Use of coloured filters with zone system

[Curtis Lowe]

I understand how the zone system works by exposing for the shadows and developing for the highlights but what happens if I use coloured filters to further alter the tones.

How does one know how many stops a coloured filter will lighten or darken certain colours in a scene after the filter factor has been applied? For example if a green filter is used on a landscape scene, the grass will be lightened and the blue sky darkened but is there a way to measure this?

Edit: I just thought should one take their spot metre readings through the filter?

[jim kitchen]

Dear Curtis,

You ask a great question, and you will get many excellent answers... :)

I am not that attentive to such detail, nor do I meter through the filter prior to the exposure, but I am attentive to the filter factor prescribed by the filter manufacturer, and I apply that factor; accordingly, prior to the exposure. Experience and the filter's effect will be your friend going forward.

I found an old filter factor chart that might help you, but if you have a moment, you should search for a filter chart that is indicative of your filter's manufacturer.

jim k

[Ken Lee]

I regularly meter through the filter with a spot meter.

[Ed Richards]

Metering assumes that your film and the meter agree - which I think is true of Tmax pretty much. But it also assumes that you can see what the differences mean in your head. I would strongly recommend shooting some duplicate negatives with different filters until you get an idea of what you want. I find that the green is the hardest to judge - it does darken the sky or lighten the foliage as much I expect, despite being a pretty aggressive filter. I generally shoot two sheets for each important shot, and that lets me shoot different filters and pick which works best.

[neil poulsen]

That was a claim made by Zone VI regarding their Zone VI altered Pentax and Soligor meters. For Tri-X type films, one could meter through the filter and get accurate results.

Otherwise, one can do some testing and determine correction factors for each filter.

[Bill Burk]

Hi Neil,

You're right about that claim. I just re-read Fred Picker's newsletter describing all the problems with meters. He did correct the Sologor II for Tri-X.

His graph would scare you into buying his meter because he showed that unmodified meters are typically 1-4 stops off depending on the scene. Especially foliage gets underexposed several stops because of the high amount of infrared reflected.

One minor detail about the Zone VI calibrated meters. He put a UV blocking filter in the meter and assumes you will put a UV blocking filter on the camera too.

He claimed the UV problem is not as bad as the infrared problem, and you might be OK to ignore it. But he recommended at least using a UV filter in the mountains.

Newsletter No.37 Nov 1983 p.8

---
Curtis,

Maybe it's oversimplifying Zone System filtering, but seems to me the color filter won't affect "that" color at all. For example (ignoring the infrared problem) if you use a green filter on grass, and open up two stops because of the filter factor... Then you really placed the grass two zones higher (because the filter shouldn't have had any affect on grass at all).

[ki6mf]

Agree with Ken Lee with the first to reply with meter through the spot meter. Recently while waiting for the sun to clear a building to get a shoot i metered the filters i carry against the same back ground in the same light and found that my red filter had a different reading than what the filter factor suggested! Through the Meter solves the problem.

[Maris Rusis]

A light meter can see colours very differently to film.

My Pentax 1 degree analogue spotmeter looking at a neutral surface through a #25 red filter indicates a light loss of 1.5 stops. The actual filter factor established by exposure testing on film is 3 stops. The light sensing cell in the meter is overly sensitive to red light and "thinks" there is twice as much light as is actually present.

Solution: meter the subject through the filter, calculate twice the indicated exposure, and place it on the pre-visualised exposure Zone in the usual way.

All my strongly coloured filters, red, orange, green, and deep blue have a "meter error" correction factor established by experiment. In no case is a direct meter-through-the-filter reading exactly right.

[Curt]

I found that with my Pocket Spot from Metered Light I can meter though the filter and get an accurate reading. I'm very pleased, I also have a Fred Picker Zone IV Soligor meter.

Curt

[mikebarger]

I haven't had any problems metering through the filter with my zone VI modified meter (tri-x 120 and HP5+ in 4x5).

Mike

[vinny]

It should be noted that modern emulsions dont react to filtration the way old films did.

