----- Original Message -----
From: "BertS" <aasainz@xxxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Sunday, October 01, 2006 3:31 PM
Subject: [pure-silver] Re: estimating filter factor: gray
card?
Shannon Stoney wrote:It might be clarifying to note that the light gathering power of a lens is proportional to its area. Since the area varies as to the square of the diameter f/stops are square law. Opening up one stop, that is, doubling the amount of light admitted by the lens requires increasing the diameter of the lens opening by the square root of 2, or 1.414. Filter factors are given as the amount of increase in light needed to compensate for the filter. This is directly proportional to exposure time but the square root must be used to translate it to iris diameter or stops.In a message dated 9/30/2006 7:35:49 P.M. Eastern Daylight Time, sstoney@xxxxxxx writes:
*"As I understand it, if the filter factor is 2X, you open up a
full stop,"* or you double the exposure time. So, opening up a
third of a stop would be the same as multiplying the time by 1.3.
For example: let's say the exposure at f/64 should be 6 seconds,
without the filter. With the filter, it would be 8 seconds, or 6
times 1.33
Shannon,
No, if the filter factor is 2 x you would open 1.41 stop not 1,
!!!! This is very confusing. Why do they call the filter factor 2 if it's really 1.4?
--shannon
It is not only confusing but also wrong. A filter factor of 2 means that it requires twice the exposure, so you open up one stop. So, if the metered exposure is f/5.6 you would open up to f/4. Notice that 5.6 divided by 5 is 1.4. The ratio of any two consecutive f-stops is 1.4 but that does not mean that you open up 1.4 stops. You open up one stop which is the same as dividing the f-number by 1.4.
So, a filter factor of 2 means give twice the exposure or one stop. A filter factor of 4 means to give four times the exposure or two stops. A filter factor of 8 means to give eight times the exposure or three stops.
So, a 0.30 neutral density filter has a filter factor of two which means it cuts the light down by one half so it has a filter factor of 2. A 0.6 ND cuts the light to one quarter of the original amount so it has a filter factor of 4. etc, etc, etc. When using colored filters things get complicated since the filter factors are approximations and depend on the color of the light, the color of the subject matter and the spectral sensitivity of the film. So, filter factors for colored filters are an attempt to express the absorption of the filter assuming the absorption of the filter as if it were a ND filter for a given light color and an average subject matter color. They are also just starting points and require the photographer to think about what is going on.
Bert
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