If you have already read the introduction, “What is HDR All About“, you will have an idea of why we may want to use HDR but I’ve not yet mentioned anything about how you actually generate a High Dynamic Range image other than to tell you that it involves several different exposures of a scene and some special software. This page aims to provide this information.
The aim when shooting any photograph is to capture all the brightness levels in the scene so that you have detail in the dark shadow areas and in the bright highlights. In many cases you will find that a single exposure can do this – look at your histogram after taking a well-exposed shot under reasonably even lighting conditions and you will see that you have none of the histogram dropping off either end of the display. Under tricky lighting conditions (bright sunlit scenes with shadows, building interiors with views towards outside windows, etc), however, exposing for one part of the scene will likely cause other parts to either become completely white or completely black and lose detail. This is the kind of situation where HDR can be put to best use.
Looking at a scene you can generally get a pretty good idea of whether you will need HDR just by considering the brightest and darkest areas. If there are a lot of shadow areas and also very bright highlights, I would shoot for HDR without thinking. If you want, you can use your camera’s spot meter (assuming it has one) to gauge the dynamic range of the scene, metering the darkest and lightest areas and, if the difference is more than 6 to 8 EVs, chose to shoot for HDR but, personally, I don’t do this – I eyeball the scene and shoot for HDR if it seems appropriate. The only cost of doing this is a couple of exposures and we all know that storage is now free so it’s not a problem to me.
There is only one camera feature that is an absolute requirement to allow you to shoot HDR – the ability to set your own aperture. This means you need a camera with either manual or aperture priority exposure modes. If you have a cheaper point and shoot, you will, unfortunately, not be able to do this – sorry. Note that many higher end point-and-shoot cameras are great for HDR. I particularly like the Canon Powershot G9 but I’m biased since I own one. If you point-and-shoot has manual or aperture priority mode, you are good to go.
One other camera feature is desireable but not 100% necessary. If your camera offers the ability to capture raw files, use this rather than JPEG. When you have the camera save JPEG images, you are throwing away about 2EV to 3EV of dynamic range information. You don’t want to do this. Shoot raw and save as much of the original brightness information as you camera can give you since I firmly believe it gives you better images in the end. To provide a counter argument, though, I have some very well-respected HDR shooting friends who swear by using JPEG capture and an 8 bit workflow (more of which later) but I respectfully disagree with them and believe firmly in not throwing away any information until you absolutely have to.
There are a couple of reasons you shouldn’t use auto or program modes when shooting HDR. The first is that you need to be able to tell the camera to take pictures at what it considers to be the “wrong” exposure (overexposed or underexposed). On some program mode cameras, you can do this using the exposure compensation setting but this is where the second reason comes in – you need to ensure that each exposure has the same depth of field (the same areas in focus) so that when you merge the images later, they will merge cleanly. Given these, set Aperture Priority mode on your camera or, if you don’t have this (which strikes me as unlikely), chose Manual but set the aperture and leave it alone.
Which aperture to use, though? This depends on the scene. I start at f8 since it is typically pretty sharp with the lenses I am using. I meter the scene and see what shutter speed the camera will chose. Since I know I will want to take one exposure that is at least 2 stops underexposed (and hence will need a shutter speed 1/4 of the metered value), I know that the initial shutter speed must be at or below 1/1000. My Nikon D90 has a top shutter speed of 1/4000 so, starting at 1/1000 or below, I know I will still be able to get a +2EV exposure without having to change the aperture. Normally, however, I try to start a bit below 1/1000 since I may find I need an additional -3EV or -4EV exposure to catch highlights so 1/250 is often a good place to be at this point.
If you are fortunate enough to have a camera capable of Automatic Exposure Bracketing, now is a good time to set it up. By default, I set my camera to take 3 exposures, 2EV apart at -2EV, 0 and +2EV. I don’t have space to tell you how to set this up on every camera so consult your manual and fiddle with the buttons and menus. If your camera can’t support this, you will need to be prepared to change the exposure between each shot, either using the exposure compensation override which most cameras have or by using manual mode and setting the appropriate shutter speed for each exposure.
Your camera settings are all set but there is one other pretty critical thing to do before you start taking photographs. You will be taking at least 3 exposures of the scene and these need to be as similar as possible except for the exposure and this means keeping the camera completely still between each shot. The best way to achieve this is to use a tripod. The software we will use to merge the exposures later will do a far better job if the camera doesn’t move (though more on this later) and you will end up with far sharper images if you use a sturdy tripod.
To further ensure that the camera doesn’t move between shots, it’s a good idea to use a cable release or a wireless remote to trigger your shutter. I have both but normally use the little ML-L3 IR remote since it’s attached to my camera strap and, hence, always available. If you are a Nikon shooter you should definitely get one of these (check to see if your camera supports it) – they only cost about $18. If you don’t have a remote release, another good idea (which I used when shooting with the Powershot G9) is to use the self timer. This way, any vibration you induce by pressing the shutter release is likely to have dissipated by the time the shutter actually fires. Many cameras have a choice of 10 second or 2 second time intervals and I use the shorter one for this.
One other thing which some people swear by but which I don’t do is to lock your DSLR’s mirror up before taking the first exposure. The vibration induced by the movement of the mirror can apparently have a noticeable effect on the sharpness of the image. I’ve not found a way of doing this with my Nikon D90 but it does have a mode called “Exposure Delay” which delays the shutter release for half a second after the mirror has been lifted. I’ve been using this recently but have not noticed any appreciable improvement in sharpness yet. I have, however, noticed an appreciable increase in hassle and confusion since I have a habit of forgetting to turn it off after use.
With the camera fixed and the exposure mode set, it is now time to shoot the images for your HDR. Start by taking the 3 images in your automatic bracket with exposures -2EV, 0 and +2EV. If your automatic bracketing only allows you to space the exposures 1 stop apart, take 5 shots at -2, -1, 0, +1 and +2. After this, DON’T MOVE THE CAMERA since you may not be finished.
With the first bracket in the can, take a look at the picture histograms on your LCD (consult your camera manual to find out how to display the histogram on your camera). These will tell you whether you have captured all the necessary information or not.
Start by looking at the darkest image. If the histogram is clipped on the right edge rather than trailing of to zero before it hits the edge, you have lost some highlight detail so you need to take another exposure with a shorter shutter speed to capture this. Turn auto-bracketing off, dial in -4EV exposure compensation and shoot another exposure. Repeat this step, moving +2EV each time until you see no clipping on the right edge.
Look now at the brightest image. If the histogram is clipped on the left edge you still have some lost shadow detail so you need to take another exposure with a longer shutter speed. Turn auto-bracketing off, dial in +4EV exposure compensation and shoot another shot. Repeat this step, moving +2EV each time until you see no clipping on the left edge.
Once you complete this process, you will have captured everything you need to build an HDR image. Practically, I find that the process is more sensitive to loss of highlight detail (people don’t mind areas of black in the picture but blown out white areas are really annoying) so if you are pushed for time, make sure you capture the highlights before worrying about shadows. To be honest, if I’m in a real hurry, I just grab a 3 shot bracket (-2, 0, +2) and keep my fingers crossed until I get the camera back to my computer.
The image below shows the original brackets for one of my popular Hamilton Pool images. The final image on the right was generated using Photoshop and manually masking together parts of each of the original exposures. You can do this if you want but the HDR one looks a lot better to me and took about 3 hours less to process!.
