Capture More Light: HDR and High Speed Bracketing

March 28, 2011

The enemy of merging images is any sort of movement (moving objects or moving cameras).

This is the second article of a 3 part series:

  1. HDR (High Dynamic Range) and how to create HDR images (click to read)
  2. High speed bracketing method
  3. Getting the most out of your HDR images (click to read)

4.    Many of today’s digital photography enthusiasts think of shooting different High Dynamic Range images of the same scene as a tripod-only exercise. Thus, many dismiss HDR photography as something suitable only for totally static subjects, shot in a slow and methodical fashion. However, with the shooting speed of today’s cameras, this doesn’t have to be the case. This article demonstrates how you can use this speed to shot HDR images in circumstances you may never have considered before.

5.    Improving the dynamic range that our digital cameras can capture is for us as important as getting higher resolution (more Megapixels). We covered in part 1 of this series how bracketed images can help to capture more dynamic range using HDR techniques.

6.    The enemy of merging images is any sort of movement (moving objects or moving cameras). That is why exposure bracketing is most often only applied to static objects and the camera is mounted to a sturdy tripod. This of course imposes limitations what we can shoot using HDR.

7.    If in theory these multiple exposures could be captured instantly both issues -- Moving objects, and camera movement -- would be no problems at all. We may actually see in the future cameras that could implement this kind of strategy: capture multiple exposures, create HDR internally and still freeze even faster moving objects.

Bracketing with EOS Mark III and Mark II series cameras:

EOS-1D Mark III, EOS-1Ds Mark III:

Use Custom Function I-6 (Number of bracketed shots). Default is 3-shot bracketed sequence; choices are 2, 5, or 7 bracketed shots. If two shots are chosen, you determine whether it's normal exposure plus over-exposure, or normal plus an under-exposed frame via C.Fn I-5-1 or I-5-2.

Original EOS-1D, EOS-1Ds, and all Mark II series:

Install Personal Function 8 into camera (connect FireWire cable from camera to computer, activate EOS Utility or EOS Viewer Utility software, go to Camera Settings > Personal Functions, and upload P.Fn 8 into camera after selecting 2, 5, or 7 bracketed shots as an option). Then, once installed, P.Fn 08 can be turned on or off in camera's menu. If user selects 2-shot bracket using P.Fn 08, Custom Function 09-0 or 09-1 provides a normal and under-exposed frame; C.Fn 09-2 or 09-3 generates a normal and over-exposed bracketed 2-frame sequence.

Speed Matters

When we first used a Canon EOS-1D Mark III camera, we did not think to use its speed (it can capture 10 fps) for our normal work. Its primary target audience is sports, newspaper/magazine, and also wildlife photographers. In general we photograph more static nature or urban landscapes. Like many digital shooters, we initially thought that fast bursts of images in general do not matter for us. However, once we got used to the 1D Mark III we started experimenting with what we call “High Speed HDR”. What does this mean? The camera is set to the highest burst rate (10 fps) and we capture bracketed shots at this speed. This means a 3 shot (e.g. -1 1/3, 0, +1 1/3 EV) bracketed sequence is shot in less than half a second (with hand-held shots your shutter speed is mostly faster than 1/50 sec). Why does it make a difference? Everything that does not move much in 1⁄3 second is frozen in all three exposures. It limits the camera movement and also captures all slow moving objects reasonable sharp. And perhaps most important: it opens the door to shooting HDR images hand-held.

Important Note: We now use the same technique successfully with many other cameras as well (Canon EOS 40D, EOS-1Ds Mark III, Rebel XTi). Of course the speed of the 1D Mark III is not matched by these other cameras but it still works quite well for us. With the 1D Mark III it is even very easy to capture 5 shot bracketed sequences like -2, -1, 0, +1, +2 EV.

We used this technique now for over a year and at least 5,000 photos (these are about 15,000 exposures at 2, 3 or 5 shot brackets). It changed our photographic style. For us it is not only a photo technique but also a photographic philosophy.

Good and bad light?
Photographers talk a lot about “good” and “bad” light. Actually, they’re usually referring to the quality and contrast of light, not whether it’s bright or dim. Often, it is bad because normal film or digital cameras cannot capture the dynamic range in the scene. Using HDR techniques (multiple exposures and tone mapping) you can capture quite a bit more dynamic range. However, freezing movements is essential. We are using “High Speed HDR” now in many light situations that we called previously “bad” light and get very pleasing results.

