Are the Zone VI meter modifications worth it?

by Paul Butzi

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Amongst large format photographers, the Pentax Digital Spotmeter is ubiquitoous.  It's small size, relatively light weight, accuracy, and reliability have made it a standout choice for field photographers for years and years.

Back when Zone VI was still in business (before it got bought out by Calumet), Zone VI sold a modified Pentax meter; they claimed that the modifications included replacing the light sensor, adding a filter pack to adjust the spectral sensitivity of the meter to match that of film, and adding baffling to reduce the sensitivity of the meter to flare.

Calumet still sells the modified meter, and Richard Ritter (see http://www.lg4mat.net/)    still makes the modifications to Pentax and Soligor meters.

The claims for the modifications are:

• True one degree spot metering
• reduced flare
• 'ultrablack' coating eliminates reflections
• filter packs match spectral response to film (specifically Tri-X) so that metering through filters is accurate
• superior photo-cell and calibration improve linearity and accuracy.

Calumet have an article written by Alan Ross discussing the modifications on their web site here. In the article Alan discussing metering through filters, and describes a test where film is exposed according the meter readings (with the meter readings made through filters, or metering color patches) and then developed to demonstrate any differences between meter sensitivity and film sensitivity.

For years, I've owned a Zone VI modified Pentax meter - I called up Zone VI on the phone, and actually had a nice conversation with Fred Picker when I ordered the meter.

Recently, I bought a used unmodified Pentax meter when a friend was getting rid of his large format gear.  Having a backup meter made sense and the price was right.

Then I realized that I actually possessed both a modified and unmodified meter, and I could actually test them against one another (and against my Sekonic L-508) to see if the modifications actually made any difference. 

(please see also zone VI reprise.htm, where I examine various literature and counterclaims)

Spectral sensitivity testing

The first thing I decided was that I would NOT test the claim that the Zone VI modified meter matched the spectral response of Tri-X.  First, I don't use Tri-X, and second, my real questions were "Are the results of metering with the Zone VI modified meter and the unmodified meter actually different, and if so, under what conditions are they different?

Beyond answering those two questions, I wanted my experiment to be repeatable and reproducible - that is, I wanted to be able to perform the same experiment again to see if my results were repeatable, and I wanted to use only equipment likely to be found in the toolkit of other photographers, so that other photographers could see if they could reproduce my results. 

That said, here's my experiment: I would meter various test targets of various colors, under various lighting conditions, both without filters and through filters, and see if I could detect any difference between the readings given by one meter versus those given by another.  If there's no difference in the meter readings, then I conclude that the modifications have no meaningful effect.  If there's a difference, I can look at the situations where the meters produce different readings, and see if there are patterns to when the meter readings differ.

Macbeth Color Checker

For test targets, I decided to use a standard for colors, the Macbeth Color Checker(tm).  Everyone knows what the colors are in the Color Checker, each Color Checker should match all the others, and photographers commonly own them, which makes it easy for other people to reproduce my experimental equipment.

My plan was to meter all 24 color patches of the Color Checker, in several different lighting conditions, without and thru filters, and record the meter readings for each meter.  Then I can compare the readings and assess the differences, if any are found.

I metered each color patch from a distance of about 2 feet, metering the center of the patch.  Each meter reading was recorded.  Every few readings, I would meter the first patch again, to ensure that I'd spot any changes in illumination. Meter readings which oscillated back and forth between two readings (e.g. between 10 1/3 and 10 2/3) were recorded as being midway between the two readings (e.g. 10 3/6).  All the meters had fresh batteries inserted just prior to starting the tests.

Finally, I tested the meters' sensitivity to flare by setting up a flare prone metering situation and comparing meter readings.  The test setup was a white sheet of matboard, illuminated from the front, with a 2" hole cut in it.  A sheet of flat black material was draped about 2 feet behind this card, with the light source placed so that the light passing through the hole fell on the material outside the line of sight through the hole.  I then metered the dark surface through the hole.

Direct Sun, no filters

The first setup was having the Macbeth Color Checker in direct midafternoon sun (my location is near Carnation, WA, east of Seattle).

