Depth of field

NPhoto - - Feature -

The size of a cam­era’s sen­sor has a big im­pact on the depth of field in your pho­tos: the smaller the sen­sor, the greater the depth of field at a given lens aper­ture and equiv­a­lent fo­cal length. This is a good thing if you’re pho­tograph­ing land­scapes, still lifes, macro shots or any other kind of pho­tog­ra­phy where you want as much depth of field as pos­si­ble. It’s not so good if you want to make your sub­ject stand out by blur­ring the back­ground. As a rule of thumb, DX cam­eras will ef­fec­tively give you one f-stop more depth of field than FX ones at a given aper­ture, as­sum­ing you’re us­ing an equiv­a­lent fo­cal length to al­low for the sen­sor’s ‘crop fac­tor’ and to get the same an­gle of view

The larger sen­sor in FX-for­mat Nikons like the D750 means that you need to use longer fo­cal length lenses to get the same an­gle of view, and this pro­duces shal­lower depth of field. In the days of film, when fo­cus­ing was rel­a­tively prim­i­tive and pho­tog­ra­phers re­lied heav­ily on ‘zone fo­cus­ing’, ex­tra depth of field would have been a bonus. How­ever, with to­day’s su­per-sharp lenses and pin-point fo­cus­ing, pho­tog­ra­phers can prop­erly ex­ploit their cam­eras’ shal­low depth of field ef­fects, cre­ative back­ground blur and beau­ti­ful ‘bokeh’ – and FX Nikons can do this bet­ter than DX mod­els.

The dif­fer­ences are sub­tle. In our sam­ple shots, you have to look quite closely to see that the back­ground in the pic­ture from the D750 is more blurred. The im­por­tant thing to re­mem­ber is that depth of field is not an ‘on-off’ thing. Sharp­ness falls away pro­gres­sively, not all at once. The dif­fer­ence be­tween the D500 and D750 is not the dif­fer­ence be­tween sharp and blurred, but de­grees of blur. Af­ter a while, though, you do start to no­tice that this tends to give full-frame shots a bit more spa­tial ‘depth’.

This pic­ture was taken at a fo­cal length of 35mm (50mm equiv­a­lent on the D500) at an aper­ture of f/5.6. The lens was fo­cused on the flow­ers, but you can see the de­tail on the news­pa­per print in the back­ground is still rel­a­tively clear. You’d need a lens with a wider max­i­mum aper­ture to blur it more than this.

This was taken from the same po­si­tion at a fo­cal length of 50mm on the D750. The lens aper­ture was the same at f/5.6, but you should be able to see that the print in the back­ground is more blurred. This shal­lower depth of field comes from the longer fo­cal lengths used to get the same an­gle of view on a full-frame cam­era.

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