Photo tip - How to take better HDR photos

07th August 2010
I’ve often read that people either love or hate HDR photography. Personally, I just like good photos, whether they’re HDR or not. So, although I have all the software and I’ve taken quite a few HDR shots, I’ve never obsessed over it like someone who loves HDR. As such, I’ve found my HDR results rather mixed until now. This article explains my epiphany with HDR and why I think I may have cracked it. Here's one of the photos I've taken since my epithany.



But first, let me explain how HDR works, for those of you who don’t know.

What is HDR photography?
HDR (High Dynamic Range) photography is where you take several different exposures of the same scene and then blend them all together. The purpose of it is to capture all the highlight detail and all the shadow detail. We’ve all taken photos where the sky is too bright and the landscape is perfect or the sky is perfect and the landscape is too dark; well this technique removes that problem. Normally you take an over exposed shot, an under exposed shot and a standard shot and the HDR software blends them together so that everything is exposed correctly. Always take HDR photos with a tripod so that each exposure matches up properly.

Most cameras have an Auto Exposure bracketing function that will fire off 3 exposures in rapid succession (if you set the timer on your camera). The first shot will be a normal exposure, the second will be under exposed and the third will be over exposed. Once the three shots are blended together you find that the dark areas are no longer featureless black and the light areas are no longer featureless white. The result tends to be an otherworldly effect that seems to look hyper-real because the photo shows a range of contrast that we’re not normally used to seeing in photography.

The first problem with Auto Exposure Bracketing
The ‘AEB’ on most cameras will only allow you to under expose and over expose by 2 ‘stops’. A ‘stop’ is effectively how much light you’re letting into the camera and a stop either way allows either double or half the amount of light in, depending on whether you are over or under exposing. This means that most cameras give you a maximum of 4 stops of light when using AEB. Unfortunately many scenes have a higher contrast than 4 stops of light; sunsets are a prime example.

The second problem with Auto Exposure Bracketing
Since AEB only takes 3 photos there is generally a two stop gap between each of the three exposures. This means that the over-exposed photo is quite significantly brighter than the standard shot and the standard shot is significantly brighter than the under-exposed shot. Therefore when your HDR software (I use Photomatix which is generally regarded as the best) blends the exposures it often gives quite a coarse / speckly transition in the high contrast areas of the photo. This is because the difference between each exposure is quite intense.

The solution
I always knew that you could run more than 3 exposures through HDR software, but I never got round to trying it, until yesterday. If I’d known what a difference it made I would have tried it ages ago. Why has no-one written about this! OK, they probably have, but it should have been somewhere more prominent, like the BBC news or something. Yesterday, in an idle moment I decided to see what it was like running 9 exposures through Photomatix. I didn’t even leave the room to try it out – I just took the shots right out of my office window. The results looked great (even if the subject is rather dull!). The transition between light and dark was smooth and the overall effect looked much more natural than usual.



But was it just a fortunate coincidence? To check, I took 3 of the 9 exposures that were each two stops apart and merged them in Photomatix to see if the effect was the same. Nope, it was as coarse as it usually is.



I’d found a better way to take HDR photos. With this new knowledge I’ll report back with some photos that are slightly more interesting than the view from my office window. Bear in mind that once you've merged the photos you still need to process the photo in Photoshop (or similar). What you tend to find with HDR photos is that they can be a bit flat because you have such a broad range of mid tones. Therefore a levels or curves adjustment is likely to be the minimum you would need to complete your photo.

I hope you found this article useful. If so (or even if you didn’t), why not leave me a comment using the form below.

Happy snapping

Dan

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