Image Sharpness

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A common question from photographers is, “How to get the maximum possible sharpness in a print?”

When seeking sharpness in a platinum/palladium print, there are several factors to consider:

  1. The sharpness of the negative
  2. Whether you use a split-back printing frame or a vacuum frame
  3. The paper you are using
  4. Your UV light source

Negative Sharpness

It stands to reason that without a sharp negative you are not going to make a sharp print.

Your Printing Frame

Split-back printing frames are very convenient, but they often struggle to get good and even pressure across the whole negative. This is especially true for negatives larger than 8″x10″.

Low or uneven pressure can result in the paper and negative separating slightly during exposure, and this leaves the print with soft/blurry areas.

When making big prints it is almost essential to use a vacuum frame.

Your Paper

Your choice of paper affects print sharpness in two ways. The first, and perhaps most obvious, is that paper fibres shift around a bit when you develop, clear and wash your print. And the paper tends to shrink when it is drying. This means that you will always lose some definition when making a platinum/palladium print. But different papers may shift/shrink by different amounts, so if you are looking for maximum sharpness then you will need to experiment here.

The other way that paper affects sharpness is through its texture. Textured paper has lots of small indentations which create micro-separations between the negative and the sensitised paper surface. These micro-separations reduce the print’s sharpness. Papers like Arches Platine and Revere Platinum have very little texture, so are fairly sharp. Papers like Buxton and Rives BFK have quite a lot of texture, so they have more micro-separations, so give relatively softer images.

Figures One, Two and Three show how this happens.

Figure One: A Large Light Source with Smooth Paper

The purple bar at the top of Figure One is a large UV light source, and the descending purple lines indicate the rays of UV light. At the bottom is a sheet of smooth sensitised paper with a negative pressed against it. For the sake of simplicity, the negative is assumed to be fully black with only a single, small spot that is transparent (signified by the little gap).

Light can pass through the transparent spot from a range of angles. Any micro-separation between the negatives and sensitised paper leads to softening of the image. With smooth paper pressed tightly against the negative, this softening is minimal.

Figure Two: A Large Light Source with Textured Paper

Figure Two shows what happens when you use textured paper. The increased micro-separation between the negative and sensitised paper leads to a softer image. This softness is quite subtle, and one of the major reasons that I use textured paper.

Figure Three: A Large Light Source with Smooth Paper and Negative Separation

Figure Three shows what happens when micro-separations become actual separation: the image becomes even softer.

There are several ways separation can occur:

  • Poor or uneven pressure from your split-back printing frame
  • Flipping your negative so the emulsion is on the top, separated from the sensitised paper by its support
  • Putting a sheet of transparent material between the negative and the sensitised paper, which is sometimes done to protect the negative from moist sensitiser

Your Light Source

Most people use banks of fluorescent tubes as their UV light source. These large light sources have many advantages, but they do tend to increase the softness caused by micro-separations or actual separation between the negative and sensitised paper.

For the ultimate sharpness, a point light source is better. This is typically a single, very bright UV bulb which is placed a long way from the negative. If the point light source is too close then the negative and sensitised paper will have uneven illumination, leading to visible fall-off at the edges of your print. For large prints you may well find that you need to position a point light source as much as 10 to 15 feet away from the vacuum frame.

Figure Four: A Point Light Source with Smooth Paper

Figure Four shows the effect of a point light source with smooth paper.