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Understanding the Technology

Photographic Technology involves understanding the physical and chemical processes that result in the capturing of an image either on film or an electronic recording (i.e. Digital). Both methods share technology; and whilst Digital has become the preferred option for many people, understanding film can also provide insights into digital.

Raking photos, whether digital or film, should always start with a knowledge and appreciation of the light you are working with.

Measuring Light

Camera light meters can be divided into three groups: "averaging", "spot" and "centre weighted". It is essential that you know what type your camera uses. In general, rangefinder cameras use an averaging system and most SLR's now use a centre weighted system.
Each system varies from the other in the following ways:

  • A centre weighted meter reads the light across the entire frame but "weights" the reading in favour of a spot in the middle of the frame, in other words, giving the light in the centre of the frame a little more effect on the reading than that in the rest of the frame.
  • A spot reading meter restricts its reading to the centre of the frame only, usually about 12% of the total frame area or corresponding to the centre focusing prism/ split image area.
  • An averaging meter reads the whole of the frame and averages the light and dark it reads to achieve an exposure reading.
Unlike conventional photography the digital sensor can change sensitivity so that, for example, in a dark situation it will increase sensitivity and therefore allow you to use more practical camera settings such as shorter shutter times or smaller apertures. Digital cameras vary across the range in what kind of metering system they use, often it is a centre-weighted meter as discussed. However some cameras use an evaluative or matrix metering system, which divides the entire image into a patchwork of zones that are all separately evaluated. This system is very accurate over a wide range of different and difficult lighting conditions. Although exposure systems in modern cameras are very sophisticated and good at delivering results, they are not perfect. Therefore it is very important to understand exposure and what is the ideal exposure for each image.



This is the study of how light interacts with the surface that captures the image; whether film or digital.

A sensitometer is an instrument for exposing a photographic material to produce a graded series of stripes, a step wedge, the values of which are accurately known.
Basically, it incorporates a light source of standard intensity and colour quality; a standard gas filled tungsten lamp operating at a colour temperature of 2950K with a filter to achieve 5500K daylight modified by camera lens.

As a means of modulating the light intensity to produce the graded series of exposures (referred to as intensity scale) a time scale shutter (sector wheel, or falling plate, or sectored cylindrical drum) can be used.

Which ever system is used, the sensitometer is deigned so that the exposures increase logarithmically along the length of the strip which shows the results of different exposure times.

Densitometers are photometers designed to measure photographic densities. 

Densitometers can be broadly classified into the following two types: 

  1. Visual densitometers are relatively simple and inexpensive.
  2. Photo-electric densitometers, also called physical or electric, vary greatly in their design:
    • single-beam instruments - uses a beam of light to illuminate a photo-electric cell.
    • twin-beam instruments - two beams are used thereby eliminating possible errors.
Digital Sensitometry
Because a camera’s sensor produces no negative, it does not suffer from reciprocity, and has a spectral sensitivity very similar to that of the human eye. But does it follow that there is no longer a use for sensitometry in digital photography? Of course there is, and we actually use it all of the time in Photoshop, Paintshop Pro, GIMP or if you are using Linux or whatever image manipulation program and operating system combination you happen to prefer.

In Photoshop, for instance, the Levels tool produces a representative histogram of the tonal range of the positive image. A dark image will have most of the tonal distribution towards the left of the histogram, and a bright one to the right. An image with an even distribution of tones will have them spread throughout the histogram. This is a representation for the positive image, while in the sensitometry you have looked at so far with film, you have considered the negative. So where in Photoshop do we find the Negative? The answer is in the Curves Tool.

If you were to open the Curves, you would not see one that you were immediately familiar with. The curve you see is a straight line from bottom left to top right. In the ones you have been used to so far, there is a lag phase, a toe, a straight line area and a shoulder before flattening off at D-Max. These portions of a film’s characteristic curve are not design features, they are flaws. If there was such a thing as the perfect film, it would have a characteristic curve exactly the same as the one in the Curves tool.

Where this curve further differs from the ones you are used to, is that it is not the fixed type produced by a photographic process, because a sensor is not a piece of film, it’s for a digital image. So, in the Curves area of Photoshop, or any other image manipulation program, you are looking at the characteristic curve of an individual image, as if it had it been derived from a negative.

However, and because we have the power to do so, we can further alter the shape of that curve as though we had increased the exposure and reduced the development to flatten the contrast, or reduced the exposure and increased the development to increase the speed of the film.

But, not only can we alter the shape of the curve for the whole piece of film, we can also do this in small regions of the curve; say in just the highlights that might be a bit blown out, or the shadows that are too dark. What we have with the Curves is the power to a change a curve as if we had adjusted the exposure and development times, to the entire curve, to a portion of the curve, or to a number of areas along the curve.

Furthermore, you can produce a different curve for every single image that you take. If you are really interested in, or have read a little about the Zone System, this would be the digital equivalent of it.

If you do not know much about Edward Weston and Ansell Adams, it makes for interesting and useful read. A young Adams used to assist Weston, in the 1930’s and the two photographers were very poor. The result being they only allowed themselves two sheets of paper per negative, one for test strips and the test piece, another for the print. To use any more than this ate into the paper allowance for the next and subsequent prints.

What they developed between them, and which was later much refined and perfected by Adams, was a process by which they could match the characteristic curve of any scene to one that would fit perfectly onto a sheet of grade two printing paper. They would take a series of 12 exposure readings from a scene and then modified the exposure and development times accordingly.)

But not only is there a Curves adjustment tool in Photoshop itself, there is also a Curve tool in ACR for when you process your RAW files. Remember from Sensitometry A, where we looked at adjustments made in Photoshop – or whatever image manipulation program you use – any changes will always result in a loss of image information. There are no ifs or buts here; this is always the case.

The RAW file can be considered as the latent, unprocessed image. When you process in ACR, you do not lose information, you effectively modify the output when you finally process the image.

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