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ND Filter Guide Numbers

AdrianTurner 14 350 Wales
14 Jul 2005 5:06AM
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?

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

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.


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

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baxter 13 164 England
14 Jul 2005 6:59AM
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 15 8.2k 1 England
14 Jul 2005 7:15AM
Must admit I wondered about that myself, now I know. Smile

AdrianTurner 14 350 Wales
14 Jul 2005 7:15AM
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!



Kris_Dutson 15 8.2k 1 England
14 Jul 2005 7:22AM
Why they just can't label them 1 Stop, 2 stop...tickle him under there, I shall never know.

AdrianTurner 14 350 Wales
14 Jul 2005 7:23AM
They couldn't do that Kris, everyone would understand and forum activity would decline.

Kris_Dutson 15 8.2k 1 England
14 Jul 2005 7:26AM
Lol, true..

chriswebb 13 893 United Kingdom
14 Jul 2005 8:20AM
So what is the thinking behind Nikon making an ND400?
Kris_Dutson 15 8.2k 1 England
14 Jul 2005 8:37AM
That's Nikon's way of saying lens cap Wink

chris.maddock 17 3.7k United Kingdom
14 Jul 2005 8:55AM
chriswebb 13 893 United Kingdom
14 Jul 2005 8:59AM
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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