Filter factor

What is the best way to determine the filter factor for graduated and polarizer filters?

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I'm using a P67 without an internal meter. I'm using Cokin filters and they do not list filter factors for many of the filters. They must assume you are using a TTL meter.

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joel

I am sure that you are aware that the purpose of a "grad" filter is to adjust exposure to part of the image while retaining normal exposure on the remainder of the image. Expose normally for the part of the image you do not want to alter and will be composing through the clear (usually bottom) portion of the filter. Reduction in exposure of the part of the image you want to alter is controlled by the construction of the filter. For example, the Cokin G2 Graduated Grey filter yields a much more dramatic image alteration than the G1.

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Polarizing filters will affect the exposure from 0 to 3 stops depending on atmospheric conditions and the amount of polarization you dial in. In most cases, I have found that I can get good results by bracketing exposures using factors of 0, 1/2 and 1 stops or 0,1 and 2 stops.

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If your meter has a small receptor (a 1 - 1-1/2 cm circle as opposed to the wide bands typical found on selenium and CdS meters), you can get some idea of the effect of "grad" and polarizing filters on exposure by taking meter readings through the meter. When doing so, make sure that the meter is close enough to the filter (or otherwise shielded) so as to be unaffected by side lighting. I saw a lady in San Francisco using a cardboard tube (about 15 cm long) with a glued in step-up adapter at one end (for her glass filters) and a shaped adapter at the other end (for her meter). I'm going to build one for my cokin P-series filters and Luna-Pro meter; if the results are favorable, I'll post a report.

Cokin lists the filter factor for each of their filters on page 37 of their yellow "cokin creative filter system" booklet.

Since you don't have an internal meter, you can measure the light loss with a spot meter by holding the filter in front of the meters receiving lens, or borrow someones SLR with an internal meter, and hold the filter in front of the lens in manual mode.

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Mike Mahoney

For the polarizer: It has a factor of two f-stops. That's it. Don't adjust for "Oh, the sky is darkening" because everything else isn't dark. Just take your meter reading, increase the exposure by two stops, and that's it.

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For the graduated filter: The density should be printed on the filter. Set your exposure based on what isn't covered by the filter. I use a spot meter, so no problem.

I'm confused about this, too!

When I point my spot meter through my Hasselblad linear polarizer, then rotate the filter, there is at least a 1 stop difference in the light reading as the sky turns dark.

It seems intuitive that we should compensate for the darker image, but maybe not?

I haven't done enough testing of this myself, so I'd like to hear what other shooters do with their polarizer settings.