Firstly I had to find a scene that had bright sunlight, a reflecting surface and an area of deep shadow with a dark surface. I used a window for this.
I shot this image at f/8 1/80s to make sure there was no highlight clipping. It was difficult to get a shot without over-exposing too much with the sun shining directly on the window frame.
The reading I got for the highlights was f/8 1/1250s. The reading for the darkest areas was f/8 1/10s. This gives a dynamic range of 7 stops which is less than I would have expected.
A more accurate assessment of the camera's dynamic range can be carried out using tools like Imatest which performs a range of tests. From looking on the internet a Nikon D90 has a dynamic range of 8.3.
When I colour sampled the pixels in Photoshop I got 7 for the shadow areas in the window and 233 for the window frame. I expect that the lower value for the highlights is down to two things; the fact that I had to overexpose the image by about a stop to ensure the highlights weren't clipped therefore making the whites slightly darker overall and secondly I shot this at ISO100.
The D90's sensor has a native sensitivity of ISO 200 with ISO equivalent available by using the Lo 1 setting. It was this Lo 1 setting I used for this exercise. The dynamic range of Lo 1 has almost one stop less dynamic range in the highlight regions which you could expect to have from an ISO 200 picture overexposed by a stop.
I changed the ISO setting to 200 and shot again. This time I did not have to adjust the exposure and was able to use the camera's meter reading of f/8 1/200s which is approx one stop in the difference. I measured the highlights and the shadow area again and got readings of f/8 1/6s and f/8 1/1250s. This gives me a dynamic range of 8 stops.
I think the most accurate way to get your camera's dynamic range would be to use the tests from Imatest. However, in the field we would not be able to shoot with precision so having an awareness of your camera's dynamic range should be enough to capture well exposed images.
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