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corn exchange, leicester market - black and white imageReducing halos with Silver Efex Pro 2

You can use an image processing plugin several times

Nik Silver Efex Pro (V2) is a very efficient tool for converting images from colour to black and white.

However, as with all processes that can include local contrast enhancement or sharpening, there is always the risk of producing unwanted halos round objects.

Keith has been looking at ways of reducing this effect, whilst still making use of the power and flexibility of Silver Efex Pro.

Update March 2016 - The whole Nik Collection is available for free from Google.

Converting to B&W

I've made a great deal of use of Nik software's Silver Efex Pro over the last few years, where it's been a key tool in the creation of some of my large prints.

If you're new to the software, do have a look at my review of Silver Efex Pro 2, which covers many of its features and how to use them.

Even using control points in the software, I've found that the settings that work for one part of an image, may not be optimal for another (the sky for example).

Fortunately, you can apply the plugin several times to a colour image and then combine the black and white versions.

You can buy Silver Efex Pro 2 at a discount via our link at the foot of this article.

digital black and white logoKeith runs a LinkedIn group (~7900 members) for people interested in any aspects of modern Black and White photography: Digital Black and White

The image I'll use for an example is one I created during the testing of a new mount for my GigaPan and the latest version of the Autopano Giga stitching software.

It's a crop from an even larger images, but as you can see, it would make for a big print...

part of the original colour source image

Stitching multiple shots together, quickly gives me the equivalent of a 500+ megapixel camera with a ~12mm (35mm equiv.) lens. Large format detail, with the equivalent of wide lenses that don't exist (or would cost more than my house if they did).

Using Nik Silver Efex Pro 2

I'll show some of the built-in conversions that you could pick. I tend only to use these to explore some of the huge range of options that the software allows.

The default conversion - I'll always look at this and see what effect basic brightness adjustments and colour filter choices might have.

basic conversion to black and white

Note how that a preset such as the 'full dynamic (harsh)' one shows distinct halos around items sticking up into the fairly flat sky.

full dynamic (harsh) setting

If I adjust the soft contrast setting from -100 to +100 you can really see the effect I'm looking to lose (mouse over the image to see).

soft contrast shows halo effect

High structure shows less halos

high structure setting

Full contrast and structure is looking good, apart from around the darker items sticking up into the sky.

full contrast and structure

The adjustments look great in the darker detailed areas

fine detail in shadows

After a bit of tweaking of sliders, I've come up with an image I like the look of - apart from the slight halos

result of first pass with silver efex

With some images it would be possible to put some control points in the sky areas and turn down some adjustments - I tried and it made some difference, but with this particular image didn't quite work the way I wanted.

Converting again

One of the really useful things to learn about if you have Photoshop (or Elements) is the power of layers and masking them to blend layers together.

I'm using a simple basic conversion with Silver Efex Pro, a second time to produce a new version of the colour original. This time I've avoided any controls that use any form of local contrast adjustment - so really just the basic brightness adjustments/filters.

It's this 'default' view that I'll blend to remove the halos.

the 'no halo' layer

photoshop layersHere are the layers I'm using.

The bottom layer is the colour original, the next is our main B&W image.

The highlighted layer is our (2nd) default conversion - note the black mask, meaning that none of this layer is visible.

Where I lighten this mask, the layer will start to show over the layer below.

The top curves layer (linked to the 'default conversion' layer is really just for fine tuning the levels of that layer, to match them more closely to the main B&W image. It's the sort of fine adjustment I might need, but for the purposes of this demo you can ignore it

With a soft brush set at no more than 5% flow, I'll lightly paint (white) on to the mask of the 3rd layer.

I'll apply it round the edges of objects where there are halos.

Move your mouse over the image below to see the effect.

removing halo

I've made the mask visible, to show just how little application of the new layers is needed.

mask layer made visible

The whole image shows the relatively subtle effect I'm looking for (mouse over image to see).

I'm not looking to remove every last trace of a halo - there is a level where it's not too obvious (YMMV on this) and it can work to clarify some image elements - you have to see the issue to be able to 'fix' it.

editied image

One thing to remember with such adjustments is that you can always turn down the opacity of the adjustment (3rd) layer afterwards. Over the years of editing images in Photoshop I've found I get better looking results if I slightly overdo adjustments like this painting in of the non-haloed version, and then back them off slightly with the layer opacity, than to try and get everything just right.

Refinements and observations

With care it's often possible to avoid creating such halos during the initial conversion process, but sometimes that means not getting the look you were after in the rest of the image.

I've exagerated some of the example here to make the effects visible in web sized images. It's important to note that the halo effect varies with the size of an image, and you should look at image parts at the size you're intending to print to see whether any sharpening/contrast halos are genuinely noticeable.

There is a variation in our eyes' sensitivity to gradations of tone with the angular size of the gradation - the upshot of this is that what doesn't show as a halo at web size, jumps out at you in a large print, or vice versa. If I'm editing an image to be viewed as a 3 metre wide print I need to be aware of this at a lot of stages in the process.

Ultimately, you need to decide what looks best to you. Obviously visible halos just grate with me - they all too easily suggest sloppy technique. It's one of the things about some HDR styled images that puts me off - indeed, in my recent look at using Nik HDR Effex Pro 2, I was pleased to see the progress towards making more realistic looking images (and yes, I am aware of the essential 'unreal' nature of working in black and white ;-)

This image was actually created as part of testing new panoramic stitching software, so not originally conceived as a B&W print. As such, lighting isn't optimal and the distortions inherent in a very wide angle rectilinear view are a bit too evident. The distortions of the statue being the most obvious.

Note too in the shot below, how I've also removed one of the spotlight poles - I really don't have an issue with this if the image is for decorative purposes.

detail of  main image

The file size shown at the top of the article is also a bit of a fib. At 32,109 pixels wide, the image needs to be kept as a Photoshop .psb file, since it goes beyond the 30k pixel limit of both JPEG and .psd files. I reduced it to 10,000 pixels during the testing, since Silver Effex Pro does start to run a bit slow with huge images.

Images this size can also have problems in Photoshop with some plugins, and your print setup needs some care (see the making of a 14m print article I wrote last year).

Once you've made the image to your taste, there is no problem with flattening all those layers (keeping the image as B&W in a colour working space) and applying Silver Efex for a third time.

You can use the plugin in 'Brush' mode to paint in a bit more contrast/detail/lightening/darkening - just be careful to keep your adjustments well out of the way of where you went to all hat trouble not to create halos...

Article first published Feb 2013 - Discuss on Google+ or below

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