How To Use Graphics Software To Optimise Underwater Photographs
The computer screen that you are currently viewing is made up of millions of pixels each one of which is subdivided into three colours - red, green and blue. These are often referred to as the 'Primary' colours. If all three colours are turned on at the same time and at the same intensity the result is a pixel that appears white. This is called additive mixing; your colour printer may use subtractive mixing in which case the colour ink cartridges will be cyan, yellow and magenta. The main thing for us to remember as regards correcting undewater photographs is that the sea progressively filters the red part of the sun's visible light spectrum - a somewhat unfortunate fact of life for those of us who enjoy diving!
The direct result of this is photographs that have a distinct green/blue cast. Not only that but relative contrast is also drastically reduced making everything look washed out. Below is an example image taken in the Red Sea at a depth of around 12m using a Vivtar 6200W six mega pixel digital camera.
As we can see this image whilst quite interesting content wise is terribly green. So what, if anything, can we do to improve it?
The first thing we're going to need is a graphics application. Adobe Photoshop is probably top of the list for most people but unfortunately it's rather expensive. An excellent alternative, one that I've been using for years, is Paint Shop Pro now currently at version X3. I use version X which I've had for a couple of years. It may prove difficult to find but version XI is available from Amazon.co.uk for around £12 - cheap as chips. Especially when you begin to discover what you can do with it.
As we know the fundamental problem here is caused by the sea's filtering out of red light and to a lesser extent blue light. There is still some red content in the image (if there weren't then we'd be in serious trouble) but it is so low in comparison to the green and blue content that to all intents and purposes it has disappeared.
Let's assume that we're going to be working on numerous images and therefore let's customise PSP a little so as to make working with these images easier and quicker. Open PSP, right click on any tool bar and choose 'Customize'. From the 'Image' catagory scroll down to 'Split to RGB' and drag it to one of the tool bars. Do the same thing with 'Combine from RGB'. Now go over to the 'Adjust' catagory and drag 'Fade Correction' to the same tool bar as the others.
Now we can start to do the magic. With the coral scene above selected in PSP I click on the 'Split to RGB' tool and PSP gives me three new images which represent the individual red, green and blue contents of the original scene.
We can immediately see that the brightest image is the green; the blue is fairly bright but look at the red - pratically nothing in comparison to the other primary colours. What we're going to do now may seem a little odd but not to worry because it works a treat. We're going to creat a new image from these three 'raster' images but in so doing we're going to build the red content of the new image from the blue content of the original image. We're not replacing the red content with the blue but rather increasing the orginal red content and using the blue channel as the reference as to how much.
Click on the 'Combine from RGB' tool icon and select the tick box - 'Sync blue and green to red if possible'. The dialog box should now look like this...
Now deselect the tick box and in the drop down box for the red channel source choose the blue image...
Click 'Ok' and PSP creats a newly combined RGB image which magically has an awful lot more red in it...
At this point we can see that the newly combined RGB image while a lot healthier looking is still a bit green and generally lacking in colour. Prepare to be amazed!! For this particular image the next step isn't too important but we'll follow it through anyway because with other images the results will be very dramatic. Click on the 'FadeCorrection' icon, select the value '5' as the amount of correction and click 'Ok' to close the dialog. This is basically just a contrast adjustment. The next step is were things really start to happen. Hold the 'shift' key down and press 'H'. This will bring up the 'Hue/Saturatin/Lightness' adjustment dialog. For the scene in question I've used '72' for the saturation level, '-70' for the hue shift and '-11' for the lightness.
I've also taken the opportunity to save these parameters under the preset name 'Underwater Correction'.
Applying these corrections to the image results in an entirely different looking scene which is far more represenative of what it would have looked like if bathed in white light. The final optional step is to sharpen the image slightly and then we can compare the original image with the PSP corrected version...
Here are some other before and after images that have been corrected using exactly the same procedure...
For this image the Hue/Saturation/Lightness settings were -70, 32, -11
For this image the Hue/Saturation/Lightness settings were -60, 4, 3. The digital noise in the image is due to the overall like of light that was available at the time of taking the photograph. The shot was taken in the Red Sea at around 15m on the Umm Gamaar reef and these particular Yellow Butterfly fish were hiding in the coral. However, all things taken into consideration it's not a bad result.
For this image the Hue/Saturation/Lightness settings were -60, 65, -4. What a shame that this beautiful Angel fish decided to pass underneath a coral fan just at the wrong moment! I'll just have to go back for another try....
The technique described here should be applicable to any quality graphics application Photoshop included. Pleae feel free to leave any comments or ask any questions.
Clive McCombe B.Eng.
is qualified to PADI Open Water Instructor level and lives with his wife in central Italy.
They run a lovely self catering holiday apartment in the Sulmona valley called MonteViste