The sample we later present in this article was shot at strong sunlight in Bryce Canyon (11am).

Notes on camera technique
We exclusively use image-stabilized zooms (most of the time in the 24 to 200mm range), and shoot handheld. The ISO range is set to a value that allows us to keep even the longest exposure in our bracket sharp if shot handheld (but also taking the image stabilization into account).

Exposure Mix & Match: Philosophy

We add this more philosophical section because some of our readers may think this is all just technical mumbo jumbo and they rather take photographs. Of course they have a point here and many different ways lead to Rome. You have to work the way that fits best for you. There is no one universally best practice.

Photography falls into two principal activities:

  • Framing and Exposure
  • Post-processing

We think both parts are equally important although we spend way more time processing than taking pictures.

Framing and Exposure
Without the right composition and an at least decent exposure there is hardly a chance to get a good print. The ideal would be to concentrate only on creative aspects like composition and leave the rest to the camera. But even with today's sophisticated cameras there is a lot to consider:

  • f-stop: Depth of field but also diffraction limits (beyond a certain point, as lens apertures get smaller, sharpness actually decreases)
  • Shutter speed: Consider moving objects and camera shake
  • ISO: Allows higher shutter speeds at the expense of more noise
  • Tripod vs. Hand-held: This is a whole can of worms and a very personal decision. Of course it even more depends on the situation. Night shots are not often done freehand (except at very high ISO values). Freehand or hand-held allows more flexibility while the tripod is a strong aid to get the best framing
  • Exposure: Mostly about not clipping (over-exposing) the highlights, while avoiding underexposures to keep the shadow noise in check

Capturing bracketed shots makes exposure much easier. We can almost always be confident in capturing one or more images with sufficient sharpness and without blown-out highlights. This gives us a chance to concentrate on the framing aspect.

About the only downside of this approach is we capture more images, and they have to be stored someplace (bigger CF cards and disks). Otherwise bracketing was and is always very useful.

Post-processing
Shooting brackets makes post-processing easier and more complex at the same time.

  • Easier, because: Having 3 or more exposures available allows finding the best possible exposure of the bracket. With only a single exposure you have to live with it and fix things that might gone bad (e.g. blown highlights — hard to fix if at all).
  • More Complex, because: Having options is always nice but also makes life more complicated. If you ignore HDR all is quite simple. But if you want to explore these options, this will clearly cost more time and work.

With bracketing, you capture more images that allow you to revisit some of these issues later (e.g. HDR, lower noise, ...). If you capture only single shots this can be just fine but it also limits your options.

Exposure Mix & Match: Technique

We created a few diagrams that discuss the preparation for you shooting and the evaluation of the resulting shots:

There are two principle different situations:

  • Tripod mounted
  • Freehand

Tripod Mounted
(You need to use all techniques available to stabilize the camera/lens on the tripod.)

  • Use mirror lockup (even better, use live preview available on some new DSLRs)
  • Let the tripod rest between shots (to avoid vibrations from previous exposures)
  • It is better to use manual focus as the camera AF may shift during bracketed shots

Freehand (hand-held)

  • Use the fastest shutter speed you can afford (this is a balance of ISO & noise vs. shutter speed)
  • Use image-stabilized lenses, whenever possible. Don't assume this does miracles. We try to be very conservative how much the IS buys us (but 1-2 f-stops is already a lot). Some more recently-introduced Image Stabilized lenses offer even higher performance, allowing safe hand-holding at speeds up to 3 or 4 stops slower than otherwise possible.
  • You may also use a monopod for extra support (light yet sturdy)
  • Fast moving objects: HDR does not really buy you anything if there are faster moving objects. Fast is in relation to the time it takes to take all bracketed shots. From tripod all movements can be considered too fast. With High Speed HDR many movements can still be captured without too much blur. Not all blur is actually a problem. This is then an artistic decision
  • Slow moving objects often don't show with High Speed HDR
  • Some movements may not bother you: Clouds, distant people
  • Highlight Tone Priority: This is a unique new feature on select EOS models, such as the EOS-1D Mark III, EOS-1Ds Mark III, and EOS Rebel XSi. It allows you to record some detail in bright highlights, in slightly overexposed shots. Remember our technique is also about getting the single best exposure from the bracketed shots
  • Enable camera auto bracketing
  • For High Speed HDR we need about 3-10 frames per second speed (the higher the fps the better)
  • Number of shots per bracket: We use mainly 2, 3 or 5 for our High Speed HDR technique. Seven or more shots are reserved for pure tripod based bracketing. (Please note: Canon’s EOS-1D and EOS-1Ds models allow choosing the number of bracketed frames in a sequence, from 2 through 7. All other EOS models [as of July, 2008] are limited to 3 shots whenever auto bracketing is activated)
  • EV spacing depends on the dynamic range we want to capture. We most of the time use about 1-1/3 EV (1.3 stops) but in some situations 2EV is better. How did we come up with 1-1/3 EV? In this case the best shot in the bracket can be only about 2/3 EV off an optimal exposure because Highlight Tone Priority allows us easily to recover at least 2/3EV overexposure
  • ISO and f-stop should be constant for all shots in the bracket. The only variable should be the shutter speed. This means shoot in Aperture-priority exposure mode

