Focus Testing

Robert Reeves suggests testing the focus of your lenses as the infinity mark cannot always be trusted. This isn’t necessarily because the manufacturer was sloppy; certain long focal-length lenses may significantly affected by temperature and at some temperatures they will focus beyond infinity (Buzz Lightyear, take note).

I decided to do a quick test of the 50-mm f/2 lens on my Pentax A3000. The test involves making a Hartman mask for the lens and shooting a set of star trails, each with a slightly different focus. The first step was to print a small label with lines at intervals of about 2-mm which I could attach to the camera lens where the distance calibration marks are located. This allows me to carefully turn the focus ring in steps of about 1-mm. I focused, per Reeves’ suggestion, starting from the closer settings toward infinity, stepping 1/2-mark without ever backing up to insure that the gear mechanism is fully engaged with no backlash.

I used Lyra as my target area simply because it was roughly overhead at the time which allows me to easily point without having any local street lights shine into the camera. Since my camera does not allow multiple exposures on the same frame, I used the “hat-trick” though in my case it was a black fleece glove. The first exposure was for 60-seconds. I then covered the lens with the glove and adjusted the focus ring. I then took a 30-second exposure and repeated. Making the first exposure longer than the others allows you to easily tell from the resulting photo which end is the starting point.

Below is a close-up of one section of the print showing Vega and Epsilon Lyrae. What I expected to see was a series of double trails for each star. Each pair would be closer together than the previous pair with them eventually converging into a single trail and possibly spreading apart again. Since I only see one trail (the double trail for the upper star is because Epsilon Lyrae is a double; there really are two stars there), my tentative conclusion is that focusing is not very critical for a 50-mm lens.

However, I’m not quite willing to stand on that just yet. Among other things, I’m not sure about the orientation of the Hartman mask so it is possible that the reason that I don’t see two trails is because they are co-linear instead of being parallel; i.e., maybe I need to rotate the mask 90-degrees and try again.

References

• Wide Field Astrophotography, Robert Reeves, copyright © 2000, published by Willmann-Bell, Inc.