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 >