Achieving focus

Achieving focus

I was discussing focussing aids with a friend yesterday, although she didn’t know it…

So I’ve been thinking about the changes in how lenses focus, and the various implications. We’ll start where I did when I was in my teens, with manual focus.

Most of the simplest and cheapest lenses focus by the front part rotating and moving backwards and forwards. It’s mechanically easy to design – just a screw mechanism moving one tube inside another.



There are, I believe, some quality issues with this, so it’s not terribly common. It’s far more usual for a lens to have a much more sophisticated mechanism that allows the whole of the lens to move backwards and forwards, usually without anything other than the focus ring rotating. This may not seem like a very big deal, but with the aperture mechanism in the moving part, the position of the aperture ring and the index line would move around: not great (but it does happen on a few lenses).

Usually, the focus ring was nice and wide, to give a good grip for the fingers, and the movement was nicely damped: no backlash at all, and enough resistance to prevent accidental movement. The feel of a good lens is a wonderful thing: silky and purposeful.



Everything changed with autofocus: as the electric motor doing the work had to work from a battery with limited capacity, the aim was to reduce resistance to a minimum. As for manual focus, it took a back seat, and early AF lenses often had a narrow focus ring. There tended to be a thorough lack of ‘feel’ and bags of backlash.

The whole thing was so iffy that Zeiss stated that it wasn’t possible to make an AF lens that met their standards: that might be why they collaborated with Yashica over the Contax AX, possibly the oddest AF camera ever, in which the whole mirror box, shutter and film were moved backwards and forwards, behind a stately manual focus lens. Fortunately, Zeiss have found a way to make it work now…

Eventually, it became apparent that even with the magic of three AF spots, some people, sometimes, would need to focus manually, and broader focus rings came back in.

And, somewhere along the way, designers discovered that you can, if you’re clever enough, make a lens focus without all this unseemly external movement, by moving a few elements inside the lens relative to each other. Loads of big pluses: less mechanical work to do, so less drain on batteries. No rotating front elements, twisting graduated filters out of true. And – importantly for digital – no tendency to suck air (and dust) into the lens and mirror box as the glass moves away from the sensor.

There’s plenty to say about how – other than AF – we check that things are in focus: but to do that properly, I’ve got to work out how to take a picture through the viewfinders of cameras. Wish me luck: any tips welcome!