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J V McLure
13-Aug-2007, 21:38
Forum,

I hope this is the right place to post a question about filters. If not, please forgive me and point me to the right place.

I am using the Cokin P system for filters on my LF lenses. I need to purchase a polarizing filter and figure out how to use it on a LF camera.

My first question, which is the best polarizing filter to purchase?

Next, do I need a Circular or Linear polarizer?

Then, how do I determine the best rotation for the filter. With an SLR, it is simple, I can watch the results in the viewfinder as I twsit the filter to the sweet spot. I'm not sure how this might work on a GG. Do I hold the filter above the lens, sight through it, twist to the sweet spot and then slip it into the holderr's slot?

Finally, how do I determinne the impact the filter will have on exposure. Do I simply increase by two (2) stops? Does the rotation play any role in the exposure?

I thank you in advance for helping educate me in the use of a polarizing filter on a LF camera!

J V McLure

Brian Ellis
13-Aug-2007, 22:13
I've never used the Cokin system so I can't answer all your questions. However, you don't need a circular filter, a linear will work fine. You adjust exposure by a constant number 2 - 2 1/2 stops regardless of rotation.

You might find things a little eaiser if you just bought a glass screw-in filter with adapter rings if necessary and didn't try to use the Cokin system with the polarizer. That way you wouldn't have to worry about figuring out how to find the "sweet spot," with the Cokin system, you'd just observe the ground glass as you rotated the filter.

Alan Davenport
13-Aug-2007, 22:31
My first question, which is the best polarizing filter to purchase?

I don't believe there's any one brand that's "best." B&W, Hoya and Tiffen all have good reputations. B&W makes some filters with brass mounts, which purportedly don't get stuck like aluminum can.


Next, do I need a Circular or Linear polarizer?

All you need for LF is a linear polarizer. If there's any chance that you'll want to use the filter elsewhere, like on a medium format or 35mm camera, you might want to buy a circular pola anyway. You can use a circular polarizer with any camera.


how do I determine the best rotation for the filter. I've tried the Cokin polarizers, and found them a royal PITA. Your idea (look through the filter to find the sweet spot, then try to insert it in that orientation) is on the mark, but fraught with error in practice. I went back to standard screw on polarizers.


how do I determinne the impact the filter will have on exposure. Do I simply increase by two (2) stops? Does the rotation play any role in the exposure?

Yes to the first question: you make the same exposure compensation regardless of the polarizer's rotation.

Two stops is a good starting point, but the actual filter factor varies some between brands. If you can find an unpolarized subject, you can simply meter through the filter to determine the actual factor (with unpolarized subjects, a polarizer acts the same as a neutral density filter.)

Rory_5244
14-Aug-2007, 00:01
I use the Cokin polarizer designed to be used with the Cokin holder. It works well enough. It has projections around the filter so that it can be rotated with your fingernails while it sits in the holder. I determine the degree of polarization by looking at the ground glass while I rotate the filter. It has been quite easy to see the changes thus far. I compensate by adding 1.66667 stops.

Andrew_4548
14-Aug-2007, 04:30
I don't believe there's any one brand that's "best." B&W, Hoya and Tiffen all have good reputations. B&W makes some filters with brass mounts, which purportedly don't get stuck like aluminum can.

It must be me but the amount of grief I get with brass-mounted B&W polarisers is beyond belief - sticking and locking up before a lot of shouting at it for it then to innocently release and screw off as though nothing's happened. I've tried pencil graphite, light oil, cleaning the threads out with a scotchbrite pad to take off any edges, burrs etc. I quite often go back to my cheapo Jessops polarisers as they're nothing like the trouble....

Helen Bach
14-Aug-2007, 05:08
A square polarizing filter (I use a Schneider one, similar to B+W I guess) is easy enough to use if you have a matte box or compendium shade with a rotating stage, but in general I'd agree with the recommendation for a screw-in type.

