Definition of filter density factors

[QT Luong]

I sometimes see filter densities expressed as 0.3 for 1 fstop, 0.45 for 1.5 fstops, 0.6 for 2 fstops...
Where do these numbers come from, and is there any way to make sense of them (other than
dividing by 0.3 to convert to f-stops :-)) ? For a moment last week-end, I couldn't remember how
to compensate for my 0.45 CF.

[Arne Croell]

It goes back to:

log(2)=0.3

So instead of multiplying factors (1 fstop=factor 2 in time), you add densities. Its a matter of convenience, since adding numbers in your head is easier for some people than multiplying, especially if its not round numbers.

[Doug Meek]

While we're on the subject, I've also noticed that my Lee grad nd's do not have the advertised light reduction effect. My .6 grad nd's (both hard and soft) only have a 1-1/3 stop reduction instead of 2 stops. Additionally, my .9 filters(again both hard and soft) only provide a 2-1/3 stop reduction instead of 3. This isn't proving to be a problem since I know the true numbers and use them accordingly. Does anyone out there know whether or not any other filter makers have grad nd's that are true to their specified numbers?

[James E Galvin]

Density is the logarithm of 1 over the transmission. f stops are also a logarithmic progression, so dividing by .3 is correct. Density .3 is 50% transmission or 1 stop, 1 is 10% or 3.3 stops, 2 is 1% or 6.7 stops etc. Put the density into your calculator, do 10 to the x (the inverse of log) and invert, you get transmission.

[Darin Cozine]

what a pain in the a$$

[Rick Heitman]

Cant you take a spot meter reading and then shoot through your filter to get a #??

[David A. Goldfarb]

You'll notice that film densities are expressed in the same units.

[Ralph Barker]

0.3 = 2 = 1 Gotta love the way the math works out. ;-)

[Karl Amo]

Doug Meek remarked that his Lee ND grad filters do not have the advertised light reduction effect.

I tested mine: 0.6 hard, 0.9 soft, and 0.9 hard. I placed them on a trasparency viewer in a dark room. I measured the light passing through them using a Gossen Digiflash lightmeter in reflective mode. I measured 2 stops, 3 stops, and 3 stops light reduction, respectively.

I guess mine are OK. Whew!

[Doug Meek]

Karl -

I find your results interesting, and am suddenly concerned by my own findings. The way I came up with my figures was to meter an object unfiltered with my Pentax Digital Spotmeter, and then measure the same object again through the grad nd's. My results were consistent: 1-1/3 stop reduction with my .6 grads and 2-1/3 stops with my .9 grads. Is my methodology flawed perhaps? Was my test too simple? Any input from those more knowledgeable than me would certainly be appreciated!

[Karl Amo]

Doug,

Your method sounds OK to me. I would try your method with the 1 % spot mode on my Gossen Starlite, but I loaned the meter to a friend.