Un­der­stand­ing lenses and fo­cus­ing tech­nique is im­por­tant for ob­tain­ing crit­i­cal sharp­ness

Digital Photograper - - Techniques -

No lens is per­fectly sharp, es­pe­cially when stopped down to small aper­tures, re­sult­ing in dif­frac­tion – the bend­ing of light rays as they hit the aper­ture blades. The smaller the aper­ture, more light rays get bent, so images get pro­gres­sively softer as you stop down.

The dilemma is that it is dif­fi­cult to achieve enough depth of field with­out stop­ping down to a rel­a­tively small aper­ture. A balance has to be struck, where the aper­ture is small enough to get the depth of field you need, but not so small that dif­frac­tion be­comes a prob­lem.

Most lenses are sharpest at mid-aper­tures, usu­ally around f8. With good lenses, f11 will be only marginally worse and at f16, you will see a dif­fer­ence, though not an ex­treme one. Be­yond that, images can be­come no­tice­ably soft. The sen­si­ble ad­vice, there­fore, is to shoot in the f8 to f11 range where pos­si­ble, but not to be too con­cerned about stop­ping down to f16; how­ever, try to avoid stop­ping down fur­ther.

Know­ing where in the scene to fo­cus is key as this al­lows you to place the zone of sharp­ness where you need it, so that depth of field ex­tends from fore­ground to back­ground. Rather than the tra­di­tional fo­cus­ing ‘a third of the way in’ to the scene, which is rather im­pre­cise, try ‘dou­ble-dis­tance’ fo­cus­ing. As­sum­ing you’re shoot­ing with a wide-an­gle lens and a small­ish aper­ture, es­ti­mate the dis­tance from the cam­era to the near­est ob­ject you want to keep sharp and then fo­cus at dou­ble that dis­tance. In prac­tice, this de­liv­ers con­sis­tently good re­sults.

You need to make sure that once you’ve fo­cused, fo­cus is locked. So ei­ther fo­cus man­u­ally, or set up ‘back-but­ton fo­cus­ing’ where fo­cus­ing is de­cou­pled from the shut­ter but­ton and as­signed to one of the but­tons on the rear of the cam­era.

Fi­nally, to make sure your lenses per­form at their best, keep them and your fil­ters clean and free of dust and be sure to switch off vi­bra­tion re­duc­tion when the cam­era is mounted on a tri­pod, as VR on tri­pod-mounted lenses can ac­tu­ally in­tro­duce shake.

to get enough depth of field, this im­age was shot at f22. it’s sharp enough but dif­frac­tion has soft­ened some fine de­tail With clas­sic ‘close fore­ground’ land­scapes, dou­ble-dis­tance fo­cus­ing is an easy tech­nique for max­imis­ing...

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