We have to consider two goals:

  • Capture a good single exposure as part of the bracket
  • Allow as often as possible to merge images for HDR In case you have visible object movements during the full cycle taking all bracketed shots you have two choices:
  • Accept the resulting artifacts when merging the exposures
  • Use on a single best frame from the bracket

A) Overexposed shot
This is often the least usable shot working freehand. Why? Because it uses the longest exposure time and so the image can easily get blurred. Always watch the shutter speed for this shot.

If the clipped highlights in this shot can be properly recovered through software intervention (Canon's DPP, Adobe Camera Raw, and LightZone all offer effective recovery tools) and the image is not blurred, you actually have all the information you need.

B) Middle Exposure
In most of our cases (using the camera’s Aperture Priority “Av” mode) we can recover this shot properly using highlight recovery to some extent. If the image does not show motion blur we have a good single shot photo. If you want to open up shadows a lot, this is possible, but you may increase the digital noise. In this case, we try to reduce the noise in the shadows by using commercial noise removal software.

C) Underexposed photo
It is often not good if you need to use only the underexposed shot because it shows the strongest noise — especially once you try to lighten it using image-editing software. On the other hand, it can still get you good results if you stay at lower ISO values or accept some noise in your image.

D) HDR Merging
If there are hardly any object movements (remember, for minor camera movements, the software alignment algorithms will take care of this) you can merge some exposures.

  • All three exposures if all three shots don't show motion blur and are in focus
  • Only two exposures if one of them shows motion blur or is out of focus (most likely the overexposed shot)

Sample Mix & Match

These five exposures were taken with a Canon EOS-1D Mark III at -2, -1, 0. +1, +2 EV. The most overexposed shot (fifth photo, +2 EV) also has the longest exposure time, and we often discard it. In this case we only used three shots: The first photo (maximum underexposure, with most highlight detail), the third photo (0. EV, middle exposure), and the fourth photo (slightly overexposed, and low noise in the shadows).

After merging to HDR and Tonemapping (Details Enhancer) we get this result:

We actually nearly always use a two-step process (described in Part 1 of this series):

  1. Tonemapping in Photomatix software
  2. Final tuning in Photoshop

Here is the final resulting photo. In this case the tuning was really minor. This is not always the case.

Conclusion

High Speed Bracketing/HDR is a new and powerful technique. Being able to capture good photos hand-held and at normal sunlight opens incredible new possibilities. For photographers whose style is to work quickly, without a tripod, it makes HDR shooting a practical option. Besides more space for images and more imaging work we don’t see a lot of disadvantages. If some of the objects move too much we can pick the best single exposure.

We also experiment with different bracketing sequences:

  • -1, 0 EV: saves space (possible only with EOS-1D/ EOS-1Ds models)
  • -1, 0, +1 EV: would allow getting a good optimal single exposure that cannot be more than 1⁄2 f-stop off
  • -2, 0, +2 EV: if you want to captures a larger dynamic range. The +2 EV exposure may often require the use of the tripod though
  • -2, -1, 0, +1, +2 EV with very fast cameras like the 1D Mark III
  • -1 1/3, -2/3, 0, +2/3, +1 1/3 EV optimal fine exposure spacing (use only with very fast cameras)

All the following elements are very important:

  • Fast frame rate
  • Highlight Tone Priority (a Custom Function on select EOS models)
  • Merging to HDR
  • Tonemapping

We honestly can say that we got a lot of fun using this technique. Only our disks complain about the data volume we produce. We have more data because of the bracketing but also a higher keeper rate.

The CDLC contributors are compensated spokespersons and actual users of the Canon products that they promote.

All images are copyright Uwe Steinmuller

http://learn.usa.canon.com

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