The data for this setup are:

patch	Zone VI	Pentax	Sekonic	Pentax 'error'	Sekonic 'Error"
dark skin	14.33	14.33	14.60	0.00	0.27
light skin	16.17	16.00	16.40	-0.17	0.23
blue sky	15.67	15.33	15.60	-0.33	-0.07
foliage	14.67	14.67	14.70	0.00	0.03
blue flower	16.00	15.83	16.00	-0.17	0.00
bluish green	16.33	16.33	16.40	0.00	0.07
orange	15.67	15.83	16.00	0.17	0.33
purplish blue	15.33	15.00	15.40	-0.33	0.07
moderate red	15.33	15.33	15.70	0.00	0.37
purple	14.33	14.33	14.50	0.00	0.17
yellow green	16.00	16.00	16.10	0.00	0.10
orange yellow	16.00	16.00	16.20	0.00	0.20
blue	14.67	14.67	14.70	0.00	0.03
green	15.33	15.33	15.40	0.00	0.07
red	15.00	15.00	15.30	0.00	0.30
yellow green	16.33	16.33	16.60	0.00	0.27
magenta	15.67	15.67	15.90	0.00	0.23
cyan	15.67	15.33	15.50	-0.33	-0.17
white (.05)	17.33	17.33	17.60	0.00	0.27
Neutral 8 (.23)	16.67	16.67	16.90	0.00	0.23
Neutral 6.5 (.44)	16.17	16.00	16.20	-0.17	0.03
Neutral 5 (.70)	15.33	15.00	15.20	-0.33	-0.13
Neutral 3.5 (1.05)	14.33	14.00	14.20	-0.33	-0.13
Black (1.50)	13.00	12.67	12.70	-0.33	-0.30

 

The patch titles are those given on the back of the Macbeth Color Checker.  The numbers are the meter reading on the Zone VI and Pentax meters, and the value given by the Sekonic L-508 meter with the film speed set to 100.  The columns labeled 'Pentax Error' and 'Sekonic Error' are the readings given by the unmodified Pentax (and Sekonic, respectively) with the readings given by the Zone VI modified meter subtracted.

Some interesting observations - the unmodified Pentax meter reads .1 stop lower than the Zone VI modified meter on average, and that the Sekonic L-508 reads 0.14 stop higher.  In no case in this setup does the deviation between meters exceed 1/3 stop.

A better assessment of the difference between meters might be the RMS difference between the readings; for the Zone VI and unmodified Pentax meter in this setup, the RMS difference is 0.18, for the Zone VI and the Sekonic, the RMS difference in this setup is 0.28.

Open Shade, no filters

The next setup was with the Macbeth Color Checker set in open shade.  The data are:

Patch	Zone VI	Pentax	Sekonic	Pentax 'error'	Sekonic 'Error"
dark skin	10.00	10.00	10.30	0.00	0.30
light skin	11.67	11.33	12.10	-0.33	0.43
blue sky	11.00	10.83	11.40	-0.17	0.40
foliage	9.67	9.83	10.30	0.17	0.63
blue flower	11.00	11.00	11.70	0.00	0.70
bluish green	11.33	11.33	12.00	0.00	0.67
orange	11.17	11.00	11.70	-0.17	0.53
purplish blue	11.00	10.67	11.30	-0.33	0.30
moderate red	10.67	10.83	11.40	0.17	0.73
purple	9.67	10.00	10.30	0.33	0.63
yellow green	11.17	11.00	11.70	-0.17	0.53
orange yellow	11.00	11.00	11.80	0.00	0.80
blue	10.33	10.00	10.80	-0.33	0.47
green	10.67	10.67	11.20	0.00	0.53
red	10.33	10.67	11.10	0.33	0.77
yellow green	11.67	11.67	12.30	0.00	0.63
magenta	11.00	11.00	11.70	0.00	0.70
cyan	10.67	10.67	11.30	0.00	0.63
white (.05)	13.00	12.67	13.50	-0.33	0.50
Neutral 8 (.23)	12.50	12.33	12.90	-0.17	0.40
Neutral 6.5 (.44)	12.00	11.67	12.20	-0.33	0.20
Neutral 5 (.70)	11.00	10.67	11.20	-0.33	0.20
Neutral 3.5 (1.05)	10.00	9.67	10.10	-0.33	0.10
Black (1.50)	8.67	8.67	8.70	0.00	0.03

 

In this case, on average the unmodified Pentax meter read 0.13 stop lower than the Zone VI modified meter, and the Sekonic read 0.49 stop higher.  The maximum difference between the Pentax and Zone VI modified meter was 1/3 stop, the maximum difference between the Sekonic and the Zone VI was 0.8 stop. 

