News and Views from Dave Wilson

HDR Tip #6 – How Many Exposures Should You Shoot?

by on Mar.05, 2011, under Photography

Yes – I’m copping out on this one since it was published a month or so ago on Scott Wyden’s blog as a guest post. However, here it is again, with a few updates and corrections, since I still think it’s a good tip…

St. Mary's Roman Catholic Church, Lanark
I’m often asked how many exposures I use for my HDR images and tend to make myself unpopular by answering “As many as it takes.” This sounds glib but it is, in fact, the truth. Some people appear to be under the impression that you can get by with always shooting, for example, 3 images spaced 2 stops apart. This may work most of the time but that will likely be by luck rather than design. An outdoor scene will probably be captured fine using exposures from -2EV to +2EV but inside a cathedral with bright, stained glass windows, for example, it can take 11 shots (or more) spaced 1 stop apart (-5EV to +5EV) to capture the full dynamic range.

The point of bracketing for HDR is to ensure that you capture the entire dynamic range of the scene – as much shadow detail as you can get and all the highlights. To do this, you really have to check your histogram after taking a basic set of bracketed exposures and add to this set if needed.

These days, I typically start with either 5 or 7 images bracketed by 1 stop. The initial choice will depend upon the scene I’m shooting and my assessment of the dynamic range but it doesn’t really matter as long as you follow the next step. Once the initial bracket is done, make sure you don’t move the camera but review the brackets on your LCD with the histograms visible. The darkest image should have no blown out highlights (if you have the ability to turn on “blinkies” to warn of this, that’s really helpful) and you should see no significant bunching of data on the right of the histogram. The lightest image should have no solid shadows (data touching the left edge of the histogram). To be extra safe and to reduce noise in the shadow areas, I always try to ensure that my lightest exposure has no data at all in the bottom quarter of the histogram.

One other thing to beware of when looking at your histogram is whether any particular colour channel is blown out. The standard, single luminance histogram doesn’t give you the whole story here but most cameras allow you to look at a display containing independent histograms for red, green and blue channels so it’s a good idea to use these instead and make sure you apply the rules across all three. No channel should have right edge clipping in the darkest shot and no channel should have significant data in the left quarter of its histogram in the brightest shot. Thanks to Dave Nightingale for this particular suggestion!

If you find that your outer exposures don’t meet the criteria given above, carefully turn off auto-bracketing (if you were using it) and dial in exposure compensation to allow you to shoot exposures on either end of your original bracket. If I shot a 5 image bracket (-2EV, -1EV, 0EV, +1EV, +2EV) , for example, and see that my darkest image still has blown out highlights, I will dial in -3EV and take another shot before checking and repeating as necessary. Most of the time, I shoot in manual mode with my camera set to change shutter speed in half stop steps since it makes this process a bit easier. I can turn off auto-bracketing then click my thumbwheel the correct number of times to change the exposure as required for the other shots I need to take.

This brings up another comment I hear frequently from many photographers who state that their camera can only shoot 3 frames in auto-bracketing and, hence, they can’t bracket any wider than that. Remember that auto-bracketing is merely a tool that makes things a bit easier. Even if your camera can only take 3 shots in a sequence, you can still turn the automatic mode off and dial in the required exposures by hand as I do to widen your bracketed exposure range. Another option, if you have money to spare and a compatible DSLR, may be to purchase a Promote Control
which will allow you to dial in the larger number of exposures without touching the camera at all but, even without this, a bit of control tweaking is all it takes to capture the full dynamic range of any scene you are shooting.

By making sure you capture the full dynamic range of the scene, you are certain to notice better final results – lower noise in the shadows, fewer blown-out highlights and generally cleaner images. Next time you are asked how many exposures it takes to produce an HDR, remember to answer “As many as it takes” and be ready for a longer discussion!

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