HDR Tutorial: Bracketing

Muizenberg beach huts

 

Muizenberg Beach in Cape Town has a very pretty collection of beach huts for changing in. I don’t think I’ve ever actually changed in them, but I have photographed them often. Lurking around change rooms with a camera is probably not a terribly bright idea, come to think of it.

This was photographed at dawn, so no-one was near the huts. I was very surprised to see one family swimming already and there were several joggers around. It’s hard enough to get up this early to photograph, so I don’t know how you do it for exercise. I think the HDR technique does a good job of bringing out the texture in the weathered timber.

 

This week I continue with part four of my HDR Tutorial series by looking at an essential part of capturing an HDR image, bracketing.

What is bracketing?

Once upon a time, in the days of film photography, photographers didn’t have the benefit of being able to preview a shot immediately after taking it, like we can on our funky little LCD screens. Which means they didn’t do that amusing thing we do where we squint at the back of our cameras every single shot. But it also means they couldn’t be sure they were getting the shot right until they had developed their film.

To make sure they had a better chance of getting their camera settings correct for difficult shots, they developed a technique called bracketing. The idea was to shoot a range of images of the same scene while just varying the one setting that was uncertain. For example, if you weren’t sure of the exposure settings for a scene, you would take several shots with different exposure settings, adding to the the most likely setting with ones that were likely to be a little under- and overexposed. You would end up with a sequence of shots from too dark to too light, with hopefully an ideal one somewhere in the middle. This is a bracket set. The problem is that in difficult lighting situations there often isn’t just one ideal exposure, even if you shoot a bracket set. Fortunately, now we don’t need to choose one shot anymore, we can take them all and combine them into one HDR image.

You can use the concept of bracketing for other settings on your camera too, such as white balance or depth of field, but for the purposes of HDR we are just interested in exposure bracketing. The whole point of HDR is to extend the dynamic range of a single image, so we deliberately shoot a range of exposures from too dark through to too light, and let the software sort it out.

How many shots should I have in my bracket set?
Before you can set up a bracket set on your camera you need to decide how that set will be made up.
The convention of a bracket exposure set is that it’s usually three, five, seven, or sometimes even more shots, with the middle one being the normal exposure you would have taken. From the normal middle shot, you vary the exposure up and down by a fixed amount, usually by an Exposure Value (EV) of 1 or 2 EV. The middle exposure is called ‘0’ because it has zero deviation from the normal settings. An exposure underexposed by 2 EV is called ‘-2’. Likewise, an exposure overexposed by 2 EV is called ‘+2’. A simple bracket set could then be referred to as -2, 0, +2.
Deciding what bracket set to shoot for HDR is a topic of much debate and personal preference. I have found that for 90% of my HDR images, I can get the results I want with a -2, 0, +2 set. It’s a matter of choice, but for starting out I would recommend you keep it simple and shoot your images with a bracket set like this. It’s important to remember that you want to change your exposure by changing your shutter speed, not your aperture, so make sure you are set to Aperture Priority mode. If you change your aperture with each shot, all your shots will have different depths of field, which will be hard to combine later.
I increase the size of the set for shots with trickier lighting. So for a dimly-lit interior scene with windows looking onto a bright exterior, I would probably shoot a -4,- 2, 0, +2, +4 set. The most hardcore lighting is shooting directly into the sun, say for a sunset, and in that case I would consider an even bigger range. On trickier shots, I also sometimes use a 1 EV spacing in the set, so that the HDR software has an even greater range of exposures to work from. I don’t find that making a huge difference, so I wouldn’t recommend that while starting out.
Something to think about though. You’ve come all this way to take this photograph, you’ve taken the time to set up your tripod, you won’t be here again for a long while, so why scrimp on your bracket set? Shoot more than you need, you can always delete the ones you don’t want later. Not that I’m very good at deleting anything. But I’d rather suffer having to buy another hard drive than missing a shot because I didn’t shoot enough images.
Muizenberg beach huts

 

How do you shoot a bracket set on auto?
Many cameras, especially digital SLRs, can shoot a bracket set automatically. Have a look at your manual to see how to set it up for your particular camera, but don’t despair if you don’t have an auto setting for it either.
You will typically be able to shoot three or more images in a set, and you should be able to set your EV spacing to at least 1, or maybe 2 EV. My Nikon D7000 lets me only shoot three shots on an auto bracket set, but I can set the spacing up to 2 EV, which is why I usually choose -2, 0, +2 as my standard set. On a camera that won’t go higher than a 1 EV spacing, rather shoot a -2, -1, 0, +1, +2 set.
It’s very easy to do it this way, but if your camera can’t do auto-bracketing, or not as big a set as you would like, move on to the next section for help. If you are hand-holding your HDR then this is by far the best way to set up your bracket set, especially if you set your camera to high-speed shooting. You can get a three shot bracket in about a second, and they will be fairly well aligned if you have a steady hand.

 

How do you shoot a bracket set on semi-auto?
I find this the easiest way if I want to shoot more than a three shot bracket set on my camera, and it should work on most cameras. Simply use the Exposure Compensation setting on your camera to dial in the settings for your set. So if you want to shoot a -4, -2, 0, +2, +4 set, then set your exposure compensation dial to -4, then -2, 0, +2 and finally +4 for each consecutive shot, and you will have a perfect bracket set. Easy as that!