[Ken Lee]

Meter designers try to obtain a linear spectral response curve.

Film manufacturers try to obtain a linear spectral response curve.

Photographers try to get every exposure perfect.

Given that the color of light changes throughout the day, one could spend considerable effort trying to eliminate all possible variables, which amount to a fraction of an f/stop. By that time, the light will have changed, or the clouds will have moved. :)

In my humble opinion, as long as the meter isn't picking up IR or UV light, I think it should be plenty close enough.

[kev curry]

You should read about the method devised by Gordon Hutchings on metering through filters and then adding the filter factors that he advises. Its all explained in Steve Simmons book 'Using the View Camera'. Never fails and makes life simple. There will definitely be threads dealing with this if you can find them.

[Heroique]

...as long as the meter isn't picking up IR or UV light, I think it should be plenty close enough.


You should read about the method devised by Gordon Hutchings...Never fails and makes life simple.

Both Ken’s method (and Jim’s in post #2) are my field methods, but I’ve always noted that they often contradict what I “learned” from the book Using the View Camera and its section about “Hutching’s Factors” – that is, factors that one adds after metering through the filter.

According to the book, the reason to apply this “extra” factor is to help prevent under-exposed shadows. “Shadow areas,” it explains, “are primarily illuminated by blue light, and blue light is most severely curtailed by yellow, orange, and red filters.”

This additional factor is not negligible:

For example, the Hutching’s Factor asks you to take a meter reading through the filter, and then add an additional stop of exposure for filters #11 (light yellow-green), #16 (medium orange), and #21 (light red) – and to add an additional 2 stops for filters #25 (medium red) and #29 (deep red).

It recommends no additional exposure for yellow or medium yellow filters.

I suspect that even devout believers in “Hutching’s Factors” typically ignore them when their compositions contain no significant shadows.

[Ken Lee]

I am not familiar with Hutching's methods, but if we meter the shadows through the filter with a spot meter - or any other portion of the subject for that matter - there is no need to further compensate for whatever darkening the filter may introduce. We are metering it directly, not estimating.

If we only metered a standard gray card - rather than shadows lit with blue skylight - then we would miss the minus-blue effect of the filter. But that would defeat the whole point of metering through the filter: to measure directly.

If you think about it, blue skylight is not always the same color, and not always the same depth of blue. Applying a correction via formula will get us close, but measuring the shadows through the filter, is the direct method.

Short of metering at the film plane (which can be done with a Sinar metering back) that's as close as we can get, to directly metering what the film sees.

[Andrew O'Neill]

Go to this thread:

http://www.largeformatphotography.info/forum/showthread.php?t=45535

[Bill Burk]

I can't imagine a subject without an important shadow, especially since that's where I meter.

I checked spectral response of TMY-2 - it starts well-responsive to UV and blue, slopes down then drops off sharply in IR.

I checked specs for my L-758DR, no spectral curve provided but it uses silicon photodiodes and Wikipedia shows a typical spectral curve for a silicon photodiode: low-responsive to UV slopes up deep into IR then drops off - but a few hundred nm higher than the film dropped off.

I just did a quick check to see if it's sensitive to IR. Looks like it is. I'm cooking dinner right now so I pointed the meter at the electric stove on medium high (not glowing red). The hot burner in deep shadow reads 30 f/1.4 while an area in light also reads 30 f/1.4

So for blue shadows and infrared reflecting foliage, I can see where the meter is going to be a problem without IR and UV cutoff filters. So I can buy into the Zone VI Studios sales pitch.

I read the thread, Andrew, and the tests John performed are a service to the community. But I suspect metering a gray card, even a Macbeth color checker target, isn't going to tell us what's happening in subjects with excess IR and UV.

[Brian Ellis]

There was a recent thread that discussed the use of filters and the zone system extensively. Sorry I don't have a link but it would be worthwhile to try to find it by searching. I think it was within the last four or so months.

