[pure-silver] Re: estimating filter factor: gray card?
- From: "Ralph W. Lambrecht" <info@xxxxxxxxxxxxxxxx>
- To: PureSilverNew <pure-silver@xxxxxxxxxxxxx>
- Date: Sun, 01 Oct 2006 02:23:22 +0200
Shannon
The math shown in the post is not correct. Each doubling is an f/stop. Try:
f/stop = log(filter factor)/log(2)
Filter factors of 2, 4 and 8 must return 1, 2, 3 f/stops respectively.
Regards
Ralph W. Lambrecht
http://www.darkroomagic.com
On 2006-10-01 01:35, "Shannon Stoney" <sstoney@xxxxxxx> wrote:
>> In a message dated 9/30/2006 7:55:15 A.M. Eastern Daylight Time,
>> sstoney@xxxxxxx writes:
>>>
>>> I can't really bracket when shooting large format film.
>>>
>>> I am thinking that although the filter factor is supposed to be 2,
>>> that seems excessive to me given that it only changes the reading by
>>> a third of a stop. I am thinking something like 1.5 would be more
>>> accurate.
>>>
>>> I wonder if it's better to meter a gray card, rather than meter the shadows?
>>>
>>> --shannon
>> I'm joining this thread late and don't really know what went on before
>> but.... the factor indicated by an X on the filter ring and is only a
>> factor, not an adjustment. The adjustment is the Sq. Root of the factor: 5
>> X factor = 2.24 stops. A filter factor of 2 X = an adjustment of 1.41 stops
>> (close enough to your gut feeling). The adjustment can be made in either
>> time or aperture.
>>
>
>
> Actually what I meant was, an adjustment of .33 to .50 stops seems about right
> to me. How would that translate into filter factors? The math doesn't seem to
> work going the other way.
>
> 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.
>
> Here's my plan for figuring out what the filter factor really is. I am going
> to shoot a test scene without the filter, then with the filter but no exposure
> adjustment, then with a factor of 1.33, 1.66, 2.0 etc and see which one is
> closest to the exposure with no filter and no adjustment. This test scene may
> not be exactly like every other scene, but it will be better than nothing.
>
> --shannon
>
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