High Filter Factor CPL

Hey Folks,

 

I busted ANOTHER ND filter outphotographing waterfalls yesterday. Dropped onto

rocks and cracked. I'll be replacing it...but... Looking at the photographs,

I'm seeing some shots that could also benefit from the careful use of a CPL.

Unfortunately, I still haven't purchased a CPL for my EF-17-40/4L. I have one

for my previous WA lens, but it just hasn't been a priority.

 

Most of the time, I could probably get by with just a CPL with a one-stop

exposure compensation, but I prefer to carry a two or three stop ND filter for

my waterfall photography these days, due to the drought conditions requiring

generally longer exposure times. While I intend to replace my 8x ND

immediately, I'm looking for a CPL for my 77mm that could serve as a standalone

filter.

 

My question is... Does anybody know of a CPL that has an exposure compensation

greater than 1 or 1.5 stops? That way, when I use it for water situations that

needed a CPL, I can avoid stacking filters on the 17-40/4L. On a 1.6x body I'm

guessing stacking won't pose a vignetting issue, but that's more glass than I'd

like to add to my lens.

 

I'll be looking myself, but if you know of anything, let me know. Thanks for

the help.

Don't most of them eat up about 1.5 to 2 stops of light? Mine certainly do.

 

 

Kent in SD

Kent,

 

Of the ones I've checked the specs on, B&H lists them as having a filter factor of +1.3. This includes filters from Cokin, Tiffen, Hoya, and B+W of various price classes. Either they aren't putting the actual specs on their page, the + refers to greater than rather than a lengthening of exposure, or all of them designed to be around 1.3 stops for some arcane reason.

Polarizers intrinsically have a filter factor of around 1.5. You can't make one with much less due to the physics involved. Unless you use an ND filter to make it, you won't get a polarizer with a filter factor over 2. As far as I know nobody makes a combined polarizer/ND filter.

 

Kent - I don't think they are specifically designed to be 1.3, it's just that that's the natural value thay they have. If you assume light is an equal mix of horizontally and vertically polarized light and you filter out one polarization (half the light), you'd have a filter factor of 1. Add in some inefficiency factors and you get the other 0.3

I don't think anybody makes a combination filter either, although I'd certainly buy one if they did. Thanks for the help guys, I really appreciate it.

Some neutral density filters are rated in optical density. For example if the density is 1.0, the transmission is 10%. If the density is 1.3 the transmission is 5%. Obviously this is quite dark and would not likely be mistaken for stops.

Alex,

 

I'm somewhat confused by your post. I think what you are getting at is that I might be conflating the ND ratings with the exposure compensation rating for the CPL filter. Not the case. The specs on B&H's site for CPLs clearly indicate 1.3 stops, and I try to always use the stop ratings for my ND filters.

 

However, I am aware that ND filters are often rated in three different ways, an optical density rating (0.9, 1.3, etc...), a stop rating (1,2,3 stops, etc...), and a multiple rating that reflects the stops (2x, 4x, 8x).

 

I generally use a three stop (8x or 0.9) ND filter. I was hoping there was a CPL out there that required a 2-3 stop exposure compensation, which would close to the equivilent of the ND filter I typically use. There is no such filter.

Oh, and sometimes the 8x is reversed, and described as 1/8 ISO.

<cite>The specs on B&H's site for CPLs clearly indicate 1.3 stops</cite>

 

<p>If that's the case, then they are citing what they believe to be typical. There is no fixed filter factor for a CPL. There cannot be, because the amount of light it cuts depends on how strongly polarized the lighting in your scene is and how you orient the CPL. You can easily get a difference of a stop with some scenes simply by changing the orientation of the CPL, and on other scenes, the amount of light lost may change very little as you turn the CPL.</p>

 

<p>As others have said, the only way to make a CPL that has a higher light loss than others would be to make a combination CPL/ND filter. It's certainly possible to do this, but as with the others, I've never heard of one, either.</p>

Steve,

 

I've got what you are saying, and it explains why they cite a single, probably middle-of-the-road figure.

<I>There is no fixed filter factor for a CPL. </I><P>

 

Yes and no. The filter factor that is given for polarizers, is the factor for the filter when used with non-polarized subjects. With non-polarized subjects, the angle of the PL doesn't make and difference and the factor is constant.