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Underwater HDR: not ready for prime time!

Published on 2012-10-26 22:45:26

HDR stands for High Dynamic Range. This tutorial details the basics of HDR shooting and explains their applicability to underwater imaging.

So I got this really nice firmware update for my Canon T2i. It's called Magic Lantern. It extends the existing Canon stock firmware with zillions of functions. Two of them were of particular interest to me and were worth the risk of bricking my camera: Extended HDR bracketing and Intervalometer. The later is for timelapses and I may write a tutorial about this photo technique even though its application to underwater photography is not immediate. The first is what we are here for today: High Dynamic Range (HDR) photography.

But first the basics!

In a nutshell, an HDR photography is a compound picture created by merging several images of the same scene shot at different exposures. Most camera have a feature called "bracketing" which automates multiple exposure shooting for off-line HDR combination. On my Canon G12, bracketing is limited to 3 shots at -1EV, 0EV and +1EV taken by the camera with only one press of the trigger. On the T2i without Magic Lantern, you are limited to 3 shots up to 2 EVs apart. If you are not familiar with the notion of "EV" think of it as a unit of "exposure value" that represent how dark or bright your picture is compared to the "normal" exposure. There is a relation between f-stops and EV but it's not the purpose of this tutorial for us, point-and-shooters, to talk about.

The idea of HDR is to cover the widest range of illumination as possible. If you have ever tried to take a picture of say, a forest with a bright sky above it, then if you expose on the forest the sky gets burnt (over exposed), if you expose on the sky, then the forest is black (under-exposed). By taking pictures at several exposures and combining them together in a clever way, one can make the details of both scenes, the forest and the sky, appears. The problem with the 3-shot limit of most cameras is that the range of exposure is greatly limited. With a firmware such as Magic Lantern or with pro contraptions connected to your DSLR, you can extend that limit to be able to cover a wider range. With Magic Lantern, up to 9 shots can be taken to create an HDR compilation. With special gizmos, there is no real limit, also one can argue that usually 9 shots will cover the whole dynamic range, so yay! to Magic Lantern.

The figure below illustrates the principles of HDR:

Figure: HDR composition

So let's do this!

One of the main challenges of HDR photography is to be able to take several shots of the same scene: on land, the use of a tripod and a constant focal is recommended. However, most HDR combining software are capable of aligning several shots and allow hand-held HDR shooting. Constant focal is not mandatory but allow the same part of the scene to be in focus, regardless of the exposure.

If the scene changes, ghosting will occur. The following figure illustrates ghosting: in the scene being photographed, people walk in front of the camera during the 7 shots taken. With shot being about 1 second apart, there is clearly a difference between the first shot at 0 EV and the last one at -1.5EV.

Figure: Ghosting Effect

Ghosting effect can be corrected by HDR combining software. It is a painful manual process where ghosted zones are selected by the user (you!) and the software does not combine shots for these zones. This creates non-uniform zones in the pictures which may become noticeable if they cover non negligible areas of the final shot.

And now underwater...

So what does all of this mean for us, avid point-and-shoot underwater shooters?

Well, nothing really good actually.

The main problem underwater when it comes to HDR, is that everything moves! Even rocks move underwater, relative to the camera, because, you, the photographer, move with the current. Even with top notchbuoyancycontrol, there is absolutely no way you can shoot the same scene at 1 second interval without moving. You can of course bring a tripod underwater, but remember our moto: dive light! So the solution is to shoot stuff that won't move by themselves, like rocks or wrecks and pick up conditions with little to no current and shoot "landed", i.e. not floating around...

The results? Well, not splendid I'm afraid... Here's a couple of shots taken in La Paz last month with the G12 at -1EV, 0EV and +1EV:

So even with my best effort, this is definitely not worth the cover of a magazine!

But the technique is great, and yields amazing results topside!

Oh, and by the way do you remember the time when we had to shoot on *film* and you would not really know what a shot looks like until you're back home and havedevelopedthe films... Well with HDR it's about the same thing. You pile up shots, and how they will look once combined remains a mystery until you put them back together in the digital darkroom...


 

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