This exercise was designed to measure the dynamic range of my camera for a high contrast scene. The instructions suggested using a scene with at least one brightly reflecting surface, bright sunlight and an area of deep shadow with a dark surface. I set up a situation similar to the image in the course material with a sheet of A4 white paper next to the back door. After waiting for the sun to create a shadow across the scene, I spot metered areas of the scene using the camera set to AV.
F9.5 1/180 ISO 100 14mm
I spot metered three areas and made a record as to what I was seeing. The lowest value was 1/20, the highest value was 1/180. Between this there are 7 stops:
1/20, 1/20, 1/45, 1/60, 1/90, 1/125, 1/180
Camera forums suggest my camera (Canon 1000D) has a dynamic range of 11 x F stops. Although my camera suggested 7 stops for this exercise, I think it varies with the brightness of the light (for example, on the day I did this exercise, the light was changing rapidly. I had to wait for the sun to come out from behind the clouds at which point the camera speed was faster (1/250 compared to 1/180) but the shadow remained the same speed.
As I took the image in RAW, I was able to view the colour values of the white paper in photoshop. R = 249, G = 243, B =242. As these add up to less than 255 x 3, the area is not overexposed. In the shadow area, adjusting the exposure of the window frame until it was visible meant increasing it by 2 stops. I am not really sure why I did this, apart from the fact it was part of the exercise. Following feedback from Assignment 1, I installed Lightroom on my computer. This shows the histogram and the areas of highlights (burnout) and shadow detail which may be unrecoverable in different colours and I have found it invaluable for checking whether a photo I thought was correctly exposed actually is. I felt at the end of this exercise that I perhaps needed to something similar again, so I sought out a similar type of scene as part of the following exercise (scene dynamic range)
I read around the subject of dynamic range. I knew already that when films were processed the image was built up like using layers in photoshop. I was aware that some digital cameras have the capability of taking three similar images like auto bracketing and merging them together to create an HDR image (merging highlights and lowlights). I had read that software was available to do this in the processing stage for cameras which did not have this capability. Freeman (2011) explains that “as the charge in the photo-diode “well” is filled up, the highlights are blocked and it is as if you have a hole in the image – or triple 255 when measured in RGB.”
From learning about the histogram and dynamic range, I observed that the histogram on the back of my camera clips when one or two of the channels are near the top of the graph. This was backed up by Freeman (2011) who explained that “clipped highlight warnings displayed in a camera’s LCD are likely to be at a value of less than 255”
Freeman. M, (2011) The digital SLR handbook, Ilex press, Lewes, UK (p46)
Freeman. M, (2011) The digital SLR handbook, Ilex press, Lewes, UK (p48)
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