I rarely use a polarizing filter for darkening skies etc, but I do use one for 'seeing' past surface reflections when required. In that case, which seems to be often overlooked, the exposure compensation is not always fixed. You are exposing for whatever is behind the reflecting surface, with an allowance for losses through everything between the subject and the lens. If I don't have TTL metering I spot meter through the filter in that case, being careful to avoid reflections off the back of the filter (my spot meter has a rubber lens hood, so I can hold the filter against the hood safely).

Best,
Helen

Bob Salomon
14-Aug-2007, 06:15
Forum,


I am using the Cokin P system for filters on my LF lenses. I need to purchase a polarizing filter and figure out how to use it on a LF camera.

My first question, which is the best polarizing filter to purchase?

Heliopan has the most neutral foils. They also have fully calibrated rims so you can easily determine the best position while holding the filter to your eye and then rotating to the setting that worked best visually.

Next, do I need a Circular or Linear polarizer?

You can use either. But if you are also going to use it on a camera that utilizes a beam splitter in its focus or metering system then that camera would need a circular. As a rule of thumb a circular works properly on all cameras under all lighting conditions. A linear will not work properly under all lighting conditions if a camera utilizes a beam splitter for focus or metering. So if in doubt and you will use it on multiple cameras get a circular.

Then, how do I determine the best rotation for the filter. With an SLR, it is simple, I can watch the results in the viewfinder as I twsit the filter to the sweet spot. I'm not sure how this might work on a GG. Do I hold the filter above the lens, sight through it, twist to the sweet spot and then slip it into the holderr's slot?

Finally, how do I determinne the impact the filter will have on exposure. Do I simply increase by two (2) stops? Does the rotation play any role in the exposure?

Filters are marked with the exposure factor. Just try the marking on the filter and try it. If you would like a darker or lighter exposure modify from there.


J V McLure

Marko
14-Aug-2007, 08:04
Then, how do I determine the best rotation for the filter. With an SLR, it is simple, I can watch the results in the viewfinder as I twsit the filter to the sweet spot. I'm not sure how this might work on a GG. Do I hold the filter above the lens, sight through it, twist to the sweet spot and then slip it into the holderr's slot?

You can also paint a bright yellow or white mark - a dot or a tiny line - somewhere (anywhere) on the ring and another one at 180 degrees apart.

Start by looking through the filter from the same position and angle as your camera does with the mark at the top. Rotate to find the best angle, note the new position of the mark, mount the filter and turn until you have the mark in the same position as noted.

Brian Ellis
14-Aug-2007, 08:24
A square polarizing filter (I use a Schneider one, similar to B+W I guess) is easy enough to use if you have a matte box or compendium shade with a rotating stage, but in general I'd agree with the recommendation for a screw-in type.

I rarely use a polarizing filter for darkening skies etc, but I do use one for 'seeing' past surface reflections when required. In that case, which seems to be often overlooked, the exposure compensation is not always fixed. You are exposing for whatever is behind the reflecting surface, with an allowance for losses through everything between the subject and the lens. If I don't have TTL metering I spot meter through the filter in that case, being careful to avoid reflections off the back of the filter (my spot meter has a rubber lens hood, so I can hold the filter against the hood safely).

Best,
Helen


Helen - If I understand your statement about exposure compensation correctly I think we're talking about two different things. The OP asked (I thought) whether the position of the rotating polarizer affected exposure compensation for the polarizer. In other words, does the filter factor remain the same regardless of the degree to which the polarizer may darken a subject? It's always been my understanding, based largely on statements by Ansel Adams in his book "The Negative" that regardless of the degree to which portions of the scene may be darkened by the polarizer, the filter factor of 2 - 2 1/2 stops remains the same.

You seem to be talking not about exposure compensation based on the filter itself but rather on the fact that when a polarizer is used to remove reflections the subject behind the reflection then is revealed and may be darker than it appears. So that has to be taken into account when determining the exposure. That's certainly correct. But after it's taken into account I think you still apply the same filter factor of 2 - 2 1/2 stops for the filter itself (assuming non-TTL metering) regardless of the degree of darkness of the subject after the reflection is removed, which is what I thought the OP was asking about.

