The early bird gets late to work. On focal depth.

A very short night, arising at 5am to take Mr T to Stansted Airport. Arriving at 6:15am, the airport seemed remarkably quiet, without the usual massed queues around the check-in desks of the bargain flight airlines. Even the queues through security were shorter. Having ensured that there were no last minute glitches to Mr T's departure, I wandered outside to the viewing area to the south. The intention was to record the noisy take-off of the passenger jets with my new audio recorder. The wind noise was too great and I lacked the microphone windshields, which were still in the post, so I desisted from further recording attempts.

I was therefore back in Milton by 8am and went back to bed for a couple of hours catch up sleep.

Ironically, this meant a late start to work, despite an early rise. Working on a design for a German client's flyer therefore extended into the evening, with the draft not entering the internet till past 8pm.

Yesterdays photos of patterns created by airbubbles in microscope slides were uploaded. Each scene was photographed as a series of images at different foci. Using an indispensable tool, the program Helicon Focus, These were assembled and a composite generated, featuring all elements that were in focus. The results were pleasing on the most part but the process was time consuming.

The process is necessitated because of the very narrow depth of field at high magnification, a consequence of the microscope objective lens being so close to the sample (the higher the magnification, the closer the lens is to the subject). The principle can be illustrated simply. The reader is invited to look out of the window and fix their gaze on a plant or object 30 or forty yards away. Without shifting the centre of attention, note that objects behind the centre of attention are in focus for a considerable distance beyond.

Now peruse a book or screen at a reading distance of about 18 inches and attempt to gauge the clarity of objects beyond without changing the focus on the nearby object. In this instance, objects such as a finger held closer to the nose or flowers at the other end of the room are out of focus. The focal depth, equivalent to the distance over which objects appear in focus, is decreased.

A low power microscope objective may be a couple of centimeters from the sample and the focal depth is reduced to no more than a millimeter above or below the focal point. At higher magnifications, the focal depth is within a tenth to a hundredth of a millimeter, so that in effect one is viewing optical slices through a transparent object.

With a microscope, the viewer gains a perception of the subject by moving the focus up and down. The phenomenon can be mimicked by a video or film recording of the same process. However, a still photograph will be a slice at one focal plane. Software such as Helicon Focus computes the areas in focus from a number of different images of the same object at different focal planes, creating a so called focus stack. It works particularly well with opaque objects and generally well with transparent ones.