In general filters won't make a difference in your use of the zone system because all you care about with the zone system is the darkest important shadow area and the lightest important highlight area. It would be somewhat unusual, though not unheard of, for a filter to change the darkest important shadow or the brightest important highlight from one spot or area to another by enough to seriously affect your exposure or development time, i.e. the filter probably won't cause some whole new area to become the darkest or the lightest or if it does the difference won't be so great that exposure or development will be significantly affected.

As an example, suppose you're metering a red apple surrounded by green leaves. Without a filter you determine that some part of the red apple skin is the brightest important area and some part of the green leaves is the darkest. If you're going to use a red filter to darken the leaves and lighten the apple (and thereby better separate the tones with b&w film) the red filter will only make the already-brightest area of the apple brighter and the already-darkest area of the leaves darker. If OTOH you were using a green filter to separate the tones, that would cause the brightest area to become darker and the darkest area to become lighter. But probably not by enough to cause a complete flip flop or reversal of the brightest and darkest areas or to make some whole new third area the brightest or the darkest, at least not by enough to matter.

I never meter through a filter when using a spot meter, I use the manufacturer's filter factors. On balance they work out well most of the time, at least as well I think as if I tried to test myself since the conditions of the test are unlikely to be duplicated when I'm making a landscape or architectural photograph.

[Bill Burk]

Sekonic wrote me back...

The spectral sensitivity curve for Sekonic light meters begins at 430nm, peaks at 560nm and ends at 700nm.

I guess I "flooded" it with IR when I aimed it at the stove. Probably not a suitable test.

[Brian Ellis]

I am not familiar with Hutching's methods, but if we meter the shadows through the filter with a spot meter - or any other portion of the subject for that matter - there is no need to further compensate for whatever darkening the filter may introduce. We are metering it directly, not estimating.

If we only metered a standard gray card - rather than shadows lit with blue skylight - then we would miss the minus-blue effect of the filter. But that would defeat the whole point of metering through the filter: to measure directly.

If you think about it, blue skylight is not always the same color, and not always the same depth of blue. Applying a correction via formula will get us close, but measuring the shadows through the filter, is the direct method.

Short of metering at the film plane (which can be done with a Sinar metering back) that's as close as we can get, to directly metering what the film sees.

Years ago View Camera magazine published a table of compensating adjustments to be made after metering through the filter. I used the table for many years to make compensating adjustments when I metered through the filter with my Pentax 67 system. Worked great.

[kev curry]

According to Hutching's, filter manufacturers dont take into account the altitude, colour temperature or the colour of the subject. Cant say I know anything much about all that but I use his method and his compensations and they work consistently.

For an example Hutchings recommends adding a two stop exposure increase when using a #25 medium Red Filter, that is after metering the important shadow areas through the filter. Metering the shadow areas though the filter is an important part of his method.

In my experience this method would provide me with detailed printable shadows.
Without the 2 stop compensation the zone III shadows would be lost in zone I.

[Curtis Lowe]

Thanks for all the replies, lots of useful info to read over now.

[Robert Budding]

The Zone VI modified meter is marketing hype:

http://www.butzi.net/articles/zone%20VI%20reprise.htm

"Not everything that counts can be measured. Not everything that can be measured counts."
~Albert Einstein

[Bill Burk]

Hi Robert,

I read that article. I feel Paul Butzi went a bit over the top trying to "disprove" Zone VI.

I suggest there is a problem using a Macbeth color checker (or even a gray fence or gray card) because I think the problematic subjects are foliage (for their excess IR) and sky / shadow illuminated by sky at high elevation (for their excessive UV).

Fred Picker did his original research on a Soligor II. Paul Butzi might have pegged it when he suggests Pentax selected better cells for later models and certainly by the time of the digital spot they would be using the best available. So would Sekonic today.

I do wonder about IR though. My quick test proved there is some detection in IR and that could over-rate foliage, leading to underexposure. I have at least one meter that exactly matches my visual response (the SEI).

I think its time for me to try a test. I'll see if I can find a green ink sample held against a green foliage subject, matched in visual brightness by SEI but reading differently by an electronic meter. I'll have to wait 'til tomorrow for the IR.

