[pure-silver] Re: estimating filter factor: gray card?

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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