[rcheshire (Rowland)]

I've been working on a focus mask that is easy to make and easy to use with my Canon 200mm lens.

The bow tie pattern doesn't block too much light and with substantial obstructions, ~5 mm, produces good spikes. Other dimensions as required.

I followed the (Chris) Lord method of removing the grid typical of most focus masks and used the Carey mask style to produce spikes at slightly different angles. The spikes need to be separated by a degree or two - it's easier to judge the spacing.

Focus is achieved when all four pairs of spikes are spaced evenly. The bottom image shows out of focus. Turning the focus ring clockwise - in this case a very small amount - to achieve focus, as in the top image.

Cut from a flat section of clear bubble packaging, it's relatively easy to get nice sharp straight edges with a very sharp hobby knife/scalpel. Mounted in a step up ring for attachment to the lens - tape will do.

Please modify at will. I will be playing around with it until I'm perfectly happy with it's performance.

EDIT: added better images - duplicates up the page. Note the additional perpendicular spike in focus.