[Ken Lee]

I just aimed my Zone VI Modified Pentax spot meter at the sending end of my ATN Viper Infra Red viewing device.

It didn't register any reading whatsoever. I tried this in a well-lit room, and in the darkroom.

That's reassuring, since the "Panchromatic" films I use, can't see Infra Red anyhow. This graphic is taken from http://www.infraredphoto.co.uk/guide/

http://www.kenleegallery.com/images/forum/spectrum.jpg

The Viper emits light in the range of 850-890nm.

HP5+, according to the official Ilford HP5+ tech sheet (http://www.ilfordphoto.com/Webfiles/20106281054152313.pdf), is insensitive to light at those wavelengths.

FP4+ is also insensitive at that portion of the spectrum, according to the Ilford FP4+ tech sheet (http://www.ilfordphoto.com/Webfiles/2010712125850702.pdf).

According to the Kodak tech sheet (http://www.kodak.com/global/en/professional/support/techPubs/f4016/f4016.pdf), TMY and TMX are also insensitive to that spectral range.

[tgtaylor]

When metering thru my filters (Lee, Hi-tech, B&W and Cokin) I always get the same reading as stated by the manufacturer. But it is extremely important that the filter is perpendicular to the axis thru spot meter's lens/cell. If the angle is off by even a small amount you will get a bogus reading.

[Curtis Lowe]

When metering thru my filters (Lee, Hi-tech, B&W and Cokin) I always get the same reading as stated by the manufacturer.

Do you mean you get the same reading after applying the manufacturer's filter factor?

[Brian Ellis]

Do you mean you get the same reading after applying the manufacturer's filter factor?

He probably means that if he takes a reading without the filter and then through the filter he gets the same exposure adjustment as the manufacturer's filter factor would call for.

[Curtis Lowe]

I have been reading up on Hutchings method and I'm not sure if I follow it correctly. Am I right in saying I meter the important shadow area through the filter, reduce the reading by two stops to go from zone V to zone III, and then add the filter factor as per Hutchings table?

[kev curry]

Yes thats it...right on!

[Doremus Scudder]

I've elaborated on this in other posts, so I'll give just a brief recap here.

I, too, apply factors after metering through the filter. These, however, are not "Hutchings' Factors," rather a compensation for the changed response of different films to different colors of light. (FWIW, the logic behind the Hutchings' Factors seems flawed to me; a red filter should keep blue light from reaching the meter cell and give a correspondingly lower reading, which would then be placed appropriately to retain the desired detail.)

From what I understand, and from what I've learned from empirical testing, films respond differently in regards to contrast and sensitivity to strongly-colored light as opposed to white light. This seems to be dependent on the sensitivity characteristics of different films, probably due to the different ways of sensitizing panchromatic film to the green and red portions of the spectrum. One film's red-sensitive portion may be contrastier and slower than the film as a whole, for example.


Strong filters such as dark red, dark green and dark blue limit the spectrum of light reaching the film the most, and these are the filters that seem to require the exposure/development factors. I test the strongest filters with the films I use to arrive at these "fudge factors," which include both exposure and development compensation.

I find that not all films behave the same way. Tri-X, for example, needs additional exposure and reduced development with a #25 red filter (for me, + 2/3 stop and N-1). TMY, on the other hand, needs more development in addition to extra exposure to reach the same contrast. With dark green filters, TMY needs less development, but Tri-X needs more...

Fortunately, testing is fairly easy and needs to be done for only the strongest filters one uses (I didn't find appreciable difference with #8, and #11 filters, for example to warrant a factor). Most of us don't use dark blue or cyan (and if so, then less often), so the dark red, orange and green filters are the most important and the ones we can start with.

Test neutral subjects with and without the filter and compare exposure and contrast and adjust exposure and development for the filter so that these parameters match (very similar to Zone System calibration). Neutral subjects are necessary, so that additional changes in tone are not introduced into the tests by the filter's transmission of different colors.

Once you have the factor for each filter and the films you use, you're good to go.

This refinement makes metering through the filter a really reliable and useful tool.

Best,

Doremus Scudder