PhotoPro: Backscatter elimination

Before: left part, after: right part of the photo Step 1 Step 2 Step 3 Step 4 Step 5

Photographers who brave the cold UK waters prior to the spring bloom are often rewarded with decent visibility and seasonal subjects such as lumpsuckers and mating cuttlefish. There are many reasons to praise the photographic potential: my favourite is when I get a ‘Was that taken here?’ reaction to an image that reveals a creature or colours that non-diving folk just can’t believe come from Blighty.

One of the biggest challenges of low-visibility photography is backscatter – the underwater shooter’s longstanding nemesis. Underwater, we need strobes to illuminate our subjects, but our flashes light up more than we intend. Water suspends particles and these reflect the light back to the camera, looking like bright pinpricks when in focus, and blurred, aperture-shaped blotches when not. At best, they distract from the subject; at worst, they obscure it.

Most backscatter can be eliminated by correct in-water photographic techniques. However, in my opinion, people spend too much time fussing over correct strobe positioning, and not enough on correct photographer positioning. It is a mistake to think that eliminating backscatter is just about angles; specifically the angle between your strobe and your lens. Some believe that a mystic angle exists where all images flourish in spot-free nirvana. It doesn’t. But before you get too depressed, the truth contains even better news.

To beat the enemy, we must first understand it. With backscatter, we need to think in three dimensions, not two. We must think of volumes, not just angles. Backscatter is created where the volume of water the lens is looking through overlaps with the volume of water that the strobe shines through. Think of them as cones, getting wider as they extend out in front of the camera and strobes. Once we can visualise these cones, it becomes obvious that the easiest way to reduce backscatter (the overlapping volume of the cones) is to position ourselves as close to the underwater subject as possible.

So where’s the good news? Well, once we get correctly close, backscatter isn’t really an issue, wherever we position our flashes. Instead, strobe positioning is about creating a style of lighting that suits our objective. Lighting is a big subject and one we’ll cover in detail in the future. For now, the message to take home is that the further we are from the subject, the further out from the camera the strobes need to be. Think of those cones overlapping. It becomes obvious.

That said, I’d never claim that every picture I take is free of aquatic acne. Sometimes we have to shoot first and tidy up later. So, particularly for those searching for an excuse to skip those early-season dives, I want to dedicate the remainder of this column to Photoshop’s selection of anti-zit treatments.

I took the example image off Plymouth and I have cleaned half of it so you can see it before and after retouching. Underwater, I did most things right, but still conspired to cock it up. Conditions were great, and I got very close using a fisheye lens, but unfortunately in my excitement I did not take care on my approach and I stirred up some sediment. Backscatter galore!

As is typical of Photoshop, there are several ways of doing anything. There are at least five tools for backscatter elimination and, for the highest quality and quickest results, understanding the subtle differences between them and applying them in combination is best. Below is a typical five-stage backscatter removal workflow. This is just an example. Each image is different: some will require all the tools, others just one or two. Try these tools on your own images and you’ll soon learn which ones you need most regularly.

Now underwater photographers have the power to eradicate our nemesis, we must ask ourselves: should we? The ocean is naturally filled with particles and, particularly because backscatter can be so easily removed, leaving in a little in can sometimes add atmosphere and realism to our shots and save them from looking too sterile.


Image data
Spider crab, backscatter cleaned from half. Nikon D2X SLR. Subal housing. Tokina 10-17mm fisheye @ 14mm. 1/50th at f9. 2 x Inon Z240 strobes


Backscatter - death by Photoshop
1 To remove small specks over large areas, use the Dust & Scratches filter (Filter>Noise>Dust & Scratches), which is particularly effective on backgrounds. Select the area needing treatment. Photoshop has many methods for masking images, but the Lasso or Magic Wand tools are usually best. If you use the wand, make sure you expand the selection to pick up all the spots! You may also want to feather the edge of the mask slightly. Then copy and paste the selection, creating a new layer.

2 If the backscatter is mostly white, set the blending mode of the new layer to Darken in the Layers palette (Window>Layers), which helps targets the de-spotting (if not, leave it Normal). This filter blurs detail, and targeting it leaves darker details untouched. This filter has two sliders, Radius and Threshold. Start with them both on their minimum values, then increase Radius until the small specks of backscatter disappear (don’t worry about the larger specks). If other details have blurred, increase Threshold until they return, but the backscatter doesn’t. Apply the filter and merge the layers.

3 Large blemishes on relatively plain backgrounds, typical of out-of-focus backscatter, are best eradicated with the Patch Tool (in the Tools palette). Select the tool, then draw around the mark and drag the selection to a similar-looking area of background. The tool seamlessly patches over the scatter, blending the edges.

4 For individual specks of backscatter, there are three main tools in the Tools palette. Use these last, to save time. The most popular are the Healing Brush Tool and Spot Healing Brush Tool, both of which cover and blend. The Spot Healing Brush Tool is the fastest, as it only requires you to click on each spot. Near edges, its automatic blending gets confused and the standard Healing Brush Tool, which allows you to select a sample area, is best.

5 The Clone Stamp Tool is best in areas of detail or on boundaries. This requires you to select a sample area to clone over the backscatter. You can alter the tool’s mode to Darken to target the white specks. With all tools, adjust the brush to be soft-edged and slightly larger than the scatter.