Advanced Exposure Blending
Post Production & Digital Imaging Techniques
Apr 28th 2012
UPDATE: This article is the first of a few I've written on using Tony's Luminosity Masks. To find and read the others, just enter 'Luminosoty Masks' in the search box to the right, or on the main blog page, simply filter the blog entries by 'Post Production & Digital Imaging Techniques'.
Ever since I realised that digital photography meant mastering digital editing (AKA that-damn-machine (TDM)) I’ve been looking for ways to use Photoshop as effectively as possible in my photography.
As much as I resent time spent in front of TDM I do love turning my RAW files into the final piece I worked so hard to capture. That said, achieving that end is a long and, often arduous, process involving some fairly complicated techniques.
My perennial problem has been blending exposures together seamlessly. That is where I’ve got a bright sky and a darker foreground, I take one shot to record the sky looking nice and one for the stuff below the horizon, splice them together and produce a finished image that blends the best bits from each.
When I was using film, the best way to do this was via graduated neutral density filters. I do still use these (there’s a whole post about them here) but I also use exposure blending in Photoshop for many shots nowadays because it offers a number of advantages.
The filters when used well produced a seamless transition from bright sky to the darker area below the horizon and, crucially, they didn’t involve any further post production work/time spent in front of TDM. This is partly why I still use them when the scene is appropriate. But often there is good reason not to use them-increased flare when shooting into the sun and elements of the scene being partially covered and partially not making things awkward. This is where exposure blending comes into play.
Saddle Tor at dawn. The Hawthorn tree and Tor are above the horizon line so putting a straight ND grad over the sky will 'clip' these parts and darken them. With exposure blending I can blend the uneven edges of the sky with those of the ground. EOS 5DMKII, f22 0.5 secs & 1/60 for the sky, ISO 100.
With exposure blending I’ve always found it hard to merge the part I want in the sky with the nice bit in the other photo for the foreground. A while back, I had the good fortune to spend a day with the graphic designer who produces all the ads for the musical Chicago which you see on the stairs of the London underground. He’s a PS whizz and he taught me some useful techniques, including the use of Alpha Channels for making selections. I’ve had some success with these but more often than not, for the really tricky transitions where the edges are soft (distant horizons being a perfect example) they are too strong and leave ugly fringing lines along the edges. Done poorly, exposure blending looks, frankly, rubbish and completely devalues both the photograph and the photographer.
This is a close up of the Tor and if you look down the left hand side you can see an ugly green fringe. This often occurs in RAW files in the transition from a bright to dark area unless you apply lens correction at the RAW conversion stage but it occurs again if you use a strong Alpha Channel to make a selection of the darker rock from the brighter sky before painting. Trust me, if you printed the pic for the wall someone would spot it and they wont see anything else after that!
Typically what happens is you get poor transitions on the edges of your selections and the image looks at best weird with strange transition lines and, at worst, plain awful with a totally unnatural look about it. By this I mean halos and odd lines around the edges of things you’ve tried to select and blend with another area of the image.
My basic method for exposure blending via layers and masks is not rocket science-I just put the light on top of the dark, add a mask and paint out what I don't need and this method is fine for many types of shot. However, isolating areas of high contrast such as the brightest part of the sky (which is along the bottom of the horizon in the morning/afternoon) away from the darker part below the horizon is much more tricky. When you start painting in your ‘correct’ sky you can easily bleed some of this into the area below the horizon creating a ‘hot’ band that looks really weird as the rest of the sea is much darker in tone.
Saddle Tor after sunset. Painting in the sky against the exposure for the tree and Tor could be a nightmare done with my normal technique; I'd have to paint in between each gap in the leaves and try and get a good smooth edge around the Tor. It just wouldn't happen. Making a selection based on the luminosiy of a particular area, such as the sky frees you from this problem. Once you have it selected you can paint away and only this area will come through, leaving the darker tree and Tor unaffected. EOS 5DMKII, 17-40mm, f1 ISO 100, 1/15 & 1/125 blended via luminosity masks.
My current solution to this problem is to use some specially crafted luminosity masks available from Tony Kuyper. Tony’s a smashing chap (I’ve not met him, but he seems really nice on email…) and he’s developed a range of automated masks (which fit into your layers workflow) for selecting various parts of the images according to their tonal value. Put simply this means we can select areas that are bright, dark or somewhere in the middle via the series of masks he’s produced and they are accurate and provide clean transitions to the other parts of the image.
Tony flogs them pretty cheap to be honest-and he’s always got a special offer on via Nature Photographers website. What’s great is Tony has written very detailed articles to accompany his masks to explain how they work and how to use them. He also offers very fast and comprehensive tech support via email for any specific questions you may have.
Now, you might be wondering why don’t I just use some automated blending software, such as Photomatix Pro. Well, I’ve tried it and I don’t like the results. I don’t like it all being automated because it doesn’t know exactly how bright the various parts of my image should be and I don’t think the results are entirely natural. I prefer to have manual control over my image production and get it looking ‘just right’ which is what Tony’s masks allow.
I’ve only really scratched the surface of these masks at the moment but I’ve successfully used them to blend in selective areas from one image into another with seamless transition zones so so far so good.
If you look at this shot below, taken at dawn near Porthcurno, Cornwall, I’ve merged three exposures-one for the dark foreground rocks, one for the cloudy top part of the sky and one for the very bright band of sky in and around where the sun is. This was not something I could have achieved before. With the luminosity masks it was relatively quick and easy to do and, after checking it all at 100% magnification, I am satisfied that there not any unwanted aberrations from blending the various parts together. Tony’s masks worked a treat because I could select the dark areas (the rocks) and keep them selected while I painted in the sky and sea around them from the layer below. None of the dark rocks from this sky/sea layer appeared and the edges where the rocks are against the sea are perfect. Same applies for the hot spot where the sun is, I brought that in without any problems occurring in the transition areas.
With so much difference in the exposure values for the four areas of this image (clouds, sun burst, sea and foreground rocks) three exposures had to be made and blended together. The task of making the transition between the four areas seamless was made all the more easy by using the luminosity masks to make tone based selections of the areas to paint in. EOS 5DMKII, 17-40mm, f22 and various exposures.
I’ll be posting plenty more examples of how I’ve used these masks as I get through my RAW’s from various trips and, hopefully, you can see how I’m using the masks in my workflow and experiment with them yourself because they are an excellent solution to the perennial problems of manual exposure blending.