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Exposure control


Exposure Control
Wednesday 1st January 2003 4:04pm


The difference between a 'good' and 'fine' print is in the retention of detail in the deepest shadows and highest highlights.
Once the basic exposure for a print has been determined it is usual to apply several steps of dodge or burn in order to balance the detail in the shadows and highlights. This can become a most compulsive obsession in the darkroom.
It is not unusual to execute a dozen or more dodge and burn steps for a given print.
Each step is applied for a given time, and if the print is to be reproduced all those times must be recorded, and each individually recalculated if any of the exposure conditions change. One reason exposure may change is if the lens aperture is adjusted to give longer exposure times, so that the dodge and burns are more manageable.

Longer dodge and burn times are easier to manage because there is usually a short (1 second say) 'error' when synchronising the position of the mask with the operation of the enlarger. This error has a smaller impact when the dodge or burn time is longer.

So if you decide to perform a dodge for five seconds you may well stop the aperture down to give a ten second dodge - remembering at the same time to double the base exposure.
Sounds simple enough, but with up to a dozen such steps the calculations interfere with the artistic thought processes that are on-going at the same time.
The whole story is similarly complicated if the enlargement factor is changed, or the contrast filtration(s), or the paper batch. Considering Ansel Adams' 'how to' series, Adams states
it is useful to think of a dodging or burning-in time as a percentage
of the total exposure. We can become quite proficient at estimating the effect of dodging or burning an area for, say, 20 percent of the total exposure time.'

Adams, A (1995), The Print, Vol III The Ansel Adams Photography Series, (pp 102). Little, Brown & Company
In Adam's 'Clearing Winter Storm' example his ten dodge and burn steps use only six different times. There are two 2s dodges, one 1s, one 2s, one 3s, one 9s, and three 10s burns, with a 2 to 4s corner burn. The 9s burn is performed in three 3s sweeps and two of the 10s burns are performed through a hole rather than by a simple mask (see fig. 1).
The image of Adams' hands literally dancing in the enlarger light stream through all these steps is compelling and helps to illustrate his philosophy of the print as performance.

Figure 1 Burn Map for the Ansel Adams example (showing duration and types of dodge/burn steps)


The performance of the print, the subjective decisions about exactly where to place the mask, by how far to overlap the surrounding area, and exactly when to end the operation, are the factors that should be consuming the attention of the photographer. It is this process that makes each print unique, and it is uniqueness that is the wet darkroom's unassailable strength over the digital workflow.

It is worth then creating a method of work that simplifies the numbers, mental calculations, and feats of memory required in the print performance, so that the creative mind is at greatest liberty to realise that performance.
Using percentages helps in this respect, especially when the exposure must change, but there are still many different actual numbers to remember, the duration of each dodge or burn step. Fortunately the process can be further simplified.
First of all, in practice there is a minimum dodge-or-burn time that produces an effective result. This will vary between different material combinations but there are only a relatively few such combinations in use by any given photographer. The minimum effective dodge or burn time for a given paper/developer combination can be determined through experience. I generally constrain dodge and burn times to a minimum of 5% of the base exposure time with Ilford Multigrade IV RC Gloss developed in Multigrade Developer.
Secondly, the precise dodge and burn time is not decided rationally and absolutely whilst reviewing the proof print but rather is part of the subjective performance of the print, within bounds. What this means is that instead of thinking about three separate burns with three separate burn times of 1s, 2s, and 3s we can think of three separate burns that are around 2s. The actual difference between the burns is decided on-the-fly during the exposures.
All three burn steps are applied with the enlarger set to 2s. The first burn can then be given 'about half' the 2s exposure time. The second burn is given 'most' of the 2s exposure time. The third burn is definitely given one full 2s burn followed by 'about half' of this standard 2s exposure time.
The above scenario reduces the number of times that have to be considered and the need to constantly change the enlarger timer setting (reducing chance for error).
When creating a 'fine' print the subjective terms 'about half' and 'most' are counter-intuitive. It seems that such approximations lack accuracy and so cannot give rise to a controlled final result.
The subjective approach does however work. The absolute 'inaccuracy' is controlled by the size of the standard burn or dodge step chosen. The 3s exposure above will be given at least 2s and no more than 4s, and so will not be that inaccurate.
Also, it is unlikely that a difference between, say, 3.9s and 4s would yield a perceivable difference in the print. The dodge and burn steps need not be applied with any accuracy greater than, say, 5% of the base exposure time. For a 10s base exposure the effect of a 3s burn can be achieved anywhere between 2.5 and 3.5s. Most especially, the subjective approach works because the precise, final decision of how much of the second 2s exposure we give to the third burn is taken as the exposure is being applied. The photographer is not concerned with resetting the enlarger timer, only that the third burn should receive one full standard exposure and one partial standard exposure. During the partial standard exposure the thought processes are entirely concerned with what was perceived from the proof print, and the subjective effect desired, and how well the operation has been carried out so far - in real time.
This constrained-subjective method is further supported when one considers that there is no such thing as 'the final print'. The printing of a given negative will always change as it is informed by the development of the photographers' style. A constrained-subjective approach creates a workflow where the print is given the opportunity to reveal something new that has not been previously, consciously considered - by its very subjective nature.

