Zone VI meter mods - idle speculation

[Steve Goldstein]

I know the gist of the Zone VI meter mods, and still have and use the Pentax meter I had modified when this conversion was first introduced. But Zone VI is gone, I don't know (because I've no need to check) whether Calumet still offer the service, and besides the mod was always restricted to a very few models.

The other day I was looking at B+W filter curves and noticed the 486 filter, which was developed for digital cameras with excessive IR sensitivity. It's an interference filter with a sharp cut just above visible red and another in the UV, and is relatively flat across the visible. Has anyone played around with this as a possible "poor man's Zone VI conversion"? It might duplicate at least some of the corrections, and would be applicable to almost any meter. Or not, like I said in the title, this is all idle speculation over morning coffee.

Does anyone have a copy of the transmission curve of the Zone VI filter pack?

If they weren't so expensive I'd buy a 486 filter and experiment...

[Navy Moose]

Get in touch with Mr. Ritter here on the board, he should be able to help you.

[Eric Woodbury]

I don't know if anyone has the curve or curves for the filters. Could be in somebody's notebook somewhere, but you can calculate. The 486 is not even close.

A silicon photodiode starts weak in the blue and its sensitivity increases until about 970 nm, then drops suddenly in the near-IR. Film response is typically strong in the blue-green, weakening in the yellow-orange, and dropping off in the red. Certainly, there are films that are different, but this is a verbal approximation. Z6's mod meter had a filter designed to make the meter 'see' as does Tri-X. This means that the response curve should be that of Tri-X divided by the photodiode response. It would be strong in the blue-green and then attenuating strongly towards the red and cutting off everything above about 670nm. Overall, unless interference filters are used (The early Z6 mods may have used interference filters. I'm suspicious there was a change in filter pack for later Z6 mods), there is overall attenuation that is compensated by a meter offset adjustment. This is not anything close to a 486 curve.

[Donald Miller]

I will need to check my Zone VI newsletters if forced to verify what Fred originally stated about the modification. But I do recall that he said that one of the aspects of the filter pack was to limit the effect of IR on the meter as it was originally manufactured.

The example that he gave was the effect of IR emanating from green foliage. The meter modification was represented to give more exposure in that part of the spectrum. I used one of these meters for years and really have no objective proof of the beneficial effect of the modification.

The person that was very much involved with this at the time was Paul Horowitz. He was involved with development of the compensating timer for both enlarging and film/paper development. If someone really wants to get specific about what needs to be done it might be wise to contact him. I believe that he is on the faculty of Harvard.

[Steve Goldstein]

Eric,

Thanks for the info, I should have been able to think this through had it occurred to me to look at a film response curve in addition to the filter curve. I recall reading somewhere that one of the modern films (TMX or TMY?) had less blue sensitivity than Tri-X; I suppose if one were making a modern version of this meter it would be necessary to at least consider whether this difference is significant enough to affect the filtering.

Donald,

Now that you mention it, I remember Fred's leaf-IR example. I sold my newsletters some years ago, or I'd have looked it up. In light of Eric's answer, I couldn't in good conscience ask you or anyone else to search for this info.

[Eric Woodbury]

Tri-x and Ilford std films are about the same response, but Ilford films tend to have more green sensitivity. Tmax film have more red sensitivity. Tech pan and probably other document films have extended red sensitivity. As you move away from Tri-x, your meter reading can be off, depending on the filter and the subject color.

[Mark Sampson]

But compared to the difference between the standard and modified meters, the differences between 'normal' films are quite small. (Tech Pan and ortho films excepted of course.) And IIRC the modified meter pre-dates the TMax/Delta films. I've used my modified meter for 17 years, and experience has proved it to be more accurate than my thinking usually is.

[Eric Woodbury]

Mark

true enough. It would strong, pure colors to fool a "modified meter" regardless of the film, whether Kodak or Ilford or other, T grains or regular grains.

[Arne Croell]

The B+W or the similar Heliopan UV/IR cut filters certainly help with the IR sensitivity. I have a modified Zone VI digital and recently got a regular Pentax one in addition. The IR filter changes the Pentax response in IR-rich lighting (Tungsten, foliage in sun, stove top) to the values of the Zone Vi version within 1/3EV. Putting the filter in front of the modified version has practically no effect on the meter response in these situations (except for the stove top test), as opposed to the regular one, so I would assume that the Zone VI filter pack has an IR filter in it, whether its interference or absorption I don't know. I have since acquired a used Gossen Ultraspot II which is also quite IR sensitive. The Heliopan filter brings its measured values also quite close to the Zone VI modified meter, within 1/2 EV, whereas it could be over 1EV difference without it.

[John Bowen]

IIRC when the original Zone VI modifications came about, I sent my analog Pentax V meter for modification. It was returned with the original photocell attached to a card. In other words, part of the modification was to replace the photocell used by Pentax with a different cell. Now THAT would be hard to replicate with a filter.

And Yes, Richard Ritter is the guy who performed more of these modifications then anyone else, so he would know what went into the "filter pack."