If a TTL meter is being used and a polarizer is on the lens (or in front of the spot meter as you mention) then of course the meter itself already takes removal of the reflections and any resulting darkening of the subject into account and so there shouldn't be any need to make further adjustments (i.e. filter factors and adjustments for the polarizer become irrelevant).

Helen Bach
14-Aug-2007, 08:48
...

You seem to be talking not about exposure compensation based on the filter itself but rather on the fact that when a polarizer is used to remove reflections the subject behind the reflection then is revealed and may be darker than it appears. So that has to be taken into account when determining the exposure. That's certainly correct. But after it's taken into account I think you still apply the same filter factor of 2 - 2 1/2 stops for the filter itself (assuming non-TTL metering) regardless of the degree of darkness of the subject after the reflection is removed, which is what I thought the OP was asking about.

...

Brian,

I agree entirely about the filter factor always being the same and about my poor explanation of what I meant. My post was badly written and open to misinterpretation. Thanks for the clearer explanation. In my defence I will merely point out that I said 'exposure compensation' rather than 'filter factor'. My thoughts were about the relative meter reading from the camera position with and without the filter - this will not equal the filter factor.

Best,
Helen

Daniel_Buck
14-Aug-2007, 08:58
I'm new at this, so don't take my words as gold. But from my short experience in LF, I have just been spot metering through the filter. Even with a 3-stop red filter and polarizer, it's still easy to meter through both of them. It seems to work perfectly! The only trick is getting the circular polarizer turned the same degree when you are shooting. For that, I just remember which 'o-clock' the logo on the side was facing, and turn it till it matches where I was metering. Yes, you can see the effect of the polarizer through the glass just fine (when wide open), very difficult without a hood though, as your view gets darker.

I determine my aperture before I actually put on the polarizer, that way I can look through the glass no problem wide open, set my polarizer rotation, then stop back down to my pre-determined aperture and get ready to expose the film.

Brian Ellis
14-Aug-2007, 10:12
Brian,

I agree entirely about the filter factor always being the same and about my poor explanation of what I meant. My post was badly written and open to misinterpretation. Thanks for the clearer explanation. In my defence I will merely point out that I said 'exposure compensation' rather than 'filter factor'. My thoughts were about the relative meter reading from the camera position with and without the filter - this will not equal the filter factor.

Best,
Helen

Helen - No need for a defense, everything you wrote was perfectly accurate and I certainly wasn't criticizing it. I just thought it might be useful to clarify for the benefit of the OP and anyone else following this thread why I said what I did and you said what you did and that there was no disagreement, just a discussion of two different things. But as always, your message was completely accurate and no apologies or defenses are necessary.

Dirk Rösler
14-Aug-2007, 19:04
There is an interesting "Vernier" polariser set made by Kenko made for non-SLR use (VC, rangefinder). You have a filter and a 'checker' (eyepiece). Both have numbers on the rim. You look though the checker, rotate the polariser for the desired effect. Then you read the number and dial it on the filter on the camera. No need for through the lens adjustments. I have not used it, but sounds neat and not too expensive either.

http://www.robertwhite.co.uk/accessories.htm#LabelKRP

Alan Davenport
14-Aug-2007, 19:49
...the amount of grief I get with brass-mounted B&W polarisers is beyond belief ...

Interesting! I only added the B&W remark because I figured if I didn't, someone else surely would. I've never felt the need to spend a B&W price when Tiffen and Hoya seem to do the job for much less. I'll try to avoid preemptive strikes in the future... :confused:

Andrew_4548
15-Aug-2007, 04:36
Luckily, I don't use the B&W ones now for LF - I've got a Lee / Formatt 100mm square system. I've just got to remember to use the bulldog clip to hold in the polariser when I turn the holder or it'll become an expensive day out... :eek:

J V McLure
16-Aug-2007, 20:25
My thanks to all for their responses. What a great place to find answers and learn!

J V McLure