Cokin ND grad filter confusion

[Matus Kalisky]

Hello,

already longer time I do consider getting a a ND grad filters and cokin seems to be the cheapest option. I have heard about some issues with them (color shifts or so), but I can not afford filters like Lee.

I am quite a bit confused what the description of the filters means. There are:
120, 121, 120L, 121M, 121S, 121F.
What exactly does it mean? What is what? AFAIK - the letters are supposed to mean Light, Medium, Soft(Sharp?, Strong?), Full - but how many stops, etc?

I do understand the meaning of the ND2, ND4 or ND8 notation - but I am not sure it fits the filters as notet on the Cokin webpage.

For example what is the difference between "121" and "121S"

What is actually the "120" like?

Thanks

Matus

[Bob Gentile]

From B&H...

121 - 2 stop GND
121S - 3 stop GND (Soft)
121F - 3 stop GND (Full)
121L - 1.75 stop GND (Light)
121M - 1.75 stop GND (Medium)

Not real clear on the difference between the 120 (G1) and 121 (G2) filters, but I'd guess one is darker than the other.

[Hiro]

Hello Matus,

Is this the "Cokin webpage" you refer to?
http://www.geocities.com/cokinfiltersystem/

It's the best info source I've ever found about Cokin filters. Follow "The Guide" link to find specific filters. Gray grads are here:
http://www.geocities.com/cokinfilter...adual_gray.htm

I have 120 and 121, and these have a rather abrupt transition from clear to ND areas with about 0.5" of overlap, or the "gradual area." The difference between the two is the strengths (the ND area of 121 is twice as dark as that of 120). As illustrated in the link, L, (M), S, and F types have more gradual transition from clear to ND across the filter with varied degrees (It's always hard to describe grad filters). I suppose you pick the right type depending on how distinctly dark/bright areas are defined in the subject scene.

Exposure is tricky when you have to meter yourself (I let the matrix metering do the work on SLRs). There is an "Exposure Chart" link from the top page of the website above. I can't offer definitive exposure guidelines, but one way to look at it would be a grad filter narrows the exposure difference between highlight and shadow areas by the specified filter factor. If the sunset sky and the foreground detail you want to capture have a 7-stop difference, a 2-stop grad, e.g. 121, will bring it down to 5 stops--manageable with slides. So you can, for example, meter the sky/cloud, add the filter factor (e.g. 2 stops for 121), then add 2-2.5 stops to find proper exposure (for slides). From the Exposure Chart it looks the clear plastic has a filter factor of 1/3, but I pretty much ignore it. Sometimes I bracket and/or make "pinch of salt" kind of adjustment by feel. At least that's how I approach when using 120 and 121. There may be better/proper way of metering, and I'm not sure about L/M/S/F types.

One more thing. As often mentioned, using "Post It" to cover half the filter to see where the "line" is seems a good way to adjust a grad filter.

[roteague]

One more thing. As often mentioned, using "Post It" to cover half the filter to see where the "line" is seems a good way to adjust a grad filter.

Wow!! That is a great little nugget of information there. I use split neutral density filters a lot and never heard of thought of this idea.