Monday, 20 May 2013

My camera's (fairly impressive) dynamic range

Photograph 1 - a high dynamic range scene
I set up/found a scene (I added elements to the scene) - Photograph 1 - with distinctly high dynamic range and all the requirements listed in the course necessary to measure my camera's dynamic range. I adjusted the camera's ISO to its lowest value (ISO 100) as suggested and turned the noise reduction setting to low (my Sony SLT-a57 camera didn't possess an off setting for high-ISO noise reduction).

   I exposed the scene so that with the highlight area (the white disc reflector) there was only minimal highlight clipping of that object. This of course meant the rest of the scene was rendered as very dark by the camera - because the scene had so much dynamic range. However, I tried not to worry about this as I would later try to 'open up' the shadows. The exposure setttings for the scene were: f6.3, 1/160 and ISO 100.

Figure 1 - an annotation of the features of the high dynamic range scene
   I then measured the highlights (the white disc reflector) and the two darkest shadow areas (a tree in deep shadow and a black-clothed dummy I had constructed also in shadow) (see Figure 1). I measured them by using spot metering and rotating the tripod head until the central spot metered the desired area.

   My recording for the white disc reflector was: f6.3, 1/500 and ISO 100. My recording for the tree was: f6.3, 1/15 and ISO 100 and my recording for the dummy was: f6.3, 1/13 and ISO 100.

   I then set about calculating the range in f-stops of the brightest part of the photo (the disc reflector) and the darkest (the dummy). I admittedly found it quite hard to comprehend the relationships between (full stop) f-stops and shutter speeds. However, after some research I understood quite clearly their relationship; namely stops and their effect on exposure. What I gathered was that in order for an exposure to gather twice the light, either the f-stop had to be opened by one full stop or the shutter speed halved. Conversely, in order for the exposure to be half the brightness, either the f-stop had to closed down by one full stop or the shutter speed doubled.

Figure 2 - a 100% crop of the highlight area (the disc reflector) and also the pixel value sampler
Photograph 2 - the scene brightened by three stops
   I first checked the white values of the disc reflector were just less than the 255 in each channel by zooming in to 100% to the disc reflector and using the pixel value sampler (see Figure 2). I found the white values to be adequately close to the desired 255 pixel value mark. Then I adjusted the exposure of the photograph to open up the shadows and so bring the (very) dark dummy into play (see Figure 3). I was able to increase the exposure by 3 stops in the raw file (Photograph 2) until there was no distinguishable difference between the detail of the (now visible) dummy and the noise that had crept in.

   I then set about calculating the extra difference in light between the dummy and the disc reflector. Because the variable in the exposure triangle for my dynamic range test image was the shutter speed, I calculated the difference in stops between the disc reflector and the (unbrightened) dummy from the shutter speeds for each I had recorded earlier. There were 5 full stops (1/500, 1/250, 1/125, 1/60, 1/30, 1/15) difference in brightness between the two.

Figure 3 - a 100% crop of the shadow area (dummy) with a careful compromise between detail and noise

   I then added this value of 5 stops to the 3 stops gathered from brightening the dummy and converted this into the requested f-stop values. This gave me a range of 8 stops or the equivalent of f2 to f22 in f-stops. Measuring my camera's dynamic range in a practical way like this made me even more aware of how powerful and useful it was to have a camera that could 'open up' shadows like in the scene I set up, while still maintaining an acceptable level of noise.

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