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salva
Joined: 20 Mar 2003 Posts: 29

Posted: Tue Oct 14, 2003 1:49 am Post subject: 


Hello I need some help whit ND filters.
I wonder if you can give me some advise.
I want to use a 4x5 film rated ISO 100 and I wont to reduce the ISO factor under 1 ISO (minimum 1 ISO).
I want to shot 90 minutes (aprox.) minimum 1 hour. Off course will depend also of the light available (exteriors).
I know some people have use a 16x ND filter to get such long exposition.
I was looking for ND filters and I find the B+W ND filter series, they are the only one I find whit high factor.
Bud I get a little confuse whit the filter factor and the fstop reduction.
which filter I should use? a B+W 106, 110, 113, 120?
This is the link of B+W
http://www.schneideroptics.com/filters/filters_for_still_photography/neutral_density/more_information/
I will really appreciate if you have some experience to share.
By the way, which holder system you recommend, certainly I will use other filters like Graduated ND, Polariser, B&W and temperature correction.
I shoot 4x5 and my first lens is a Rodenstock SironarN 210/5.6 MC if someone know what is the filter size for this lens I would really appreciate.
Thank you very much Salvador 

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Les
Joined: 09 May 2001 Posts: 2682 Location: Detroit, MI

Posted: Tue Oct 14, 2003 2:59 am Post subject: 


I'm not going to ask why you want to, I'll just try to get you there.
Lets see how many stops it takes to get to ISO of 1.
Every time we cut the ISO in half, we lose one stop, so
100 50 25 12 6 3 1.5 .75 .37 .19 .085
Okay so a 10 stop reduction gets us to ISO of 0.085
One stop of density is measured out as .3 so 10 stops will be 3.0 or as B&W calls it a 110.
Now what's our exposure in bright sun?
well the sunny rule of f16 says "bright sun on sand or snow...f16 @1/ISO so.....
with out any ND we'd be at 1/100 @f16. So at 10 stops we are at f16 at 1/ 0.085 or 11.76 seconds.
That's not even close to 90minutes! Let's add another 10 stops of ND ...
ISO 0.19 .095 0.48 0.24 0.12 0.006 0.003 0.0015 0.00075 0.00038 0.00019
Okay the inverse of 0.00019 is 5263 seconds or 87 minutes. Hey this stuff adds up fast!
So the mathmatical answer to your question is "20 stops, 6.0 ND or a B&W 120 filter... but hold that credit card!
What we haven't calculated is reciprocity and in these areas, there will be reciprocity in abundance! I"m not even going to try to calculate reciprocity in this area, I'd go on a hunch.....
Here's what I would do....
Buy Wratten ND Gelatin filters and a "technical filter holder" this is one that hinges like a book and holds the filter nice and flat. Then I'd figure a way to mount the holder on the inside of the camera if possible
(My calumet 4x5 has a big lens board so I glued wood blocks to the lensboard near the rear cell. A little velcro on the blocks and the holder and it's nice and snug and I don't have to fiddle with step up and down rings)
The filters I would be are 10 stop (3.0ND) a 2 stop (0.6 ND) and a 4 stop (0.9 ND)
This way you'll have a 10 , 12, 14, and 16 stops. One of those combinations along with f16f32 should get the time where you want it.
Now go out and TEST! Just don't try and bracket!
_________________ "In order to invent, you need a good imagination and a lot of junk" Thomas Edison 

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salva
Joined: 20 Mar 2003 Posts: 29

Posted: Tue Oct 14, 2003 10:03 am Post subject: 


Thanks for your help. You confirm I was thinking in the right direction.
The only discrepancy it's about the set of filters.
You say 10x 2x and x4 If I use all of them I will end whit a 16x factor, right?
Bud in the B+W they tip that the filter factors must be multiplied together, not added: 2x4x10=80x
It's this rule apply only in the B+W Filters set? I suppose not.
Or I mix factor whit spot?
Multiplain factor and spot is just added?
I will check the Wratten ND Gelatine filters
Regards
[ This Message was edited by: salva on 20031014 03:27 ] 

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worldphoto
Joined: 05 Feb 2003 Posts: 199 Location: Southern California

Posted: Tue Oct 14, 2003 1:01 pm Post subject: 


Bud is right regarding Filter Factors, multiply Filter Factors and calculate stop increases by taking the square root of the Filter Factor multiplied number to arrive at approximate stop/exposure increase.
Les's method using Neutral Density numbers is easier for me to use (0.3 ND filter for each stop increase). Add up the Neutral Density for each filter used and divide that ND number by 0.3 to determine the approximate stop/exposure increase required.
Harry
[ This Message was edited by: worldphoto on 20031014 06:15 ] 

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