Sunday, January 23, 2011

Ex. 9 Scene dynamic range

For this exercise I had another Christmas present, this time one that I bought myself, a 16-35mm f/2.8 wide angle zoom.  This is a lens that has been at the top of my "want" list, but not on my "need" list, as I have another lens with a similar range, but not f/2.8.  This years Christmas bonus was better than expected and so I had the chance to add this to my collection.  I plan to use this lens for street and indoors work, I think it will make an excellent documentary lens.  I am also considering a second present, a 35mm f/1.4 prime, expensive, but possibly the ultimate low light street photography lens.  As a result I have completed this exercise shooting all images at 35mm to get a sense of the framing and whether it would work for me.  My conclusion, 35mm is wide enough to grab a sense of space and tight enough to draw attention to the subject, very nice framing - so time to think whether to invest or not.  I have never regretted buying a fast prime, and looking at the second hand market it is not a great investment, but also not money burned.

Back to the exercise.  In exercise 8, I was able to estimate that my 5D2 has a dynamic range of approximately 9 stops.  This exercise turns the table around and looks at the dynamic range of scenes rather than the camera.  Today was a good day for this activity, a cold, but sunny January Sunday.  I am continuing to focus on the city and my immediate surroundings for this course, so took a walk around my local park and a few office buildings nearby, the same ones I used for my Assignment 1 images.

All of the following photographs were taken at 1/125s and 35mm - I allowed the aperture to float.  This allowed me to measure the relative brightness of each scene using the cameras in built matrix metering.  After each photograph I then used a hand held light meter to spot meter the scene looking for the brightest and darkest areas in the frame.  I set the meter to give me an eV reading for an exposure time of 1/125s and ISO 100.  For this exercise that was a simpler process as it would allow me to directly calculate the exposure range of a scene. Conversion back to f-stops can be easily achieved using an exposure table that maps eVs to f/stops and shutter speed:


This was the first scene I found that had a good range of brightly lit and shaded areas.  The darkest zone is inside the gas station and read at 11.8eV, whilst the brightest was the office tower in the background reading at 16.3eV - so a dynamic range of 5.5 stops.  This rather surprised me as it was much less than I expected for this scene.  As a matter of interest the f-stop equivalent is roughly: f/5.6 to f/22 - the camera metered f/9.5!


This is a brutal image and one taken deliberately to stress the camera and find a high range.  The white background contains the direct sun, but is completely blown out all across the range.  The exposure range is 9.9 to 22.7eV, a dynamic range of 12.8 stops (I think the 22.7 is too low as I could not use the meter properly due to the glare).  The camera has no chance of creating an acceptable image in these conditions, the range of the scene far exceeds the range of the camera.


This time I looked for a flat image with a minimal range, 10.3eV reading from the shaded tree bark to 14.8 in the sky - a range of 4.5 stops.


Here I have looked for an even lower range scene, very deliberately avoiding inclusion of the sky.  The range is 10.2 to 13.6eV, 3.4 stops. However, still an interesting image!


A more pastoral scene offering up a range of 10.2 to 15.4eV - 5.2 stops.


This was an attempt to find a higher range scene, including the glass frontage and its reflection of the sky.  I was surprised to find that this scene was actually quite low from 11.4 to 15.9eV, or 4.5 stops.


Turning the corner I finally managed to find some good contrast, this time a building strongly reflecting the sunshine, but not containing a direct reflection of the sun.  This time the range was 12.9 to 19eV, or 6.1 stops.


Moving along a little I recomposed to include the suns reflection and found a much higher range, from 13.5 to 22eV - 8.5 stops.

This was more than the 5 images requested, I found this a very interesting exercise and wanted to explore a little more. What surprised me was that many scenes that looked to me to be high contrast had a much smaller range than I had expected.  The cameras 9 stop range would theoretically handle any of the scenes that excluded the direct Sun, although that could only happen if I judged the lowest and highest exposure values correctly and then placed the camera at the center of that range.

The lesson learned is that any highlight clipping or loss of shadow detail is down to my poor exposure management, not the camera!

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