
N+1=150% of development?
I started a large format class this semester a couple of weeks ago. We have started learning the zone system. The instructor has told us that N+1 is 150%, N+2 is 300%, N1 is 75% and N2 is 50% of overall development time. I am curious as to what others think about this.
I have been using the zone system for over a year now with 120 film and multiple backs. Everything I have read says that adding 15%20% is equivalent to N+1. This is what I have been using for the past year. Who is correct? Having I been doing it wrong this whole time. Will doing what he says actually increase/ decease the highlights by a full zone.

my N times for TMX and D76 1:1 are:
N = 8 min
+1 = 10 min or 25%
+2 = 11:30 or 43.7 %
1 = 7 min or 12.5%
2 = 6:30 or 18.75%
Your Prof is mistaken, IMO, as far as determining basic "N" developing times. But some situations can involve extreme measures that may involve changing the agitation schemes, developer dilutions, etc...but for basic "N" compensation, no, as far as my own work goes anyway.
I have developed negatives exposed to very low SBR (1:4) for 3x my "normal" of 8 min for a 24 min developing time, that was a 300% increase in development even using standard inversion agitation at 1 min intervals. You can see the result of that negative on my Flickrsee "YellowPoplar Leaves".

Ask him what film/developer combination he is talking about.... What it takes to slide the film one stop on a scale depends on an exact combination. You can't generically say, for N+1, 50% or 20% more.

He may well be right for a specific film/dev combination as they all differ. +50% time is a big expansion for most films though.
Best way is to expose a couple test strips from a wedge and run a densitometer over it. That will give you a definitive answer on the contrast you're achieving.
If you're struggling with ZS concepts, get "Beyond the Zone System" (BTZS) by Phil Davis. There are plenty of secondhand copies floating around cheap and you can read it covertocover in a couple nights assuming you already have a passable grasp of mathematics.

A good starting place to determine N+1 development is to multiply your standard time by 1.4. For N+2 you can then multiply that time by 1.4 again. Differnet films and developers will of course generate different results, but this g=has worked as a very good starting place for me. usually getting me very close to my target densities. I might add that N1 can obtained by dividing your standard N time by 1.4. This is what I learned back in the 70s while studying the Zone System with John Dowdell, coauthor of The Zone Systemizer, along with Richard Zakia.

none of these factors work reliably.trust me, only a proper film test will do

I figured as much from everything I've experienced and read: Like I said, I've been using ZS for over a year.
Ralph, I use your book in place of the recommended text book

Darkroom317,
As has been pointed out, your instructor is mistaken. The numbers he quoted are not the kind of time series that you would find by testing (like CPorter's times, which I'm familiar with from looking at his curves.)
However, look at it from the point of view of "how bad" the advice is... It's not that bad. For the shot you need N+2, if you develop 300%, you have a fighting chance of pulling out a workable negative. The converse is true, N2, developed 50% the time, would hold the highlights in check.
And, it's very likely it will be on the test. So go ahead and commit it to memory.

It depends entirely on the film, developer, exposure, agitation scheme, and developer temperature. That's five variables that need to be controlled.
A "rule of thumb" will get you in the "ballpark." Experience will get you into the infield. Testing will get you into the batter's box.