Figure 2
Fixed Increment example burn map (note regularity in time & subjective language)

The 'standard exposure time' for dodge and burn steps is determined on a per-print basis and is a function of the base exposure time and the minimum effective dodge or burn time. Performing all dodge and burn steps with a single enlarger timer setting (or multiples thereof), instead of perhaps a dozen different timer settings, greatly simplifies the printing process.
Once the proof print has been evaluated a decision is taken regarding the relationship between the different dodge and burn amounts required and the responsiveness (i.e. sensitometry) of the paper/developer combination.
For a base exposure of 60s with a 50% sky-burn and a 25% central dodge, for example, the enlarger timer would be set to 15s. The base exposure would be achieved with 4 'standard exposures' for the print, during one of which the dodge would be applied. Two further 'standard exposures' would then be made to burn the sky area. The entire print is made with only one timer setting.
The principal is also useful for split-grade printing. If the entire print may be given 80% at grade 5 followed by 20% at grade 0 the timer would be set to 20% of the base exposure and applied 4 times at grade 5 and once at grade 0. Eliminating the need to reset the timer reduces the opportunity for error.
Such a method should not be applied thoughtlessly or slavishly. As the number of 'standard exposures' required for a dodge or burn step increases the chance of miss counting also increases, so eventually the method becomes more complex than continually adjusting the timer. The important principal is to establish some method that aids and simplifies repeatability within the confines of the print's sensitometry and the photographer's creative freedom. As a worked example the figure below shows the dodge and burn map for the photograph at the head of this article.

Because of the extreme latitude I originally found the exposure for the main window through one test strip and the left-hand wall through a second. These suggested 45s for the window and 9s for the wall. I decided to work at a fixed 9s exposure increment, with one increment for the base exposure. A series of additional 9s fixed increment exposures were applied as per the map.
The base exposure need not have been a single increment. In this case the smallest burns were 100% of the base exposure. Had they been 20 or 50 percent then the base exposure may have been made from 5 or 2 increments, respectively. Also if I had needed to dodge an area I may have arranged the fixed increment in order to provide an entire exposure step for the burn operation and one step with no planned dodge or burn. Planning to have a dodge and burn free single exposure increment creates space for last minute adjustments. Usually applied as the final exposure increment this allows for last minute tweaks based upon the printer's immediate reaction to the print as is has just been performed.
The fixed increment exposure method is much simpler in practice than in theory. By design the method is imprecise. The purpose is to give rigour to the numeric process of exposure so that there is a well-defined framework for the creative process. This means that some liberty can be taken in setting the fixed increment value. If the numbers aren't as convenient as in the example they can be tweaked. Each increment will be used in an 'about' way. The base exposure however should be an exact number of fixed increment exposures.
The benefit of the fixed increment exposure method is that during printing we need only count exposure increments whilst concentrating on the precise nature of the dodge or burn. In the example, the +4 burn of the main window illustrates this. How far into the image this burn extends, and for how long the burn maintains its maximum reach, is the on going subjective thought process during the burn operation. The outcome affects the floor in a major way.
The feel of the image can be entirely controlled by the amount of burning to this floor area.



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