The RMS difference between the Zone VI and the unmodified Pentax was 0.22, between the Zone VI and the Sekonic 0.54.

Direct Sun, B+W 090 (red) filter

The next setup was with the Macbeth Color Checker set in direct sun, but metering through a B+W 090 (red) filter.

Patch	Zone VI	Pentax	Sekonic	Pentax 'error'	Sekonic 'Error"
dark skin	12.33	12.67	13.30	0.33	0.97
light skin	14.33	14.33	15.00	0.00	0.67
blue sky	12.67	12.67	13.10	0.00	0.43
foliage	12.33	12.33	12.60	0.00	0.27
blue flower	13.33	13.67	14.00	0.33	0.67
bluish green	13.33	13.33	13.60	0.00	0.27
orange	14.33	14.33	15.10	0.00	0.77
purplish blue	12.00	12.33	12.90	0.33	0.90
moderate red	14.33	14.33	14.90	0.00	0.57
purple	12.33	12.67	13.40	0.33	1.07
yellow green	13.67	13.67	14.20	0.00	0.53
orange yellow	14.67	14.67	15.00	0.00	0.33
blue	11.00	11.33	12.00	0.33	1.00
green	12.33	12.33	12.90	0.00	0.57
red	14.00	14.33	14.90	0.33	0.90
yellow green	14.67	14.67	15.30	0.00	0.63
magenta	14.33	14.67	15.00	0.33	0.67
cyan	11.67	12.00	12.20	0.33	0.53
white (.05)	14.67	14.67	15.60	0.00	0.93
Neutral 8 (.23)	14.33	14.33	14.90	0.00	0.57
Neutral 6.5 (.44)	13.67	13.33	14.10	-0.33	0.43
Neutral 5 (.70)	12.67	12.67	13.20	0.00	0.53
Neutral 3.5 (1.05)	11.67	11.67	12.20	0.00	0.53
Black (1.50)	10.33	10.33	10.80	0.00	0.47

In this case, on average the unmodified Pentax meter read .1 stop higher than the Zone VI meter, and the Sekonic read .63 stop higher. 

The maximum difference between the Pentax and Zone VI modified meter was 1/3 stop, the maximum difference between the Sekonic and the Zone VI was 1.07 stop. 

The RMS difference between the Zone VI and the unmodified Pentax was 0.20, between the Zone VI and the Sekonic 0.67.

Tungsten Light, No filter

The next setup was with the Macbeth Color checker set in tungsten lighting.  No filter was used.

Patch	Zone VI	Pentax	Sekonic	Pentax 'error'	Sekonic 'Error"
dark skin	8.00	8.33	8.50	0.33	0.50
light skin	9.67	10.00	10.30	0.33	0.63
blue sky	9.00	9.00	9.10	0.00	0.10
foliage	8.33	8.33	8.40	0.00	0.07
blue flower	9.33	9.67	9.70	0.33	0.37
bluish green	9.67	9.67	9.90	0.00	0.23
orange	9.67	10.00	10.10	0.33	0.43
purplish blue	8.33	8.67	8.70	0.33	0.37
moderate red	9.33	9.67	9.90	0.33	0.57
purple	8.00	8.33	8.60	0.33	0.60
yellow green	9.67	10.00	10.00	0.33	0.33
orange yellow	10.00	10.00	10.40	0.00	0.40
blue	7.67	8.00	7.90	0.33	0.23
green	8.67	9.00	9.00	0.33	0.33
red	9.00	9.67	9.70	0.67	0.70
yellow green	10.33	10.67	10.70	0.33	0.37
magenta	9.33	10.00	10.00	0.67	0.67
cyan	9.00	9.00	8.90	0.00	-0.10
white (.05)	10.67	11.00	11.20	0.33	0.53
Neutral 8 (.23)	10.33	10.33	10.60	0.00	0.27
Neutral 6.5 (.44)	9.67	9.67	9.90	0.00	0.23
Neutral 5 (.70)	8.67	8.67	9.00	0.00	0.33
Neutral 3.5 (1.05)	7.67	7.67	7.90	0.00	0.23
Black (1.50)	6.33	6.33	6.40	0.00	0.07

In this case, on average the unmodified Pentax meter read .22 stop higher than the Zone VI meter, and the Sekonic read .35 stop higher. 

