Stacking ND Filters

[marsbars]

This may sound like a simple question but I am having trouble wrapping my head around this.
I have 3 different ND filters now that I got another one for my B-Day. All in all if I stack them I get 6 stops reduction. Now my question is, if I am using 100 ISO film that equals ISO of about 1.5. That is if I am doing my math right. The question I am having is if I use that much ND will my camera even be able to meter that? I want to take some pictures of landmarks that tend to have a lot of traffic and people milling around. I know that with a long enough exposure they won't appear on the film if they don't stand still long enough. I am guessing that I will have to do most of the calculations in my head to come up with a best guestiamate and add in for reciprocity failure. Please correct me if I am wrong.

 

[David A. Goldfarb]

That's about right, six stops short of ISO 100 is a bit more than EI 1.5, and yes, your camera meter is likely not to be reliable with that much ND filtration, and yes, it doesn't account for reciprocity, so you should meter with the filters off, add six stops of exposure for use with the filters, and then calculate reciprocity last, based on either your own tests or the reciprocity chart that should be on the film's technical data sheet, downloadable from the film manufacturer.

 

[marsbars]

So I guess that I had my head wrapped around it after all. I am guessing that I will more than likely only use that much filtration with a faster film like HP5 @ 400 but if I ever want to try a really long exposure in bright sunlight with a slow film then its all good then.
Thanks for the help.

 

[David A. Goldfarb]

Some films have really good reciprocity characteristics, so for long exposures, T-max 100 will in general be faster than most 400 speed films. Fuji slide films are also really good in this regard.

 

[benjiboy]

marsbars, are you aware that if you use more than one filter the filter factors of any additional filters needs not to be added, but multiplied so a 2x ND plus a 4XND is 8stops not 6, and any additional filters to the stack need to be multiplied also.

 

[Terrence Brennan]

marsbars, are you aware that if you use more than one filter the filter factors of any additional filters needs not to be added, but multiplied so a 2x ND plus a 4XND is 8stops not 6, and any additional filters to the stack need to be multiplied also.

Sorry to rain on your parade, but a 2X (ND 0.30) filter, plus a 4X (ND 0.60), is 2 x 4 = 8 times as much exposure, NOT 8 stops!!

An easier way is to add the ND values (0.30 + 0.60 = 0.90), and then raise 10 to that power. You can either multiply your exposure time by that factor, or divide your film speed by it, whichever is more convenient.

 

[Steve Smith]

Since camera controls work most intuitively in stops, it would make sense to work out your filters in stops too.

Confusingly there are three ways of marking a filter: (i) 0.3, 0.6, etc. (ii) 1 stop, 2 stops, etc. and (iii) 2x, 4x, etc.

0.3 = 1 stop = x2
0.6 = 2 stops = x4
0.9 = 3 stops = x8

every increment of 0.3 adds one stop or doubles the light reduction factor.


Steve.

 

[CBG]

I have two possible concerns about stacking three filters. It may degrade your sharpness. The lens wasn't designed to shoot through all that extra glass. Stacking three filters may also vignette your lens. You can test for both and see if what you get works OK for your purposes.

 

[marsbars]

I don't have a vignetting issue with my filters as they are the Cokin P series. I have staked multiple filters with my 24mm and had no issues. They are the .30 + .60 + .90 Filters. So if I stacked them all that would be 1.8. So that would be how many stops lost? From the newer posts I would guess that it would be more like 18 times less or closer to 9 stops lost. But my math was never very good.

 

[Lee L]

They are the .30 + .60 + .90 Filters. So if I stacked them all that would be 1.8.

That would be 1.8 log density at .3 units per stop, so 1.8/0.3 = 6 stops total just as you calculated originally and David confirmed.

Lee

 

[David A. Goldfarb]

ND values (.3, .6, .9) are logarithmic, so they are additive. .3 is one stop, so 1.8 is 6 stops.

These are the same values, incidentally, used for describing negative densities, Dmax, print densities, and such.

 

[marsbars]

So my math was on in the first place.
I don't know if I will ever use all three together but that would make for a long exposure.

 

[telkwa]

What filter thread do you need? B+W makes 6 stop ND filters, and I own one that is very useful to me at times.