7. Understanding F-Stop Printing

If you are new to f-stop printing, this section explains why the ΔStop works the way it does. If you are already a practiced f-stop printer, you can skim or skip ahead to Section 8.

The core idea is simple: the ΔStop measures exposure in stops, the same way your camera, your light meter, and your film do. Every other link in the photographic chain speaks this language. The enlarger is the one place where tradition defaulted to raw seconds, and that mismatch is the source of a lot of avoidable frustration in the darkroom.

Why Stops and Not Seconds

Look at the shutter speed scale on any manual camera:

4, 2, 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125

Now look at the aperture scale:

f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22

Neither is linear. Each step up or down the scale halves or doubles the exposure, which is what a stop is: a doubling or halving of light. To an experienced photographer, thinking in stops is second nature. You already know, without thinking, that moving from f/8 to f/11 is the same exposure change as moving from 1/60 to 1/125.

But in most darkroom tutorials, test strips are made in linear seconds, often in five-second increments: 5, 10, 15, 20, 25. The problem is that the visual difference between 5 and 10 seconds is a full stop, while the difference between 20 and 25 seconds is less than a third of a stop. Linear increments give you wildly uneven exposure steps, which makes test strips harder to read and results harder to repeat.

F-stop printing fixes this by making every step on the test strip a fixed fractional stop. A half-stop strip gives you consistent, readable intervals:

3.75, 7.5, 15, 30, 45, 60, 90 seconds

Each band is the same perceptual distance from the next. Your eye reads the strip the way it already reads exposure everywhere else in photography.

How the ΔStop Puts This to Work

The ΔStop's workflow follows the structure of f-stop printing directly:

The Base Time dial sets your starting exposure in seconds. This is your anchor, the exposure you believe is roughly correct for the negative you are printing.

The Test Interval dial sets the fractional stop step used when you make a test strip. At 1/2 stop, each band on the strip represents a half-stop exposure change. At 1/4 or 1/8, each band represents a finer change. The larger intervals cover a wider exposure range in a single strip; the smaller intervals zoom in around a known-good starting point.

The Print Stop Select dial tells the ΔStop how the final print exposure relates to the base time, measured in whole stops from -3 to +3. Once a test strip has told you which exposure you want, this dial is how you apply that decision to the final print.

You never have to calculate a stop in your head. The dials do the math, and you spend your time looking at the print, not at a calculator.

A Typical Two-Strip Workflow

Most negatives are well-served by two test strips. The first strip uses a wide interval (1/2 stop) to bracket your initial guess across a generous exposure range. The second strip uses a finer interval (1/4 or 1/8 stop) centered on the best step from the first strip, refining toward a precise answer.

For a fresh negative at f/8, a forgiving starting point is 30 seconds. A 1/2-stop test strip centered on 30 seconds with five or six bands will almost certainly include the correct exposure somewhere on the strip, even if your initial estimate was off by a stop or two. Pick the best band, set that time as your new base, drop the interval to 1/4 or 1/8 stop, and run a second strip. The second strip gives you a precisely dialed-in work print time.

From there, dodging and burning become straightforward: adjust specific areas by plus or minus a fractional stop relative to your work print time, rather than guessing how a handful of seconds might shift a highlight. The language stays consistent across the whole process.

Why This Matters

Linear-second timing can produce a good print in experienced hands. F-stop timing produces a good print faster, more repeatably, and in a way that scales with experience rather than working against it. Your notes from last session translate directly to tonight's session. A base time written in stops is meaningful even when the paper changes, the negative changes, or the print size changes, because the relationships between exposures are preserved.

F-stop printing is not a new idea. The approach has been documented in technical photography literature for decades, and Ralph Lambrecht and Chris Woodhouse's Way Beyond Monochrome is the reference most printers turn to for a thorough treatment. What has changed is the hardware. Doing f-stop math by hand breaks the flow of a printing session, and until recently, timers that handled the math automatically were expensive professional equipment. The ΔStop was designed to put that capability on a working printer's bench.

If you want to go deeper, Way Beyond Monochrome remains the definitive reference. For the purposes of this manual, everything you need to operate the ΔStop is here. The next section walks through each mode in detail.