Wednesday, May 27, 2015

ND Bracketing for LE/HDR scenes

As all photographers know the weak link in our photography, in addition to ourselves, {:-)}, is our equipment and, in particular, our lens-sensor system’s inability to capture every scene in a single image. Thus we revert to using various bracketing strategies to recover any shortfall eg:
  • We do exposure bracketing to capture very high dynamic ranges;
  • We do focus bracketing to extend our depth of fields.
We also use Neutral Density (ND) filters to capture long exposures to give an impression of time passing in a single image, for example moving water and clouds can be ‘smeared out’.

But what happens if we have an ND scene with a high dynamic range?

This is where we can turn to, what I call, ND bracketing. BTW I haven’t come across this idea elsewhere, but I don’t claim to be the first to use the technique.

To understand the ‘problem’ associated with using NDs, it is worth reminding ourselves of their ‘normal’ use. For instance, if I wish to capture an image at, say, an exposure of 2 minutes (120s), I could place the camera in bulb mode and work out the required ND number. However, as I don’t have an infinite set of ND filters, the ‘usual’ way to go is, knowing my ND number, set the appropriate exposure without the ND and then substitute the ND. Thus if I have a 10-stop ND and wish to take a 120s exposure, I would set the camera at about 1/8s and use the ND.

In theory it should work, however, experience shows that you will not have a perfect ND and thus you may have to take a couple of exposures to nail the perfect one. Of course you could use a variable ND set up, but these bring their own (calibration) complications, especially if front mounted on a wide angle lens.

In a previous post I talked about my preferred LE solution of using a rear-mounted variable ND, like the Vizelex ND Throttle. Although this only works with a mirrorless camera, it allows you to use any focal length lenses, without any ‘wide angle’ artefacts.

But what if I also need to do exposure bracketing?

This is where the problems set in. If my single image ND exposure is say, 2min/120s, and that adequately captures the highlights, then if I need to cover the shadows, I will need to take longer exposures, eg at, say, 2 and 4 stops up. But now I’m taking images at 8mins and 32mins! Not only is impracticable because of time, but also the image will exhibit different characteristics at 2mins compared to 8min and 32mins. Blending these images together could result in ‘motion artefacts’.

This is why ND bracketing is worth exploring.

In ND bracketing one exploits the variable ND filter to capture multiple exposure brackets, all with the same exposure time. Thus all brackets exhibit similar temporal artefacts, eg the clouds or water will have similar ‘structural’ characteristics; bracket to bracket.

You could also use different ND filters, eg a 10 stop and, say, an 8 stop, but the variable ND approach is more flexible.

The Sony A6000 + ND Throttle workflow I use goes like this:
  • Decide on the exposure time – let’s use 30s;
  • Compose the scene and set the focus and aperture;
  • Temporally set the ISO to, say, 1600 – but I will capture at ISO 100;
  • Set the exposure to 4 stops down from 30s (ISO 100 vs ISO 1600), ie about 2s;
  • Adjust the exposure, using the Sony’s blinkies and histogram, by adjusting the ND Throttle, ie the amount of ‘NDness’. That is adjust the ND density until the histogram and blinkies look right for the highlights, ie using an ETTR bias, albeit based on the JPEG-based histogram, unlike the Canon Magic Lantern RAW histogram;
  • Reset the ISO to 100, which means the exposure time needs to be reset to 30s;
  • Take the exposure;
  • Without changing the ISO, exposure time or aperture, adjust the ND Throttle by a couple of stops, ie moving the histogram to the right by about two stops;
  • Capture a second image;
  • Repeat the above step until you feel you have captured all the shadow details;
Process with your favourite post processing software. I use LR/Enfuse as I find the tone mapping software doesn’t handle the ND brackets very well.

As usual with these posts, here is a test image I took inside our home. The three brackets were taken at 30s with a fisheye lens on my A6000.

Bottom line: although ND bracketing is not an automatic process, it appears it might address the problem of taking LE images of high dynamic scenes, especially if you don’t wish to see temporally different features between the brackets. I would love to hear if others have experimented with this idea.

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