# What is the purpose of this formula

Discussion in 'Exposure Discussion' started by mark, Jun 20, 2004.

1. ### markMember

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I have been playing with my meter(not rudely of course) and I can't figure something out.

In the ZOne System adams used a spot meter to get his readings. He metered the important low and the important high. The difference between the two was the range. A normal range, for adams was 5 stops. Zone 3 to 8. Now The BTZS guy used an incident meter to do the same thing. If you tried to use a spot meter for his system you had to do a formula thingy that boils down to Meter difference +5.

While playing with my meter I realized that with the Spot attachment I was getting maybe a difference of .3 stop more light than with the incident part. I went to get the trusty old luna pro and got the same incident reading. SO why the hell do I have to go through some obnoxious formula when I use the spot meter? on the flip side is there something wrong with my meters if the spot part and the incident part is so close together? Since I used two seperate meters and came up with the same difference I doubt it.

Three tenths of a stop is not going to make that big a difference, especially in B/W. And it sure does not warrant adding five to the range.

2. ### JorgeInactive

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yep, the formula for arriving at an SBR from spot metering readings is:

SBR=(7*(D-N))/D

Where D is the spread and N is the development required (N, N+1, N-1 etc.)

This takes into account the development you plan to use to calculate the correct SBR.

The spread plus 5 only applies to incident metering, not to spot metering. Since incident metering averages all light in the shadows, I find it strange that both your spot and your incident readings are the same, unless you were spot metering a gray card and not a normal subject.

3. ### david bMember

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I'm not sure I understand you correctly but the concept is this:

meter for zone III and expose the film for this setting. now meter for your brightest highlight and this is zone VIII.

now if the brightest highlight is falling on zone IX or X, develope the film N-1 or N-2 accordingly.

if the brightest highlight is falling on zone VI or VII then develope the film N+1 or N+2, accordingly.

The reason one uses a spot meter is because you cannot get close enough to a subject such as a mountain or something similar.

I am using a Pentax digital spot meter with the zone dial. This makes the entire process effortless. I meter what I want to be zone III and then the dial tells me where zone VIII falls. If it does or doesn't determines my developement and I jot this info into my trusty little notebook.

If your meter will allow for a zone dial, get one.

4. ### markMember

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Thanks for the clarification. The difference plus five means that if I get an EV of say 15 in the shadow and 18 on the highlight then I have an SBR of 8 right?

In the formula for the spot meter what value do I use for N or is it left as a letter? Would it look like this:
18-15=3 so D=3
(7*(3-N))/3

I was not metering a grey card. The shadow I wanted detail in when I first noticed this was under a rock where little rocks were holding it up. I have noticed this on many occasions. the difference between the two meters is only 3 tenths of one stop. When I was playing with the meter this happened again.

David
I like the ease of the Zone system and it really is a no brainer. There has just been so much hubbub about BTZS stuff that I thought I would give it a try. Being the obsessive SOB that I am when I can't figure something out I want to get this right.

I do not have a zone dial. Is this something I can make or does soeone sell them? It would definately save me some brain work because I do it all in my head right now.

Thanks for the help folks. I have found the BTZS book to be a bit complicated. It's times like these when Dyslexia sucks

5. ### JorgeInactive

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Ok lets say you spot meter your shadow and get an EV value of 15, then you meter the highlight and you get an EV value of 18, that is a 3 zone spread. Under the Zone system you would want to give it a N+2 development to get the 5 zone spread for printing....so your N value in the formula would be 2.
So if you plug the numbers:

SBR=(7*(3-2))/3
SBR=2.3

If on the other hand you are taking an incident meter and your shadow is 15 and the highlight 18 then yes, the SBR would be 8.

So you see, if you are getting both the spot metering and the incident metering to give you equal readings, then there is something wrong as per your example the SBR by the formula and the SBR by incident metering do not match. Either your incident metering placement is wrong, or one of the functions of the metering is off. When you incident meter the highlights, make sure you are pointing the meter to the light source. I am getting the feeling your placement of the meter is what is the problem.

BTW, you should really ask this question at the www.btzs.org forum, Phil Davis answers these kind of questions personally, so there is nobody more qualified to help you than him.

6. ### markMember

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Thanks so much for the clarifications.

I think I am getting it. So since you print Pt/Pd and that is the direction I am ultimately heading a normal SBR is 7ish.

An incident reading that gives an SBR of 8 means I develope normally or maybe n-1

If My spot metering comes out at an SBR of 2.3 i need to just take the camera to another location because I would be looking at N+4.7 development or am I getting this wrong too? Ugh.

Once I get over this hurdle I think I am ready to go. It might also explain the negatives I have been getting. When I apply what I thought I had learned from BTZS the negs were dense as hell and flat. When I applied the Zone system modified to look for a 7 stop difference I got good looking stuff.

7. ### JorgeInactive

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You got it! a SBR of 7 is what in zone system we know as a scene that has normal contrast and requires N developement. As you say a SBR of 8 would be the equivalent of a N-1.

Absolutely right on the 2.3 SBR, it would be impossible to develop a negative and increase that much contrast. At least with modern films, IMO.

8. ### David A. GoldfarbModeratorStaff MemberModerator

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Regarding the difference between the grey card reading and the incident reading, it may in fact be the case that you need to incorporate a correction factor of .3 stops. Check the manual for the meter or the spot attachment.

There are also differences of opinion as to how to read a grey card. Some read it straight on and some read it at an angle between the lens axis and the main light source. See if reading the card in a different manner gives you a result equivalent to the incident reading (try flat, 30 deg., 45 deg., and 60 deg.).

With my Flashmeter III, I need to reduce exposure by 1.2 stops with the 5-deg. finder according to the manual (and my own experience), so I just add that factor along with filter factor and bellows factor to the ISO setting when I use the meter with the finder, and that way I don't leave anything out and can just take a straight reading.

9. ### markMember

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Thanks folks.

Jorge.

This explains a hell of a lot and I am off to shoot some film this week to try it, before I head for Colorado for a week.