Filters for my 20D + 10-22mm Lens

I have a 20D and mainly use my 10-22mm ultrawide lens for

landscapes. I use the Cokin X-Pro filter holder with a Singh Ray 3

stop Grad ND soft. If I stack a 2 stop Grad ND along with my 3 stop,

will that theoretically give me a 5 stop Grad ND filter? Or is this

not a good idea? Please share any insight. Thanks.

Will, when combining ND's you get a multiplier effect rather than additive. In your scenario, combining a 2-stop w/ a 3-stop will yeild a 6-stop loss of light...provided the ND areas are aligned on the same side of the frame.

I did not know that Lilly. Thanks for the insight.

Will,

 

I don't know anything about the Cokin filter system, but if there is any thickness associated with the filters (such as a mounting ring), watch out for vignetting with this lens - especially at the 10mm end. If your holder and filters are a larger diameter than the front of the lens, you shouldn't have any problem.

 

Rob

Rob, that's why I shelled out the BIG bukcs for the X-Pro series holder. The thing is huge, but it does not vignette.

"when combining ND's you get a multiplier effect rather than additive. In your scenario, combining a 2-stop w/ a 3-stop will yeild a 6-stop loss of light...provided the ND areas are aligned on the same side of the frame."

 

This is not true. The effect is additive.

Thanks David. Is there an article that explains this?

I don't know of an article that explains the additive nature of neutral density filters.

 

I like Shaw's article, but it doesn't go into much detail.

http://www.singh-ray.com/shawarticle.html

 

B&H has a helpful table here:

 

http://www.bhphotovideo.com/FrameWork/charts/comp_filter_factors.html

 

See the section on Neutral Density Filters.

 

The confusion for most people begins with the obscure term "filter factor". Filter factors describe the amount of light transmitted in relative terms. For example a filter factor of 1 means that 100% of the light is transmitted (no filter used). 1/2 as much light would be a filter factor of 2 (a 1 stop filter used). 1/2 as much light is a factor of 4 (a 2 stop filter used).... and so on.... Then they make it really confusing by multiplying by 2 each time they halve the amount of light i.e. A factor of 2 = 1/2 the light, 4 = 1/4 the light, 8 = 1/8 the light, etc. How ridiculous is this....

 

I would ignore "filter factor" and simply think in terms of stops because they are additive and easy to understand. Each stop gives 1/2 less light. This is easy to understand.

Upon further review I see I blew it. The effect of two filters is additive. Thanks for catching that...I dislike misinformation as the next person.

watch out for flare when you are using two filters like that. light gets in the gap between the

two filters and can cause problems.

Thanks for the info David.

ND filters are commonly described using several different standards. Cokin uses the reciprocal of the transmittance (2=1 stop, 4= 2 stops, 8 = 3 stops etc.), while e.g. Lee uses log (base 10) of Cokin values (0.3 = 1 stop, 0.6 = 2 stops, 0.9 = 3 stops). Be sure you understand which system any particular manufacturer uses, convert to stops and add. The Cokin system numbers are multiplicative, while the Lee version is additive (adding logarithms is equivalent to multiplication). If you encounter an ND5 filter, chances are it is designed as a solar filter that cuts the light by a factor of 100,000 (antilog(5)) - almost 17 stops.