 

How do you shoot a bracket set on manual?
It’s not actually that scary! Here’s how you would do it if your camera can’t or won’t do the previous two methods.
There’s the briefest of theory you need to know …
Each +1 EV is equal to one standard increment in shutter speed. Shutter speeds have had standardised speeds since the early days of photography. These speeds are (in seconds) 1 s, 1/2 s, 1/4 s, 1/8 s, 1/15 s, 1/30 s, 1/60 s, 1/125 s, 1/250 s, 1/500 s, 1/1000 s, 1/2000 s, 1/4000 s etc. You can see that each speed is half or twice the speed of its neighbour, and therefore lets in half or twice as much light into your camera. (Your camera might have inbetween shutter speeds which you can ignore for this.)
Let’s say that your camera tells you that the correct exposure for the image will be 1/500 s at an aperture of f8. This is the setting for your middle shot. You know that you must only vary your shutter speed, not your aperture, so the f8 setting can stay as it is. You have decided that you want to shoot a -2, 0, +2 EV set. You know that each EV is one standard increment in shutter speed, so 2 EV is two increments.
Looking at the list above, choose the shutter speeds two increments away in each direction. Those are 1/125 and 1/2000. So to get a bracket set, you take three shots at f/8, and at shutter speeds of 1/2000, 1/500 and 1/125.
Easy! There is even iPhone software which will work this out for you, such as PhotoBuddy, but it’s really very easy to do in your head.

 

TLDR and tips
  • Shoot in RAW mode. RAW files contain far more information about the scene you photographed than a JPG file would. You’re already extending your dynamic range by taking a bracket set, so you might as well also extend the dynamic range of each shot by using RAW.
  • Use a tripod as much as possible. Your shutter speeds might get quite low in a bracket set, so a tripod will help keep things steady. Also, it will make it hugely easier to align all you photos later. If you’re using a tripod, remember to use Mirror Up mode, switch off your lens Vibration Reduction, and use a remote to trigger the shot. Make every effort to keep things as steady and solid as possible. Your results will be much better using a tripod than hand-holding.
  • Shoot high speed if hand-holding. If you have decided not to use a tripod, shoot in high speed mode to ensure the best alignment of your images.
  • Aperture priority mode. Always set your camera in aperture priority when shooting a bracket set. If you’re shooting in manual, then vary your shutter speed, not your aperture. Varying your aperture will cause your depth of field to be different in each shot, which will cause problems in the HDR process.
  • Low ISO. HDR amplifies noise in images, so shoot in as low an ISO as you can to minimize noise. Don’t use Auto ISO.
  • Turn off autofocus. Your camera may decide to change focus points in the middle of a bracket set, which will ruin the set. Either keep a very careful eye on your focus, or disable autofocus once you have set the focus correctly.
  • Clean your lens. And your filters. HDR amplifies this dirt, so keep your stuff clean. You may barely notice it on your bracket set, but it will look pretty bad once you have combined several bracket images with the same specks and spots.
  • Bracket from dark to light. Set up bracketing on your camera so that it starts from the -EV settings, then 0 EV, and then the +EV settings. It will make things look a lot neater when you view them in Lightroom, and will ensure that you always keep your whole bracket set together and in order. The default of many cameras is 0EV, -EV and then +EV. That just looks messy, and it’s easier to lose images that way.
  • Your standard HDR set will be -2, 0, +2. Take more shots if conditions look trickier. As you get more experience you will build up a sense of how many images to take. If you won’t be back this way for a long time, take more pictures than you think you need, and delete the excess later.
  • Single Image HDR. You can dispense with bracketing entirely and make a (pseudo) HDR from one image, but you will get much better results with at least 3 images.
  • 0 doesn’t have to be 0.  Your middle image doesn’t have to remain at 0 EV. If you feel you camera is making a mistake you can add some exposure compensation to that image, as long as the spacing in relation to your other shots remains even. For example, you are shooting a low light scene at dusk, and your camera tries for compensate for the low light by brightening your image. You correct the camera reading by setting an exposure compensation of -⅔ EV. Your bracket set should now be -2⅔, -⅔, +1⅓. The important thing is that the spacing of shots is at 2 EV.
  • Leave out images. But only when processing your HDR later, not while shooting. Just because you have shot a huge bracket set, doesn’t mean you have to use every image. If you find that the HDR process is making your dusk scene look too bright for example, you can choose to leave out one or two of the lighter images to fox the HDR software into making your image darker.
  • Watch out for people. Keep a wary eye out for things changing in your scene while you are shooting your bracket set. Look out for people walking into view, for birds or cars etc. You may need to patiently wait until the people exit your scene before you resume shooting. Some of these things can be sorted out with the ghosting tools in Photomatix, and some can be painted out in Photoshop, but you will save yourself a lot of time to just not shoot the movement in the first place. Things like trees blowing in the wind, moving clouds or waves can also make your post-processing trickier.

 

Keurbosfontein Window, Cederberg (11 exposure set)
An 11 exposure set from -5 to + 5 at 1 EV intervals. This could have been simplified to a 5 exposure set from -4 to +4 at 2 EV intervals, and still have worked just as well.

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