As the name say it is a system. And the system consist of: 1. figuring out the film speed 2. figuring out the developer and the developing time 3. knowing our lenses aperture and shutter errors 4. knowing to use exposure manipulations to get specific tone (found in A. Adams books). It could be broken down into more parts, but I think it is enough. And if any point of the system fail the whole system shall fail too. What is offered today in the literature to learn are points 3. and 4., and they are analyzed in great details. The point 2. discussion can be found in just some specialized books (e.g. advanced photography ), but anyway it is accessible somehow. And it is in connection with the photo paper photog use in darkroom, for now. The most blur point is point 1. Some will say there are more than hundred ways to find out the film speed. These ways are, I think, the reason many say zone system does not works, or even many abandon use of the zone system and cling to bracketing. The most reliable way to find out the film speed is densitometer, which good and new is more than $1000us, but used on e-bay can be found and for $100us + custom and shipping + risk + . For most photographers around the planet, this out of reach, too much money. And more, it gets practical photographer into too technical side. The next option is to deep into darkroom and work for hours to figure out the film speed, which I found too bothering and too long. All of other ways are so unreliable that easy can fail (or error) and the whole lens opening. And more, measuring density using densitometer can yield correct film speed but wrong density-exposure curve slope, if one is not a scientist. All of these errors are heavily reflected also onto point 2. of the system which make things more complicated. This is where photography slips into science and many try, with good reasons, to skip it. As a result the zone SYSTEM do not works. And what smaller negative size is, the more pronounced errors are, so 35 mm is completely out of order if one is not a scientist. However knowing points 3. and 4. helps to understand many things around exposure no matter format and is good to know, and helps to use the zone system in some approximation, larger error for smaller negative. Well better than nothing, but it is not real zone system. Many that realized above problem clings to the systems as: measure exposure from the shade and live highlight to their own, . Is there solution, YES, always yes, and it will make that really good lenses and for 35 mm show its own and full power. Without the solution 35 mm will always be the second class.