Focus Stacking the Odds!

Achieving optimal focus is paramount to obtaining sharp images.

Most modern cameras, even the ones bundled on mobile phones, have auto-focus capabilities that are relatively idiot proof. They will deliver reasonably sharp images fairly consistently, limited only by the quality of their optics. And, what’s more, they’ll do so with minimal, if any, effort on the part of the photographer.

Those who desire to be more creative will quickly realise that ‘achieving focus’ is a far more complex entity than simply pointing in the direction of the subject taking the shot.

Two terms that will constantly beg – if not demand – to be recognised are ‘focal plane’ and ‘depth of field’.

The focal plane is the plane that is perpendicular to the focal axis and runs through the focal point. Put simply, if you focus on a particular object at a given distance away, everything else that lies in line with it at the same distance will also be in focus. Assuming that all adverse factors like camera shake etc are taken care of, images of subjects that lie in the focal plane will appear sharp.

Objects in front of and behind the focal plane will progressively go out of focus depending on how far from it they are. ‘Depth of field’ or DoF as it is often referred to, is the distance behind and in front of the focal plane in which objects still appear reasonably sharp.

DoF varies considerably and is dependant on a number of factors including, focal length of the lens, distance of the subject from the camera and aperture size.

The image above is of the Columbia Icefield Glacier in Banff National Park in the Canadian Rockies and was taken in July 2016.

The entire scene, right from the rocks a few metres away in the foreground to the  distant mile-wide glacier, the snow-topped mountains and even the clouds beyond are all in focus. This enormous DoF was achieved by using a wide angle lens with a 17mm focal length and a small aperture (f18). ( I used the complex-sounding but relatively simple concept of ‘hyperfocal distance’ to calculate the camera settings.)

Some situations require exactly the opposite, i.e. a shallow Depth of Field. Like in the image of the Tickell’s blue flycatcher below taken in Goa in July 2018. Only the bird and its perch are in focus while the background is beautifully blurred. This shallow depth of field was achieved with a tele lens (700mm) using a relatively large aperture (f5.6).

In this regard the genre of photography that really tests the skills of a photographer is macro or close-up photography. Typically, the lenses have fairly large focal lengths, using wide apertures (due to sub-optimal lighting) and, more often than not, the distance from camera to subject is in inches rather than feet.

The image of a ruler above will help demonstrate what I mean. The lens was a Canon EF100mm, aperture used was f2.8, and the distance from the camera to the focus point, (in this case the 20cm mark on the ruler) was about a foot. As you can see, the depth of field as indicated by the markings on the ruler that are in focus is incredibly shallow. Barely 6-7mm!

It’s no wonder then, especially with macro photography, focusing can, and often does, go horribly wrong as in the case of the dragonfly image below. Only the middle of the tail and left wing fell within the focal plane and hence in focus. The rest is blurred, rendering the image useless (except when used in the context of this article)!

One of the solutions to increasing the DoF is to use a smaller aperture (larger f-stop). Also, in the field, optimally positioning oneself so that the entire subject falls in the focal plane of the camera certainly helps to achieve good results as in the case of the spectacular Blue Oakleaf butterfly below that was taken at Ovelekar’s Butterfly Park in Thane, Mumbai. Note that the butterflies in the background (on the right) that fell outside the focal plane and beyond the DoF are completely out of focus.

Out in the wild where everything is constantly and unpredictably on the move, one can well imagine that macro photography can get pretty frustrating. I remember shooting hundreds of images of a spider on its web in my backyard in Goa on a windy day and not a single image was in focus!

Still life in a studio setting is a different cup of tea altogether. As an example, this Sunday I made myself a couple of Margaritas and positioned the glassed beside one another; the one on the right about 3-4 inches further back than the left one. Now that may not sound as much but, going by 7mm DoF of the ruler image, anything more than one inch is pretty humongous! As we can see by their images.

When I focused on the glass on the left (above) the one on the right was hopelessly blurred. And when I focused on the one on the right (below) exactly the opposite happened.

Leaving me with a couple of images that did not really achieve what I was looking for.

Which bring us to the concept of ‘focus stacking’.

With the camera on a tripod and preferably using a shutter release cable to reduce camera shake a number of images are taken, each one focussing at different points of the scene.

These are then loaded into a image editing software program like Photoshop and the images stacked so as to get the result below. Abracadabra, both glasses and the two limes are now perfectly in focus!

Depending on where I choose to limit the range of focus I can control the amount of blurring of the background.

That done all that was left to do was consume both the Margarita’s.

Which was a real pity as it brought me back to square one with everything progressively becoming blurry and threatening to go out of focus once more!