Thursday, January 27, 2011

Ex. 10 Colour cast and white balance

White Balance, the bane of my photographic life!  Prior to enrolling with the OCA, about 80% of the photographs I took were whilst scuba diving.  Very early on in attempting to take photographs at any depth greater than a few meters it becomes very clear that light behaves quite differently under water.  Red light can only penetrate around 10M of water, all other colours having varying absorption profiles with Blue having the least attenuation.  This is essentially why photographs taken underwater have a strong blue caste.  I have been shooting underwater for several years and know how to work around this problem, but this really trips up the beginner with a point and shoot in a plastic casing that goes to 40M.  Part of the problem is that our eyes white balance for us as we go deeper, however, the camera cannot be programmed to understand this, it still thinks it is above water.  They look at my colorful pictures and wonder what on earth I did.

Well the answer is not so easy - I have two 160J strobes attached to my camera, even then I can only get good colour if the subject is no more than 1 or 2 meters away (the path length of the light from the strobe to the subject and then back again is subject to the same absorption issues as sun light). Even then I have to mix strobe with ambient light.  The next requirement is to only shoot RAW and if possible shoot subjects that have an element of white or grey in them, or periodically shoot a white card.  In all cases WB must be adjusted after shooting.  It is possible to avoid use of strobes via red filters that attempt to rebalance the colour in the frame, a great technique for large objects such as wrecks or schooling fish.  However, there is a loss of at least two stops in light which results in a need to increase ISO and hence an increase in noise.  I suspect the current generation of FF DSLR's will manage this quite well with high ISO, but my old Canon EOS 40D struggled.

To illustrate this issue, this is a very early shot of me taken with a 2 megapixel at about 20M.  All red light is gone, the only colours left are essentially blues and greens:

Even with a DSLR and external strobes, the same basic issue remains.  Each of the following pairs of images shows the shot as it came from the camera and then the image processed through Lightroom.  I have made a number of changes, however, note how the colour cast has been removed in each case by using the WB eye dropper tool.  Turtles and Sharks are good subjects for this as they have large patches of near white or grey from which to establish a good reference point.

Without shooting RAW none of this would be possible.  Fortunately in the tropical ocean, white is usually available as there is frequently coral sand somewhere in the frame - these shots into the blue are the hardest to get right.

So it is with some experience of this issue that I turn to Exercise 10.  Currently much of my photography revolves around my local landmarks and landscape, my favorite target being the Hypohaus.  These photographs were taken on a wintry day with low sunlight illuminating the scene of the upper row of images from the left.  The bottom row of images is the frontage of a block of rather dilapidated flats.

In both cases I have used a sequence of WB: (left to right) Auto, Sunlight, Shade, Cloudy.  In both cases the Canon 5D2's Auto function has done a pretty good job and on the whole this is where I leave it for most shooting. The auto mode images are almost identical to the appropriate WB image in both cases. Clearly the Shade setting is warming up the image, suggesting a higher colour temperature for the shady areas.  The cloudy setting is slightly cooler in both cases. In all of the images here setting WB in post processing with the eye dropper tool would be very straightforward.

For the cloudy day shots I have completed 3 extra shots for fun:

The top row has the same sequencing as before, however, I have added 3 extra settings to the lower row (again left to right): Tungsten, Fluorescent and Flash.  Once more the auto shot best resembles the manually selected Cloudy setting.  In this case the difference between the shots is less pronounced.  The "artifical" lighting settings, as expected, introduce strong colour casts, with the exception of Flash.  I had not thought about this prior to shooting these frames, but it really makes sense.  The colour of light in a flash gun must be balanced to daylight, otherwise the use of fill flash would not be possible - the principal use for on camera flash photography.

Moving indoors I set my camera on a tripod poiting out of the window of my home office/studio/digital darkroom, know locally as the shed (independent of the fact it is actually a first floor bedroom - it functions as my shed).  This is one of the few rooms that still has good old fashioned Tungsten bulbs (these exercises get harder!), as I need to be able to flick a switch and generate instant lighting when working in near darkness on a still life for example.  By framing both walls and some silvered devices (my printer and scanner) in the same frame as the outside world I was able to combine natural and external lighting.  I waited until the two were in balance and shot 9 frames.  Again reading left to right from top to bottom: Auto, Sunny, Shade, Cloudy, Tungsten, Fluorescent, Flash - then grey card and manual white balance.

The mixed lighting conditions have definitely confused my camera and forced a compromise that leaves the interior slightly yellow and the exterior pale blue.  The daylight settings render the outside correctly, especially the cloudy setting, but in all cases the room is almost orange.  On the other hand the Tungsten gets the room almost right, but at the expense of a very blue outdoors.  By shooting a grey card and using that as a WB setting I achieved a final shot using a manual white balance.  In this case the walls and silver elements of the printer have no perceptible colour caste, however, the world outside is now quite blue.  This is an extreme example, combining strong interior light with weak external, there simply is no WB setting that will handle both.  I would accept a slightly yellow interior as we know that lights are yellow, however the only way to balance this scene would be a composite of two images, although the blinds would make this a tedious task.

White Balance is a key element in creating a successful digital photograph, as important as ISO, shutter speed, etc.  However, with a well calibrated camera and appropriate tools this can be a post shooting activity.  I do, however, keep a grey card in my bag just in case.  I also use a colour target to calibrate my camera when I really must get it right!

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