Focus at infinity and beyond

I have long been interested in having a go at astrophotography and found the comments made by Tom Ormerod (AP 7 August) very informative.

However, his statement that it can be really tricky to focus manually on the stars completely threw me. Wouldn’t it just be a case of focusing on infinity, or am missing something here? / David Richards

Technical Editor Andy Westlake replies: Yes, it’s just a case of focusing on infinity. But the question is how do you accurately focus on infinity, especially in the dark? It isn’t remotely easy, because barely any lenses have a hard stop at the end of their focus travel any more, let alone an accurate one. Then if you do focus on infinity, how do you make sure the lens stays there? Even a slight nudge of the focus ring will throw the stars out of focus. Also most lenses for mirrorless cameras are focus-by-wire, and tend to reset the focus position when you turn the camera on or off. Various solutions to this problem have been developed. The Samyang AF 24mm F1.8 FE, for example, has a function specially designed for astrophotography that allows you to accurately calibrate its infinity focus position and then set the lens there by pressing a button on the barrel. Certain Irix manual-focus lenses have a calibrated click-stop at the infinity position, along with a locking focus ring.

Meanwhile, Olympus users who are lucky enough to own either the OM-D E-M1X or the OM-D E-M1 Mark Ill benefit from a unique ‘Starry Sky AF’ mode, which remarkably can autofocus on stars consistently accurately.

Amateur Photographer
2 October 2021
Letters page
Page 20