Sunday, 15 July 2012

Exercise: Your Cameras Dynamic Range

The dynamic range of a scene is the number of stops between the brightest and darkest areas of the image (highlight and shadow). The dynamic range of the camera is the number of stops it can capture in an exposed frame.

Dynamic Range (8.0)
The 30D scored very well in our dynamic range test, essentially matching the performance of the Canon EOS 5D. We test dynamic range by photographing a calibrated test target, whose lightest area is more than 13 stops brighter than its darkest and analyzing the image with Imatest software. Imatest measures how many stops the image shows distinctly at various quality levels. We report the High Quality level, which includes only steps that the camera shows with less than 1/10 of a stop of noise, and Low Quality, which includes steps with up to a whole stop. Though the test reports the number of stops detected, it's important to note that the testing setup is designed to show the maximum range possible for a given camera and the results are meant to compare one camera or ISO setting with another. It is very unlikely that an image shot of a typical, natural scene would achieve these levels of dynamic range. 

Most cameras we have tested show a significant decline in dynamic range as ISO increases: the better the camera, the smaller the decline. The 30D has unusually steady performance into high ISO ranges. At Low Quality, its range at ISO 1000 is only one stop less than its range at ISO 100. At High Quality, it loses less than 1 1/2 stops over the same range. It's only at 1600 and 3200 that big drops in range occur.

If the dynamic range of the camera is less than that of the scene, then either the highlights will be blown out, or the shadow areas lost of detail. According to the text notes, we can use the camera’s highlight clipping warning to find the brightest highlight, and then look at the noise in the dark end of the tones. There is a point when it is no longer possible to determine between noise and detail in the shadow zone. This is subjective of course, which could be one reason why camera makers don’t publish dynamic ranges of cameras generally.

I found a scene with a very high dynamic range, as suggested by the exercise notes.  This scene is in my garden and includes a white reflective surface (trampoline signage) and the dark part of the image is the indoor view through the window, as suggested by the course notes. I turned off noise reduction in my camera and set the ISO to 100. I took a couple of photos until there was no highlight clipping of the white sign.

f/4, 1/30sec

However when I loaded this image to my computer, there is very clearly some highlight clipping according to Lightroom, so I am a bit confused by this difference.

Clipping areas highlighted by Lightroom software

I then set the metering mode of my camera to spot, and measured the aperture/shutter speed combinations of various parts of the image. These are labelled in the image below and show the range is between 1/8000 and 1/20 (with constant aperture f/4), which I calculate to be 9 stops (also shown in sketch below).

Labelled shutter speeds


IMG_2789 is the first image that I open in Photoshop that has values a little less than 255 in the white label. This image is exposed at 1/2000sec, which is ~6 stops less than the image I took with my camera first which suggested little or no clipping of the sign. (??)


Perhaps I failed to correctly analyse the clipping on my camera when I took these images (a month ago now, as I have not had time to write up these notes). I then go to the dark areas of this image (there are many, in fact, it appears heavily underexposed!) and try to bring them up to look at the detail and noise, but it all appears quite noise and it’s hard to determine which is which. I’m not sure how I’m then meant to read off the range between the two ends as suggested in the notes.
The image that I took has roughly a range of 9 stops. There is no shadow clipping at the low end, but a lot at the high end. That means that my camera cannot capture the 9 stops of range.  Although I cannot conclude the exact dynamic range of my camera, I have certainly, through this exercise, become much more aware of dynamic range in the camera. I generally actually choose to only shoot scenes with a limited dynamic range as I am aware of this problem, however, it’s been interesting to have blown-out areas and see the impact of this on the image. I may try to use this more in my photography to push the boundaries somewhat.

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