The maximum difference between the Pentax and Zone VI modified meter was 2/3 stop, the maximum difference between the Sekonic and the Zone VI was 0.7 stop. 

The RMS difference between the Zone VI and the unmodified Pentax was 0.30, between the Zone VI and the Sekonic 0.40.

Overcast, B+W 061 (green) filter

The next setup was with the Macbeth Color checker set in overcast lighting.  Metering was done through a B+W 061 (green) filter.  By this time, I was tired of the Sekonic meter, which is a hassle to read, so I dropped it from testing.

Patch	Zone VI	Pentax	Pentax 'error'
dark skin	9.33	9.33	0.00
light skin	11.00	11.00	0.00
blue sky	10.33	10.00	-0.33
foliage	10.00	9.67	-0.33
blue flower	10.67	10.67	0.00
bluish green	11.67	11.33	-0.33
orange	10.67	10.67	0.00
purplish blue	10.00	10.00	0.00
moderate red	10.33	10.33	0.00
purple	9.00	9.17	0.17
yellow green	11.33	11.33	0.00
orange yellow	11.33	11.00	-0.33
blue	9.00	9.00	0.00
green	10.67	10.33	-0.33
red	9.67	9.67	0.00
yellow green	11.67	11.67	0.00
magenta	10.33	10.33	0.00
cyan	10.67	10.33	-0.33
white (.05)	12.33	12.00	-0.33
Neutral 8 (.23)	12.00	11.67	-0.33
Neutral 6.5 (.44)	11.00	10.83	-0.17
Neutral 5 (.70)	10.33	10.00	-0.33
Neutral 3.5 (1.05)	9.33	9.33	0.00
Black (1.50)	8.33	8.00	-0.33

In this case, on average the unmodified Pentax meter read .14 stop lower than the Zone VI meter.

The maximum difference between the Pentax and Zone VI modified meter was 1/3 stop.

The RMS difference between the Zone VI and the unmodified Pentax was 0.22.

Overcast, B+W KB12 (blue) filter

The next setup was with the Macbeth Color checker set in overcast lighting.  Metering was done through a B+W KB12 (blue) filter.

Patch	Zone VI	Pentax	Pentax 'error'
dark skin	10.00	10.00	0.00
light skin	11.67	11.33	-0.33
blue sky	11.33	11.00	-0.33
foliage	10.33	10.00	-0.33
blue flower	11.83	11.67	-0.17
bluish green	12.00	11.67	-0.33
orange	10.67	11.00	0.33
purplish blue	11.33	11.00	-0.33
moderate red	10.67	11.00	0.33
purple	10.00	10.33	0.33
yellow green	11.33	11.33	0.00
orange yellow	11.00	11.33	0.33
blue	10.67	10.33	-0.33
green	10.33	10.67	0.33
red	10.00	10.33	0.33
yellow green	11.67	11.33	-0.33
magenta	11.33	11.33	0.00
cyan	11.67	11.33	-0.33
white (.05)	13.00	12.67	-0.33
Neutral 8 (.23)	12.33	12.00	-0.33
Neutral 6.5 (.44)	11.67	11.33	-0.33
Neutral 5 (.70)	11.00	10.67	-0.33
Neutral 3.5 (1.05)	10.00	10.00	0.00
Black (1.50)	8.67	8.67	0.00

In this case, on average the unmodified Pentax meter read .09 stop higher than the Zone VI meter.

The maximum difference between the Pentax and Zone VI modified meter was 1/3 stop.

The RMS difference between the Zone VI and the unmodified Pentax was 0.29.

Sources of error, some tentative conclusions about spectral sensitivity

Taking all those meter readings taught me a bunch of things about meter readings, particularly with the Pentax and Zone VI modified meters. 

