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Metering through filters


kevin_kemner

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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?

 

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Thanks

 

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Kevin

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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

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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!

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

 

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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.

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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.

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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.

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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)

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Just a thought.

 

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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?

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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

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A 1-stop difference wouldn't surprise me. Spectral responses of films

and meters differ. See also <A HREF="http://www.photo.net//bboard/q-and-a-

fetch-msg.tcl?msg_id=000bYj">B&W contrast filters and TTL

sensitivities</A> in photo.net Q&A.

 

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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?

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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).

 

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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.

 

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DJ

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