My question is about metering through filters. I recently returned from a photo trip where I decided to try metering through the filter rather than adjusting the exposure according to the filter factor. (as I normally do) Now that I'm developing, the shots where I did this are fairly underexposed. I'm guessing about one stop to two stops depending on the filter. I use a Gossen Luna-Pro F with the variable angle attachment and Bergger film developed in Pyro. I haven't had problems with this combination before so it must be the change in technique. Does anyone have any thoughts?

Thanks

Kevin

-- Kevin (kkemner@tateandsnyder.com), September 21, 1999

Answers

The spectral response of film often differs from the spectral response of the meter cell. Which means readings from brightly coloured areas or through a filter must be interpreted with care. Typically, metering through a red filter will underexpose by about 2/3 stop with most panchromatic film, depending on the meter cell. Filter factors are based on the reproduction of a middle grey. Personally, I don't even know if filter factors are necessarily the best compromise either because in many shooting situations, I find the effects of filters on local contrast extremely difficult to predict with any precision. For e.g., I know that a red filter will darken a blue sky but I'm never able to predict by how many zones it will darken the sky. I sort of makes stabs at it by metering with and without the filter. That, I think, is the information metering through a filter can perhaps give you. However, for exposure, the filter factor is perhaps a better compromise to make, at least for most field shooting situations. If I have a very complicated (for me) subject, I take the lazy way out and shoot a couple of sheets and develop them one by one. DJ

-- N Dhananjay (ndhanu@umich.edu), September 21, 1999.

I also find that metering through the filter doesnt give me the proper compensation factor. When I first started using a polarizer, I metered through the filter and it gave me a 1 1/3 stop factor, but using 1 1/3 stop still gave me an underexposed chrome. By trial and error, I finally came to a 2 stop compensation factor, which seems about right (for my polarizer, anyway). So... good idea, it just doesnt work!

-- Ron Shaw (shaw9@llnl.gov), September 21, 1999.

And so the spectre of Zone VI modified meters rears it head....

I believe it was Gordon Hutchings that developed a filter factor technique that has been published in Steve Simmons book, Using the View Camera and also in View Camera magazine, It may be on the V.C website. If interested I can look it up for you so e-mail me.

-- Sean yates (yatescats@yahoo.com), September 21, 1999.

Not much to add, except to say that I too have found metering through filters to be not as reliable as I would have thought, esp. for polarizers. Could be due to spectral sensitivity of the film, meter, or whatever. Now I shoot according to the data book (colour compensation filters only, no black and white work) and it seems to work.

-- Mark Brown (mark@jmail.co.jp), September 21, 1999.

At what are you aiming the meter when you take a reading through the filter? If you were to aim the meter at a target that was the same color as the filter, the filter would transmit most of the light striking it. On the other hand if you were aiming at a complimentary colored target, the filter would block far more light. A good illustration is the technique commonly used for shooting sand dunes at dawn. Yellow filter, no compensation, place highlight of dunes in Zone VI, give N+1 development. The lightstruck areas of the dunes are very near in color to the yellow filter. The filter passes pretty much all of that light. The blue light in the shadow area is partially blocked by the yellow filter, creating a thinner negative in that area. This, plus the extended development, which pushes areas placed in Zone VI into Zone VII, enhances the contrast of the negative, thus transforming what, in color is somewhat flat, into a contrasty, dynamic black and white image. Perhaps aiming the meter at a grey card would yield exposure offsets that would more accurately reflect the specified filter factor. One other note: Very often a filter's factor is a suggested range as opposed to a single number. Depending on the spectral response of the film, the color temperature of the light source or some of the other variables (like the color of the subject) can influence the practical factor for that situation.

-- Robert A. Zeichner (razeichner@ameritech.net), September 21, 1999.

For more than you ever probably wanted to know on this subject, see http://members.aol.com:/workshops5/zsfilter.htm.

-- Chris Patti (cmpatti@aol.com), September 21, 1999.

Hint! I have a Zone VI modified spot meter (Soligor) which give me very accurate readings in all light situations, with and w/o filters. Pat

-- pat j. krentz (krentz@cci-29palms.com), September 22, 1999.

Hate to say it but the modified Zone VI meter really does let you meter accurately through the filter....

-- Mark Eban (markeban@compuserve.com), September 22, 1999.

