Tutorial on focus stacking images

Words and images by John Gravett of Lakeland Photographic Holidays.

With PhotoShop CS4, Adobe introduced a new application within it's package – focus stacking – a facility primarily designed to help the macro photographer, but with a little planning in the field, it's use can be greater that that.


The basic technique:
The focus stack function in CS4 and 5 is designed to combine a set of similar images, where the focus is set at different points in each image, to effectively extend the depth of field.

In the first example of the beetle on the grass, it was a very still day, and my first shot, taken at f/22, showed no subject movement despite the slow shutter speed of 1/8th second. It was evident that there was insufficient depth of field. Rather than try an even smaller aperture, which would run the risk of reduced picture quality due to diffraction, I opened the aperture up to f/14, which I knew would give me optimum picture quality, manually focused on the beetle's back, and quickly took a series of five pictures, moving the plane of focus slightly further through the beetle on each picture. The resulting five pictures – taken over only about two seconds, would overlay perfectly, the only difference being that the first picture had the focus point on the top of the wing casing, the fifth, the focus point on the end of the legs. I checked the images simply by scrolling over them on the back of the camera, and it did take a few attempts before the beetle and the grass remained sufficiently static through the five pictures. Needless to say, all pictures are taken on a tripod, to ensure they line up perfectly.

The PhotoShop workflow:
1. Under file>scripts>load files into stack, a pop-up screen appears, with a browse option.


Browse in the appropriate folder and click on the first file in the list; hold down the shift key and select the last file, which highlights – in this case – all five files.


Focus stacks can be created with any number of images from two upwards, and it works with RAW files as well as jpegs. The selected files are listed in the selection box, which has an option tick box for “attempt to automatically align source images” tick this, as it cuts out an additional action.


2. PhotoShop will spend a few seconds opening and layering the files all in the same image (which you can see by looking at the layers palette), then it will align the different pictures, this is one of the most important elements in focus stacking. The focal length of a lens is measured at infinity, and as the lens is focuses closer to its minimum focus, the effective focal length changes (some zoom lenses can “lose” as much as 30% of their focal length at their closest focus points). PhotoShop re-aligns these layers to counteract the change in focal length. Which, if you look carefully at the edges of the various layers, will result in a “stepping” to the edges between frames – we'll deal with that in a moment.


3. The next step is to make all the layers active, simply click on the top layer, and holding down the shift key, click on the bottom layer – you'll notice in the layers palette that all layers are now active.


4. edit>auto blend layers, will activate another pop up window offering options to stack images as a panorama, or to stack images, choose the latter, and tick the box option for “seamless tones and colours”.


PhotoShop will chew through the data for a while and create a different layer mask for each layer, selecting the sharpest data on each layer shows the five different layers with their associated layer masks.


5. If you then activate each layer individually, you'll find the top or bottom layer has a large transparent border, this is where PhotoShop resized the layers to align perfectly. Take the crop tool and crop the image to exclude the border – which will avoid unusual edges around the picture.


That completes your focus stack, is an enlargement of the middle of the picture, and shows the precise sharpness of the beetle – right down to the end of its legs. As a comparison, fig 11 is a single exposure of the beetle at f/20, which does not exhibit as good a depth-of-field on the legs.

Focus-stacking is not something I would usually use (or recommend) on live subjects which are prone to move, but the technique is excellent for any static macro subject, especially ones with a significant depth, such as various fungi. It offers the added benefit where by layering a range of pictures at say, f/11, total sharpness can be achieved over the stem and cap, but the background can be rendered more out-of-focus that a straight shot at f/32.


I have also used it in non-macro situations, where I have either needed enormous depth of field in a landscape, or another way I use it is where I wish to isolate an element using differential focus, but have perhaps two narrow planes of focus I want sharp, whilst rendering everything else in the picture unsharp. A single shot would not be able to keep those two elements sharp, but a focus-stack works perfectly.

So add another PhotoShop tool to your arsenal, and have a go at focus stacking.

Words and images by John Gravett of Lakeland Photographic Holidays.


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