of 0 is 1.

    So therefore to 1 decimal place
    Log(base 10) 2=0.3
    Log(base 10) 4=0.6

    Playing with the power button on a calculator will make it easy to see what you are doing. You might also look for the log button, but beware there are two base numbers used, base 10 and another - e. I don't want to go into this!!!!!

    Decibels for noise work in the same way. 3dB is twice as loud (deci means tenths).

    Hope this helps.

    Kris_Dutson
    14 Jul 2005 - 7:15 AM

    Must admit I wondered about that myself, now I know. Smile

    Kris.

    AdrianTurner
    14 Jul 2005 - 7:15 AM

    Cheers Baxter, long time since I did anything with logs (except burn them) but I get the idea. So, as I thought, just different ways of expressing a value.

    " ! "

    They could make it easier for all to understand and express it all as HEX values!

    lol

    ;0)

    Adrian.

    Kris_Dutson
    14 Jul 2005 - 7:22 AM

    Why they just can't label them 1 Stop, 2 stop...tickle him under there, I shall never know.

    Wink

    AdrianTurner
    14 Jul 2005 - 7:23 AM

    They couldn't do that Kris, everyone would understand and forum activity would decline.

    ;0)

    Kris_Dutson
    14 Jul 2005 - 7:26 AM

    Lol, true..

    Kris.

    chriswebb
    chriswebb  9893 forum posts United Kingdom
    14 Jul 2005 - 8:20 AM

    So what is the thinking behind Nikon making an ND400?

    Kris_Dutson
    14 Jul 2005 - 8:37 AM

    That's Nikon's way of saying lens cap Wink

    Kris.

    chris.maddock
    14 Jul 2005 - 8:55 AM

    ROTFLMAO Wink

    chriswebb
    chriswebb  9893 forum posts United Kingdom
    14 Jul 2005 - 8:59 AM

    No doubt the world's most expensive lens cap.

    I can see it could have a use, for example if you need an exposure of 1 second to blur movement but the correct exposure is 1/500 for your chosen aperture, or if you need an aperture of 1.4 to minimise depth of field but are getting f22 for your chosen shutter speed, but I wonder why they chose a factor that doesn't work out to an exact stop, such as 256 or 512. Also, it is a bit extreme; you would think there would be a need for one or two intermediate values.

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