I have to agree with the Zone VI meter faction here. I own two modified Zone VI meters, a Soligor and a Pentax digital, and routinely meter through them with good results. There are, however, some other factors which affect film speed, etc. to be considered when using filters besides matching the spectral sensitivity of the meter to that of the film one is using. A look at the spectral response curve for any B&W film will show you that the film is more sensitive, i.e. faster at certain wavelenghts and less sensitive, or slower at others. A sharp cutting red filter (that means, photographing with red light only) can slow some traditional B&W films by up to three stops! Other colors have different effects. This is also the reason why B&W films have different speeds in daylight and tungsten light. One could get very scientific about the whole thing and compare spectral curves of filters and films and different phot cells and probably earn a Doctorate at RIT. Empirical testing, which takes much less time, shoud get you in the ballpark so that you can use your meter to read through filters (whatever it is, but I recomment the Zone VI since it approximates the sensitivity of B&W films more closely) and then apply the appropriate film speed "fudge factor" which has been determined from tests. Contrast can also be affected, so test that too, you may need a development factor as well. Using filters is unpredictable and inexact, but one can reduce the inconsistencies significantly and have a practical working method that produces consistently printable negs. For more info see David Kachel's article on exactly this subject. You can find it and many other interesting articles at: http://members.aol.com/workshops5 Hope this helps. ;^D)

-- Doremus Scudder (ScudderLandreth@compuserve.com), September 22, 1999.

Just a thought.

Metering through the filter is exactly what several million SLR users do, apparently with OK results. So what difference can a hand-held meter with the filter make? The only one I can think of is if there is airspace between the meter and the filter, which is allowing off-image light (can we call this flare?) to reflect off the filter and be read. Maybe if the filter was hard against the meter cell things would be different?

-- Ken Munn (ken@pcm-marketing.com), September 23, 1999.

I have used filters extensively in my photography and have learned how much exposure compensation to assign to each filter. There are a lot of factors involved in determining your exposure compensation for each filter. What film type, sun angle and intensity, color of the light, subject color and reflectivity, ect. All of these should be taken into consideration and learned. And the easy way to get the correct compensation is to meter through the filter you will be using. That pretty much takes care of the majority of problems. But now you have to consider and then meter for contrast range within the scene. Then all things will be good. I'm not familiar with all the different types of Gossen meters but I would like to know if the meter you were using was an incident or spot meter? If it was a spot meter then there must have been something else going on to give you that much underexposure. even 1 stop is a lot of under exposure for any filter mistake. But if you were using an incident meter then there is your problem. It doesn't take into account the contrast ratio of the scene itself in regards to the reflectances of the different components in the scene. If you are using an incident meter, discontinue using the filter in conjuction with it. Just learn to compensate with the info you get from youir prints. james

-- James (James_mickelson@hotmail.com), September 24, 1999.

A 1-stop difference wouldn't surprise me. Spectral responses of films and meters differ. See also B&W contrast filters and TTL sensitivities in photo.net Q&A.

Kevin: On the Gossen meter, I assume you are using the correct mark for the 'spot' attachment, i.e. the red or green circle, rather than the yellow triangle?

-- Alan Gibson (Alan.Gibson@technologist.com), September 24, 1999.

Just to clarify. There's normally at least two problems with metering through filters. One is what has been alluded to already i.e., film sensitivities and meter cell sensitivities can differ (the Zone VI modified spot meters are supposed to help combat this problem). The second problem is perceptual and depends on what color and tone the area you're metering is and how well you're able to perceive these. If you use a yellow filter (something like #8 which passes about 90% of yellow light) and meter off a yellow object, your meter will obviously not suggest a ton of compensation. However, if you meter off a blue object, your meter sees a lot less light and will suggest a large increase in exposure which can block up the yellow parts of the image. You could try metering off grey cards but why bother when that is what the filter factor is meant to do - reproduce a middle gray. Add to this the fact that filters are said to change the contrast index of the negative (which can again affect local contrast which is what we're trying to control with the filter).

I've come to the conclusion that working with filters has all the perceptual problems involved with metering. When you meter a subject, there are always perceptual problems involved. What area do I meter? How do I want it reproduced i.e., which zone (its worth remembering that subjects don't occupy zones, we assign them to zones)? How does this relate to the reproduction of this other area? And so on. Given all of this, looking for this level of precision (one overall factor for all situations or one working method like metering through the filter) is a bit of a wild goose chase. I think you're much better off thinking about your picture and how the filter affects various parts of the picture with different filter factors. In other words, I suspect that better results are obtainable by understanding the components of the scene and what the filter does to each of these components. Good luck.

DJ

-- N Dhananjay (ndhanu@umich.edu), September 24, 1999.