F-Stop Based Enlarger Timing

If I were to pick the one change to my darkroom processes that has had the biggest positive impact, it would have to be f-based approach to enlarger timing. I was first introduced to this technique by the Scottish photographer Stuart Low back in 2018. At this point I was wrestling with 16x12 prints in a makeshift darkroom, and wasting lot of paper …

The Basic Method

Traditionally enlarger timers are calibrated in linear time, e.g., advancing by 1/10s. This completely disregards everything we know about how exposure works, making the time adjustments an error prone guessing game that wastes both time and paper. But there is a simple fix: apply what we know about exposure from working with cameras, including the concept of reciprocity, to the darkroom process. This is usually referred to as the ‘f-stop based method of enlarger timing’.

The core principle of f-stop based exposure is this: changing an exposure value by 1 f-stop will transform a given shade of grey to another shade of grey in a deterministic fashion regardless of the actual exposure time used. This in turn makes it possible to quantify, and hence reliably anticipate, how much the print will change with a selected adjustment.

The smallest exposure change that is perceptible to the human eye seems to be about 1/12th of an f-stop, i.e., you will be able to tell which of two prints the exposure of which differs by 1/12th of an f-stop is darker, but only just. This makes the 1/12th of an f-stop the optimal step for adjusting enlarger exposure.

The best way leveraging the full potential of the f-stop method is to use a timer that is designed this way (on that below), but it can also be applied perfectly well by just using an exposure table. For anyone who wants to try out this method, there are numerous tables, and calculators, available online, but they generally unnecessarily overcomplicate the matter. All that is required is a single pre-computed 1/12th f-stop scale. Here is one used by the timer I use (RH Designs Analyser Pro), which is computed using 15s as the centre value, adjusted from in both directions:

(There are, of course, many such scales possible, you could base such a scale on a 10s, or 12.3s, or whatever, centre value, but this doesn’t matter — the biggest possible difference between any two such scales is 1/24th of an f-stop, which is inconsequentially small from the point of view of the human observer of the final print.)

Exposure Metering

The great thing about the f-stop method is not only that it produces deterministic results, but that it works well with exposure metering, which largely eliminates the time and paper wasting associated with working out appropriate exposure by the traditional method of making a series of sequential test strips.

Because the f-stop based adjustments produce quantifiable changes, it is possible to match metered values to the characteristics of any particular paper, and give a visual indication of the expected printed tone of any particular part of an image for a given exposure — the RH Designs devices have a greyscale on them, printed in 1/4 of f-stop increments, that show where the metered values fall for the selected exposure and (for variable grade papers) grade, and you can see how this will change when adjusting the time and grade parameters. That is, if you choose the metered points sensibly, you can determine both the appropriate exposure and grade selection without doing any test strips. (In practice, I still end up doing some test strips prior to the first full test print to fine tune the exposure, as there are slight variations between paper batches, etc., but this is infinitely more convenient, and less wasteful, than the traditional method.)


The f-stop method combined with metering makes scaling images very simple: I just re-meter the same spots and then apply any adjustment I did for the 10x8 print in the 1/12th f-stop units.

For example, let’s say I metered a 10x8 print at grade 2, then decided I needed to use grade 3 and exposure of +3/12th of an f-stop over the metered values to get the desired results. To scale up to 16x12: I setup for the 16x12 enlargement, open the lens by 1 f-stop, meter the same spots I did originally at grade 2, change to grade 3 and apply +3/12th of an f-stop correction, and that’s pretty much it. I’ll do a strategically placed test strip, in case some further adjustment is needed (e.g., due to variation in the paper batches, as well is difference in the scale of the metered points), but such adjustment will be small, and I am ready to do my first full-sized test print.

In practice this means I can do most of the experimentation and decision making cost effectively on 10x8 test prints and then transfer those to a bigger size of a print with minimal paper waste.

F-Stop Based Devices

I can highly recommend the RH Designs Analyser Pro timer, which combines exposure metering with an f-stop based timer. The device can be calibrated for multiple variable grade papers on the standard 00-5 grade scale, allowing both exposure times and grade to be adjusted in a seamless fashion with a visual indication of where metered values would fall. The Analyser is not cheap, but I consider it money well spent, recouped in saved time and reduced waste of materials. The device is well made and comes with decent documentation. Its only flaw is that the metered values do not survive powering down, my printing sessions often span multiple days, which means I either have to leave the device on overnight, which is not always practical, or re-meter each day, if I need the visual indication of how the print changes (the device, can, of course, be used without metering as a simple f-stop timer).

(RH Designs also produce the cheaper Zonemaster device, this is basically the analyser part of the above device, which you can use in combination with a conventional timer — TBH, if you decide to go down this route I think the additional convenience of the Analyser is well worth spending the extra money.)