Originally Posted by MartinP
Conversely, testing with a a constant time interval is a big waste of time IMO. But, that's how most students have been taught and they don't realize there are other methods that may work better until someone leads them through it. Likewise, trying to guesstimate and creep up on the proper exposure is the common modus operandi for a beginner. Sometimes they are even unaware that a black tone lurks unfound in their printed negatives. They often underexpose a print and get a too-light image that is flat and gray yet they are happy with it. Hopefully that changes with additional experience and guidance. The fractional f/stop method almost guarantees a beginner will be able to determine the correct exposure on the test print somewhere between the two extremes of severe over- and underexposure. It has to be in the middle if one side is too light and the other too dark.
The lightbulb usually doesn't turn itself on when it comes to the f-stop sequence. All those standard f-numbers are useful to memorize for other reasons. The Inverse Square Law and memorization of the f/stop sequence makes light placement a snap in the studio. Want to make a light twice as bright? Move it from 11' to 8'. Need to calculate an extension factor with a view camera? Those f/numbers thought of as extension distances will tell you the proper exposure correction on your analog light meter. No calculation needed.
Another connection usually long in arriving is that the f/stops on a camera lens work the same way as the f/stops on an enlarger (which is also a camera). It's true. Going from f/8 to f/11 halves the exposure. Another simple, but for some reason a really tough concept to grasp unless directed to think about it is the "Sunny-16 rule." Why do many beginning photo students think they will get a proper midday exposure outdoors at 1/60sec at f8 with ISO 400 film when their camera meter is reading off the scale and flashing warnings at them? It's because they haven't made the connections yet on their own. Someone needs to point out that a camera meter really isn't needed for most outdoor exposures (or indoors if they always shoot under the same engineered lighting in similar rooms). If their camera is telling them something different than a stop either side of the Sunny-16 Rule on a sunny day with frontlit subject, there probably is a malfunctioning camera at fault or pilot error is occurring.
Another reason for the large "density swings" is the additional information provided on such a strip for burning and dodging. Or, when making larger or smaller prints. Or, using a different paper. The method I outlined earlier can be adapted to any time range - one doesn't have to start at 8 seconds for example. You could begin anywhere on that table, adjust the f/stop as needed, and do fewer strips if desired. If the geometric change between each strip is too much, stopping the lens down one stop lets the refinement be in 1/6 stops relative to the initial 1/3 stop sequence.
Granted, experience teaches that most negatives properly exposed and developed will print pretty much the same and a trusted starting point can be used and you wing it from there. But once you reach that level, you've experienced many other related epiphanies. Experience trumps. If you have it, good for you. Others need some direction and a method that produces good results or those connections may never be made. I think the f/stop method is a great way to teach and learn and standardize and become consistent in technique.