UniWB – A good tool for good HDRs

Well, I must first admit that this topic is neither new nor my own finding/tip. I did however find out about it a while ago, and have been using it for a while now. This along with the other tips on HDR and Skin tones will bring out the best from your RAW image…

It all started (back in 2006 I think) when someone started complaining about how the histogram in the back of your cameras don’t really reflect the RAW capture, but that of a JPEG (Yes, RAW images have JPEG embedded inside them as well). So, technically speaking – the histogram you see on the back of your camera is not reflecting the captured image by the sensor.  Study the links in the references…they are excellent sources of info on this topic.

So, why is this important?

  1. To ensure that your exposure is accurate
  2. To make sure that you don’t clip the red and blue channels (Note that there are twice the amount of green photo receptors compared to red and blue…which is why, green is usually very well captured and controlled).
  3. To use the technique of ETTR (expose to the right) to get the most out of your RAW capture. This  technique requires you pushing the levels in the histogram to the right side as much as possible, without clipping the highlights.
  4. For HDR photography, so that you know exactly what is being captured in your camera and to determine what additional exposures are needed in order to capture the full dynamic range of the scene.

So, the story is that – a few intellects decided to fool the camera into creating this embedded JPEG with a linear gamma (just as the sensor sees it). There are a few ways to do this – One method is to use in-camera curves (I had custom tone curves back when i had the D70). There is an another option – and is easier to use in the field.

The technique is described as below:

  1. Take a dark frame shot (Manual exposure with lens cap on).
  2. Apply the WB setting from this for all your future shots. In the Nikon D300/700/D3 you can select Preset WB – slot d-1 through d-3, and set the WB to specific image of your choice. In this case – set it to the dark frame shot.

That is there to it.  Now, every time you shoot with this PRE WB (d-1 on my PRE setting), you will create an embedded JPEG with neutralized WB (which kinda looks ugly with a green color cast), but gives you a pretty accurate histogram of the RAW image capture.

Of course, when you import the image with your RAW converter of choice – You may need to re-adjust the white balance (WB) to your preference. You can optionally shoot a gray card as your first frame every time you use this technique…to nail that WB in post processing.

Let me elaborate a bit on its use in HDR photography. Regardless of how you shoot your +/- exposures (auto bracketing or manual) – you will need to rely on your camera’s meter to get that best average reading (middle shot). This is where UniWB is important. The UniWB setting allows one to get a good ETTR shot for this middle exposure. From here – the +/- shots pretty much capture the details outside of this “best” shot, giving you the best images to work with for your extending the dynamic range.

Take a look at the few images below:

  • Image 1 is a screen capture that compares the WB multipliers b/w the white saturated frame and a dark frame. In my case – they are identical, and very close to 1.0. The way i captured the white frame is by exposing a solux lamp from about 1 feet with f2.8, 30 seconds. The resulting image was pretty much a white frame. The black frame was shot in a dark room, with lens cap on with exposure of f22, 1/8000 second. The resulting image was a good dark shot. As you can see from dcraw results, the WB multipliers are identical and close to 1.0
  • Image 2 and Image 3 are the “before” and “after” shots. The before is how the image appears when you shoot an image with UniWB. The “after” shot is after processing the image in Lightroom (LR). The image is pretty accurate in terms of its color rendition.
  • Image 4 is an example of where HDR is needed. If you look at the sliders for histogram and exposure (-4) – this is everything that the RAW was capable of capturing. There simply is not enough Dynamic Range in the camera to capture the blown highlights (which is quite easily visible from the white spots towards the bottom left side of the image). This can be only corrected by an another shot that is over exposed by 4EV (which would have enough DR to capture the details only in the highlights).  A blending/merging process would then be necessary.
  • Image 5 and 6 are just example images that were shot with UniWB and processed with my skin tones technique.

References:

  1. http://luminous-landscape.com/forum/index.php?showtopic=37853&hl=
  2. http://luminous-landscape.com/forum/index.php?showtopic=22250
  3. http://www.guillermoluijk.com/tutorial/uniwb/index_en.htm
  4. http://photography-on-the.net/forum/showthread.php?t=485349&page=6
  5. http://www.roesand.com/blog-posts/173-nailing-exposure-with-uniwb-for-the-nikon-d700
  6. http://manishbansal.wordpress.com/2007/12/15/making-your-dslr-show-linear-histogram/
  7. http://www.malch.com/nikon/UniWB.html
  8. https://www.luminous-landscape.com/tutorials/expose-right.shtml
  9. http://www.adobe.com/digitalimag/pdfs/linear_gamma.pdf

197222 pages viewed, 527 today
32104 visits, 42 today
FireStats icon Powered by FireStats

8th Cross is Digg proof thanks to caching by WP Super Cache