First, meter reading are highly sensitive to even minor changes in point of view - that is, changing the angle to the metered target by moving your position even a small amount can easily make a change of 1/3 stop in the meter reading. 

Second, with both the Pentax Digital Spot and the Zone VI modified Pentax Digital Spot, the readings often rise if you hold the button depressed for more than a second - once I realized this, I started waiting for the readings to 'settle'.  Often the change was on the order of 1/3 stop.

Finally, lighting changes.  I attempted to double check readings, periodically backtracking and checking readings to make sure lighting hadn't changed.  A fair number of these checks detected changes on the order of 1/3 stop.  While I attempted to detect and correct for all lighting changes, it's likely that a few changes (especially ones smaller than 1/3 stop) slipped through.

My tentative conclusions from the metering tests are that the differences between the spectral sensitivity of the Zone VI modified Pentax Digital Spot and the unmodified Pentax digital spot are not significant.  On the other hand, in a number of cases, the differences in readings between the Sekonic L-508 meter and both the modified and unmodified Pentax meters can be on the order of a full stop, a significant difference. [update: I no longer feel that these conclusions are tentative.  Please see zone VI reprise.htm for details.]

Flare, low,  and accuracy of metering spot

Finally, I tested the meters' sensitivity to flare by setting up a flare prone metering situation and comparing meter readings.  The test setup was a white sheet of matboard, illuminated from the front, with a 2" hole cut in it.  A sheet of flat black material was draped about 2 feet behind this card, with the light source placed so that the light passing through the hole fell on the material outside the line of sight through the hole.  I then metered the dark surface through the hole.  The white card metered EV 12 with both meters.  With the white card not illuminated, the flat black metering target read EV 0 with both meters.

Flare test target from metering position.  Dark circle is about 6 degrees wide.

With the white card illuminated (and reading EV12) I metered the center of the dark target through the hole.  The target read EV 4 with the unmodified meter, and EV 5 with the Zone VI modified meter.  I attribute the difference between readings taken with the white card not illuminated and with the card illuminated to flare from the white card.  I conclude that the unmodified meter is LESS flare prone than the Zone VI modified meter.

I've heard several times that the Zone VI modifications also diminish the low light capabilities of the meter.  Since the flare tests required a darkened room I took a few low light readings and found that in general, for values below EV 3 it was quite difficult to get any reading at all from the Zone VI modified meter, although often readings were possible from the unmodified Pentax.  When meter readings of low light targets were possible with both meters, the Zone VI modified meter generally read 2/3rd stop or more higher than the unmodified Pentax meter.  These tests were not carefully controlled or notes taken but I'd tentatively conclude that the Zone VI modifications might well reduce the low light ability and accuracy of the meter.

Finally, using the flare testing setup, and by checking readings with a bright target on a dark background, I checked the accuracy of circle depicting the metered area for both the Zone VI modified meter and the unmodified Pentax meter.  Accuracy of the spot for both meters seemed about the same; the Zone VI modified meter having the spot very slightly to the left of the actual metered area, the unmodified meter having the spot about the same distance to the right.  In neither case would I consider the error significant.  The size of the metered spot seems the same for both meters.

IR sensitivity

There's much speculation that the real benefit of the Zone VI modifications to the meter consist of the added IR filters, which diminish the sensitivity of the meter to IR (infrared) light which would not be recorded by film.

I have performed a fairly informal test of the IR sensitivity of the modified and unmodified meters by using them to measure the intensity of a handy IR light source that contains little or no visible light.  The handy light source I used was picked from the collection of IR remote controls that are used to control the various devices around my home.  I used a digital camera to examine the light emissions (digital cameras are HIGHLY sensitive to IR!) and selected the remote with the brightest and most constant illumination from the IR emitter.

I then metered the emitter with both the Zone VI modified Pentax Digital Spot Meter and with my unmodified Pentax Digital Spot Meter .  Because of the IR from the remote is pulsed, the readings flicker between zero and a maximum value - but for both the Zone VI modified meter and the unmodified one, the maximum value read was EV 6.

A better test would be to rig an IR LED as a constant IR light source, and to meter that.  I'll do that test in the future, but for now, my tentative conclusion is that if there's a difference in the IR sensitivity of the meters, the difference is small.