Pixiq

Over the past year or so I have been experimenting a lot with image stacking – mainly because I love the effect of incredible depth of field in the foreground: it is that old quest for sharpness in pictures of  small living  organisms.

It is easier to create composite images indoors where you have control over the sorts of things that send successive pictures out of alignment. However, there is no reason a series of images for a stack has to be recorded using a focusing rail or under the controlled conditions of a tabletop or studio.

I have looked at aspects of stacking in two earlier posts (Helicon Remote and Image Stacking and Jurassic Park) but in what follows I have taken a look at some do’s and don’t’s for making the process work in the field. This post is taken from my new Lark book Digital Close-up Photography Q&A ( that will be on the shelves very soon…) . As usual, it is a distillation of lots of experiments tried (and often failed) in the field!).

It is possible to work with ambient light in the field. You can do this at wider apertures (f/5.6 to f/8) so shutter speeds can be fast enough (1/250 second or faster) to freeze camera shake or slight subject movement—movement within the scene will spoil the ability to register among the series of images when stacked. Here are a few points you might find it useful to remember when attempting to record image stacks in the field:

• Turn autofocus off and focus manually other wise you adjust: the camera fights back.
• Set your camera on a firm tripod on a still day and fire it using a cable release.
• Plant stems, though “elastic,” do not always return to the exactly the same place between pictures when there is a breeze. A clear acrylic (Perspex) screen can be used to shade a plant or other subject.
• Flash is an option, but many insects flinch with flash and change position slightly from one frame to the next .

• There may be a significant delay when using a built-in or accessory flash that is not very powerful, due to their charging cycle. The flash must be at the same power for each successive shot to have the same exposure. You may be able to adjust obvious discrepancies in Lightroom or Photoshop when you look at a stack before combining them.

• Please work with living creatures and treat them with care—this is a test your of photographic skill.  There are far too many pictures of insect corpses on the internet from people unaware of just how obvious it looks to any half-awake naturalist.

An additional strategy is to take the some studio equipment out to the field – even a laptop to control the camera via Helicon Remote, although that means more to carry. You can set up outdoors with flowers, or even insects on a piece of plant material on a calm day.

 My early results varied tremendously in quality and in the final results there were far too many artefacts visible where the stacking program could not make a transitions smoothly. Remember that the focus steps need to be very small with highly magnified subjects; perhaps 20 or 30 in a stack with the focus shifted almost imperceptibly between shots because the dof is very small and each step must be less than that. This is to create some overlap in sharply focused details between successive images.

To get an idea of the size of the steps between shots in the stack, use a depth of field table for example at Magnification (M) = 2x the d.o.f is 0.28mm at f/5.6 and  0.4mm at f/8. This decreases rapidly at higher magnifications and a rack and pinion drive on a focusing rail is not always fine enough for this type of macro work. Focus slides that use a micrometer screw can do the trick and one, the Stackshot, can do this under control from Helicon Remote.

Critical Sharpness

Sometimes individual images in the stack don’t seem sharp anywhere. This is often happens with magnified images, and there are two things to consider:

1.  Vibration, which is caused not only from camera shake or subject movement, but also from the camera's mirror bounce (on DSLRs). Use your mirror lock function if you have one, if it eliminates live view. At higher magnifications, however rigid the stand you use and the table it is on, you still do not damp this vibration.

 2.  Diffraction softening, due to the dispersal of light rays as they travel through small apertures. There is a marked or set aperture for a lens used with a bellows or tube (e.g. A reversed lens, a true macro, a microscope lenses, or a macro such as Canon's MPE 60mm), but you have to consider the effective aperture because of the extension's effect on diffraction. 

 Suppose the lens is set to a marked aperture of f/8 and an extension is used produce a magnification of 5x. That changes the light intensity by about 6 stops so the aperture is now effectively f/45. Even opening the lens to f/4 produces an effective aperture of f/22. Any smaller and diffraction becomes obvious, although you can try unsharp mask/smart sharpen  for a degree of correction.

Stacking is not as confusing at it may sounds - it works with tiny, incremental focusing changes to give very encouraging results. When you get hooked on this technique (and you will), you will strive for perfection…whatever software you use

 Remember, those large stacks (20 or more images) just eat up space on a hard drive!