[RichC]

I've been asked for more detailed instructions, so here they are. If anyone tries this and messes up, don't come round acting like Basil Fawlty and poke me in the eye!

Do practice on some scrap brass or aluminium!

1. Ideally, remove the mounting flange from the lens. If this cannot be done, use masking tape to protect the lens from swarf. It’s best to have both hands free, so use masking tape to fix the lens or flange firmly to a desk or table. When doing detail work like this, I wear +4 reading glasses (even though I don’t need glasses to read) to magnify the view.

2. All six pits should be centred over the sensor: make a template from thin card, using the camera mount as a guide, to transfer the sensor position to the lens flange. Then, draw lines with an indelible marker on the flange, to indicate accurately the start and end positions of each pit. I used match Technical’s Coder Kit template as a guide. If you don’t have the Coder Kit or an already coded lens to follow, the distance between the centres of the first and sixth pits is 10.5 mm, so the centre of each pit is 2.1 mm from its neighbour; each pit is 2.5 mm long (these measurements are taken from the Coder Kit template). Note that the pits are arranged linearly, not along a curve – the sensor is straight.

3. The spherical engraving bits used at the Dremel’s lowest speed setting provided good control and gave the best results, creating oval pits with a regular shape and no burring. Dragging the Dremel towards me, as opposed to pushing the bit forwards, gave the most control. The two smallest Dremel spherical bits have ball diameters of 0.8 mm and 1.6 mm: use the 0.8 mm bit to make shallow indentations at the exact start and end points of each pit, then connect these indentations to make a groove with rounded ends; the depth isn’t critical (just deep enough to hold their shape – about a third of a millimetre), and the width of the groove (about 1 mm) should be that created naturally by the bit without moving the bit sideways. Once all six pits have been ground and are identical in shape and parallel to each other, that’s the end of the first stage – the pits now need to be enlarged. Note: don’t be concerned if the grooves vary slightly in shape or have slight errors that can’t be corrected without making them too large – if minor, these will disappear during the next stage.

4. Change to the larger 1.6 mm bit, and gently enlarge the pits; very little pressure is needed – pressing too hard makes the grinding process harder to control and takes off too much metal too quickly, and may even make the bit grab and jump (conversely, do hold the Dremel steady enough to have good control!). I did try using the 1.6 mm bit only, but found that without the ‘guide pits’ made using the 0.8 mm bit, the pits were harder to position accurately and their shape more ragged.

5. Finally, run a piece of 1200-grade wet-and-dry paper lightly over the pits, to take off any sharp edges.