Infinity

Infinity

My maths teacher, Mr Witcomb, said that plus and minus infinity met up round the back of the blackboard… Infinity is a difficult concept sometimes, and we need to play tricks with our minds to get to grips with it.

Lenses with a focussing scale have a symbol at the long end, a sort of figure eight on its side, and it’s the point at which the lens is focussing rays of light that are arriving parallel to each other on the film or sensor. You may have noticed that the scale on the lens is different on wideangle and telephoto lenses – a very wide lens may have markings for 1 foot, two feet, six feet and infinity: a moderate telephoto lens will have markings that go up to fifty or sixty feet. There’s an equation in my Ilford Manual of Photography that explains this (I think), 1/u + 1/v = 1/f, where u is the distance of an object from the lens, v is the distance of a sharp image of the object behind the lens, and f is the focal length of the lens.

I’ll leave that for the theoretically-minded to ponder on, draw out on paper, and understand. For everyone else, let’s get back to a lens or two.

In the good old days, before autofocus, most lenses had a physical stop in the focus mechanism. If you wanted infinity focus, you turned the focus ring all the way to the stop, and it was right. Modern lenses usually focus a little bit beyond infinity… This is because they allow for errors in the lens mounting, wear in the mechanisms, and possibly for the way that the AF mechanism hunts backwards and forwards before locking on.

Now, because designers have taken advantage of the way htat an AF mechanism can take up the slack, even manual focus lenses designed for AF cameras – like this Samyang – focus ‘beyond infinity’:



And now there are lenses completely without a focus scale… This first appeared as a lack-of-feature on consumer zoom lenses, but it now extends to very respectable glassware. My Sony 85mm f/1.8 is smaller, lighter and cheaper than the wonderful G-Master, but the performance is quite outstanding. Do I miss the focus scale? Not really. While I was used to one for a very long time, I didn’t use it often: maybe for manual flash exposure setting, and occasionally for infrared film… (With infrared, the light comes to a focus in a different place from visible light, so you need to adjust focus slightly. Most older lenses had a little red line next to the focus index mark: focus using the usual mechanism (rangefinder, SLR screen or AF), and then turn the focus ring so that the red line points at the distance that the focus index was next to.

See the red line by the left-hand f/8 depth of field mark? That’s the IR focus line:



Infinity – sometimes, it’s best not to think about it, and get on with shooting…