EXR ..Debunking the Mystery .. Pt 3

The EXR sensor… DR mode.

After having explored the EXR HR & P modes of the HS20/30 we come to what is perhaps the jewel in the crown of the EXR range, EXR DR mode, but is it really all its said to be?

Below is a image of how the DR mode of the EXR sensor works. As you can see half of the pixels indicated are darker than the rest. This is the key to how this mode works. In simple terms the sensor acts as two 8Mp sensors , one taking a low sensitivity image and one taking a high sensitivity image. This happens at the same time, not consecutively as this would lead to image blur. The EXR processor then combines both elements of the image into one 8Mp image. At this point one may start to question how the camera puts out a 8mp image from a 16Mp sensor. Shouldn’t the image be 16Mp when re-combined? The answer is no. We can use a 35mm film analogy here, take for example two negatives from a 35mm roll of film, shoot them identically except for their exposures values. One negative will be slightly over exposed and one slightly under exposed, lay the two images one atop the other and process the image as a single image with increased dynamic range and depth of color , contrast and tone. The trade off for doing this is the same as the EXR in DR mode. You lose some resolution for the gain in dynamic range & noise reduction. The 35 mm film would also lose some resolution because the film has grain & texture. Modern day astrophotgraphers use this very technique digitally to enhanced multi exposure images. Its called image stacking, a good beginners guide can be found here – Astro stacking.

Click the image for more information

What the EXR sensor is doing can also be thought of  as  a method of pixel binning, which is another method for obtaining higher DR in images. The linked explanation while a little confusing does show how this method works, whereas the EXR version is based on pixel  sensitivity rather than the combining of pixels into a larger single pixel. Both methods however do have the same result, increased Dynamic Range at the expense     ( although rather small ) of reduced resolution. This is a outline of how the system works and is not intended as exploration into the mechanics of the process and which method is better. Suffice to say  the above information is really all the photographer needs to know about the actual process. More importantly from the photographers perspective is in using this mode to get the best result.

To answer this, one basic fact should be remembered, EXR DR mode is primarily of greatest benefit when used in very well light (bright) environments where there is a great deal of contrast between highlights and shadows. The object of using this mode is to maintain as much highlight as you can while bringing out hidden details in the shadows of an image. The images below are a good example of how this works. I shot these images today, approximately 20 minutes prior to writing this article. They are as shot, but have been resized to better suit image load times.

The first of the images is shot in EXR HR @ 16:9 format, large file size as were the EXR DR images but at medium file ( image ) size. Don’t be fooled by the sizes your camera is showing if working in 16:9 format. The camera is still taking the image at the full 4:3 image size and simply cropping off the top and bottom of the image. To see this in operation one only has to shoot Raw + Jpeg then view them on a computer.

  • EXR HR
  • Metering = Spot
  • EV = 0.0
  • Dr% = 100
  • ISO = 100
  • Shutter Speed = 1/832

  • EXR HR
  • Metering = Spot
  • EV = 0.67+
  • Dr% = 100
  • ISO = 100
  • Shutter speed  = 1/739

  • EXR DR
  • Metering – Spot
  • Ev = 0.67+
  • Dr% = 400 (Manually  set )
  • ISO Auto = 100
  • Shutter Speed  = 1/609

Even with just a quick glance its easy to see that there is a great deal of difference in the three images above. At 0.0EV in the first image its really quite dark and sombre.  If I was shooting for mood this image would be a keeper. Lifting the Ev value to 0.67+ in HR mode has brought out a lot more foreground detail that was in shadow, and this is an effective technique to use if you are shooting in HR mode and want as much detail & resolution as possible.

The third image demonstrates just what can be achieved when shooting in DR Mode. There is a good deal of increased detail in the foreground, and the cloud detail while lifted hasn’t been over exposed to the point that all the highlight is lost. This photo was taken from our balcony at approximately 8.00 am local time, still early morning but plenty of daylight, but heavy showers were passing through, an ideal time for a few shots of crepuscular rays.

So does DR work? In a word, Yes. In situations like that above or in highly lit areas DR mode is definitely an advantage. Resolution loss using this method is minimal, and the smaller file sizes will benefit the write times shot to shot as well. For most of my work I prefer EXR HR as I’m more concerned about detail retention, however when I want to record the sort of image above then DR is the mode for the job.

Higher DR is always available ( 800 & 1600% ) through all the ISO settings. The only thing to remember when using higher DR settings above 400% is that the output is software based whereas the 100 to 400% range is hardware based from the sensor. Results at higher settings can be somewhat more variable as software interpretation may not be as reliable as the sensor based output. To date I haven’t seen enough of a difference between images using a DR setting above 400% to warrant setting the camera to one of these settings. As always, its worth a little experimentation for those wishing to tinker a little with their settings.


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