Before the Coffee

HDR tips

A photographer sent me an email, “Ferrell, I am enjoying reading your book on HDR photography.  I have a Nikon D700 and thought perhaps that it might be possible to take one photo and the bracketing take place in the camera using the menus. Thanks in advance. Paul Fretts

Hi Paul,
A common approach among new HDR enthusiasts is to take a single image and in post processing create an over and under exposed image, save each one, then follow with HDR processing. This approach is incorrect. It’s not possible to create more dynamic range than the sensor captured simply by adjusting exposure in-camera or in post processing. The dynamic range of a single image is limited by what the camera sensor accurately records. To illustrate this, let’s say your sensor captured a single image with blown highlights. Those blown pixels are void of accurate data, you cannot recover the blown pixel data by doing an exposure adjustment of that image. Sure the mid-tones will get respond nicely to the adjustment but the bad ones are gone bye bye.
You may say, “What if I don’t have blown pixels and my shadow detail is good, then why can’t I use the exposure adjustment idea on a single image, save each image, then process to HDR?”  You can, but I’d argue that HDR-style processing is not needed, you’ve already captured the dynamic range of the scene in a single image, you can tone map that image if you like.

Two important scene indicators of High Dynamic Range.

The Sun – Compose any landscape or cityscape image with the sun in the scene and you’ll have High Dynamic Range. Start thinking in terms of 7 or 9 images at 1EV spacing.

Cave Effect – The cave effect is created when sunlight is blocked from all directions except the one you are looking. Of course the severity of the cave effect will vary based on it’s depth and overall % of the composed scene it occupies. The important thing is to be aware that you are dealing with High Dynamic Range and take 5,7,9 images at 1EV spacing. Here are a few examples: Store front where the interior is visible, tunnel entrances, the local vegetable/fruit stand, natural cave openings, and underside of a bridge with lots of I beams. You get the idea, take a moment and try to think of a few yourself.

The HDR knowledge base tells us that dynamic range can be as high as 1,000,000,000:1

That’s 1 billion to one and what does it mean? It’s simply the difference in EV from the center of the sun’s disc on a clear clear day to the darkest cave you can imagine, a place where there are no photons of light.

Is it common in nature? Nope, due to so much diffused light.

Should I worry about it? Nope, unless your a scientist taking measurements. It’s widely accepted in photography that the disc of the sun and surrounding area can be overexposed. But watch those clouds.

I often see posts that say “tone mapping a single image is not true HDR.”

Our primary goal as a photographer is not to capture HDR image sets with little thought about the scene. But through scene awareness we should concentrate on capturing the FULL dynamic range of the scene. It may take 3, 5, 7, or 9 images OR it may take just 1 image.

Single image tone mapping is not an inferior technique to the full blown HDR process when the single image captures the full dynamic range.

So where is the going? Process the full image set to HDR but also try to process a single image in the set. Choose the image based on the histograms and what areas of the image are important. I like to take 5 images at 1EV spacing so I have more options to chose from. There is one thing for sure, when you tone map a single image, ghosting and alignment issues will never be a problem.

5 Comments »

  1. “Single image tone mapping is not an inferior technique to the full blown HDR process when the single image captures the full dynamic range”

    From my fairly limited understanding, I’m not sure this is true. Yes, you may have fully captured the dynamic range of the scene (i.e. the histogram stays within the upper and lower limits) BUT you would not have the detail in the shadows and highlights that would be available from merging with an underexposed and overexposed image.

    Happy to be shown wrong on this.

    Ross

    Ross, If noise is an issue when you tone map the single image it is an indication that the dynamic range of the scene is approaching the upper limit of the effective dynamic range of your camera sensor. It’s interesting to research dynamic range of sensors. Researches vary in their conclusions, anywhere from 5-10EV depending on their tolerance for noise. Higher EV means higher noise levels. Even if you grab the full range of light there can be a question as to whether those pixels are high in quality and that only some of them lie within the sweet spot.

    Let’s look at these simple examples – shoot a gray card and your sensor will have no problem capturing the full dynamic range, put a few bends in it and light will start to vary across the surface. Compose a shot that is all sky and does not include the sun. You will capture the full range of light and provided your exposure is correct there will be little to no noise. Take a picture of your car door or a highway billboard with no reflections and you’ll capture the full range of light. These examples are not common photographic subjects but they can allow you to start to see that not all scenes benefit from multiple exposures and the hdr process.

    That said, I always shoot an image set, 5 images @ 1EV spacing and also try a single image tone map for comparison. Having 5 images to choose from is nice, it’s not always the 0EV that tone maps the best.
    Ferrell

    Comment by Ross — March 10, 2008 @ 9:31 pm

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  3. Dear Ferrell,

    Enjoyed your book, especially the photos that were shot with multiple strobe pops. It gave me the idea of using that technique to photograph some antiquities. It’s a recent project in which I need to capture subtle color variations and tones in carved stones. However, color accuracy is also important. Do you think using the multiple strobe technique and HDR could work for this? Would shooting a grey card or Gretag Macbeth Color Chart in the scene help in this matter?

    Thanks,

    Joseph Chiang

    Comment by Joseph Chiang — September 1, 2009 @ 12:44 pm

    • Hi Joseph, I certainly would shoot a grey card for the record, even a white and black card. HDR processing will skew colors, most feel the skew is acceptable but when color accuracy matters go the extra mile. Also, compare the HDR image to the single image. I think it would be neat to see your flash merging images. Feel free to send them to me. Ferrell McCollough

      Comment by beforethecoffee — September 1, 2009 @ 5:52 pm

  4. Hi Ferrell,

    Thanks for the feedback. I just got everything set up in the studio, but haven’t processed anything yet. I guess my current workflow will be shoot, white balance each file in LR, then merge to HDR and then white bal again. However, because the shots will involve the flash coming in at different angles for each shot and the Color Chart must be parallel to the focal plane, I’m not sure how I can White Bal off of the Chart for each shot. Any thoughts on that? Perhaps I should create a White Balance shot without the subject in it and use that file for White Balance only.

    In any case, I have not done this before because most of my previous work was nature landscapes, where White Balance was not critical. If it looked good, it was good enough. My sister had been trying to shoot these antiquities for some time now and had trouble capturing the subtle color variations and tones of her subjects (stone carvings).

    Thanks again,

    Joe

    Comment by Joseph Chiang — September 1, 2009 @ 6:01 pm


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