Monday, 24 October 2011

Exercise: Your camera's dynamic range

This was an interesting exercise to determine the dynamic range of our camera, mine is a Canon EOS 40D. To measure the range we are to set up a scene in sunlight with a piece of white card to provide the highest relfectance and hence the highest highlight. The scene should also have a dark area  with detail, I chose a summerhouse in sunlight where the windows were almost black but able to see some detail, I used a piece of A3 card and placed it in both shade and direct sunlight.

With the camera set at 1/800 exposure at ISO 100 with all noise reduction off, I found that an aperture of f/4.5 just prevented the camera's clipping warning on the white card, signifying the brightest part of the scene. This image is shown below.

We were then asked to measure the brightest part of the image and two or three dark parts. I changed the metering of the camera to spot and zoomed into the card which measured f/11. The lowest part of the image I measured was f/4, this however was also on the limit of the lens I was using but seemed to be a good reading.

With the image open in Photoshop I used the colour sampling tool to measure the RGB channels on the card, these were almost at the maximum with typical readings on all channels in the 25x range showing that there was no burn out in the image and it was indeed exposed as best as possible.

To find the lowest exposure capable of the camera I opened the image in RAW and turned off any noise reduction and zoomed into the darkest part of the image. Adjusting the exposure slider so that the dark areas were lightened until the level of noise introduced could not be distinguished from the detail. Here is a snapshot of that scene. The noise introduced on the wood is not not distinguishable with the detail.

This adjustment gave a further 3.5 stops. This only gave my camera a total dynamic range of 6.5 stops, reading online literature it is actually around 8.5 stops, but I think that although this exercise shows how to judge the dynamic range it is probably not as accurate as the testing methods I have reviewed. If time permits I will repeat this again with a lens that has a lower f-stop, I think that is probably where I lost a couple!

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