Saturday, 2 June 2012

Exercise 23 : Control the strength of a colour

The first exercise in the Colour projects deals with how exposure settings can control the strength of colour in a photograph and requires that five photographs of a subject with a strong, single colour are taken at different exposure values.  The single colour is needed to fill the entire frame of the pictures and the frame ideally should not move between exposures, so I set this up on a tripod, using a bright red backdrop and a cricket ball.

In my usual shooting mode of Av (aperture priority), altering the f-stop would result in the shutter speed changing automatically to maintain the ‘correct’ exposure according to the cameras metering system.  It was therefore necessary to set the camera to manual so that the same shutter speed was maintained and only the aperture changed – obviously this is going to result in over and under exposure.  The camera metered the subject that I selected for this exercise as 1/350 sec at f/5.6, so I took the first exposure at this setting.  With manual mode then selected I took further images at f/4 (so at a wider aperture than the metered setting), and at f/4.5, f/5.6, f/6.7, f/8 and f/11, all at 1/350sec.

These photographs were imported into Lightroom 4.1 and examined in their RAW state without any manipulation or correction.  Increasing underexposure gave an increasing strength of colour as can be seen from the sequence of photographs presented below.  Using LR to mouse over the same point of each image allowed the RGB values to be recorded; these were as follows, and it is interesting to note that the R:G and G:B ratios remain fairly constant throughout the series of images.

f-stop  Red     Green Blue

f/4.5     71        44        37

f/5.6     61        35        28

f/6.7     53        30        25

f/8        42        23        19

f/9.5     34        18        14

f/11      27        11        8

Freeman (2007) in The Photographers Eye (p106) discusses the value of over and under exposure in either diffusing or concentrating attention of the viewer, and shows how intentional underexposure can be used to focus on specific bright elements of an image and restrict the eye from wandering away from the prime purpose of the photograph.

I took this exercise one step further and used a sequence of images I had taken previously allowing the camera to make its own adjustments to exposure as I stopped down the aperture from f/5.6 to f/22 to demonstrate just how technically efficient the metering system is at remaining almost identical colour as the settings changed (aperture and shutter speed in the instance).  It is the bonnet of a Lamborghini (unfortunately not mine ..) and appears in the sequence of images immediately following those for exercise proper.  Mousing over the same spot on each image in the sequence demonstrated almost zero variance in numerical RGB values.

It is worthy of note that brightness and saturation are often mixed in photographic interpretation and it needs to be recognised that they are not the same thing.  Changing the exposure, as in the exercise here, will alter the brightness of the colour in the images, but the actual purity of the colour itself will not alter.

This struck me as quite a formulaic exercise when I started, and I don’t really get inspired by the “do this, now do that” approach, but I think I got quite a lot out of it once I had the images in front of me and the chance to look at impact of changing the f-stop and the impact of that in turn on the numerical RGB data.  I chose red because that was what was most easily accessible, but I plan to return to this and repeat it with a couple of other colours to see what that turns up.
f/4.5 and 1/350sec

f/5.6 and 1/350sec

f/6.7 at 1/350sec

f/8 at 1/350sec

f/9.5 and 1/350sec

f/11 and 1/350sec

f/5.6 at 1/750sec in Av mode

f/8 at 1/350sec in Av mode

f/11 at 1/180sec in Av mode

f/16 at 1/90sec in Av mode

f/22 at 1/45sec in Av mode

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