[pure-silver] Re: estimating filter factor: gray card?
- From: "Ralph W. Lambrecht" <info@xxxxxxxxxxxxxxxx>
- To: PureSilverNew <pure-silver@xxxxxxxxxxxxx>
- Date: Mon, 02 Oct 2006 11:57:39 +0200
The square root function is not used to turn filter factors into f/stops or
visa versa? I think this has caused some confusion before.
The conversion can be done using the following:
x = 2^N
log(x) = log(2)*N
N = log(x)/log(2)
where x is the filter factor and N is the amount of stops.
Regards
Ralph W. Lambrecht
http://www.darkroomagic.com
On 2006-10-02 05:43, "Richard Knoppow" <dickburk@xxxxxxxxxxxxx> wrote:
>
> ----- 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:
>>>> 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
>>
> 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.
>
> ---
> Richard Knoppow
> Los Angeles, CA, USA
> dickburk@xxxxxxxxxxxxx
>
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